Estimates of Freshwater Discharge from Continents ... · this discharge from land along with...

660 VOLUME 3J O U R N A L O F H Y D R O M E T E O R O L O G Y

q 2002 American Meteorological Society

Estimates of Freshwater Discharge from Continents: Latitudinal andSeasonal Variations

AIGUO DAI AND KEVIN E. TRENBERTH

National Center for Atmospheric Research,* Boulder, Colorado

(Manuscript received 22 March 2002, in final form 12 July 2002)

ABSTRACT

Annual and monthly mean values of continental freshwater discharge into the oceans are estimated at 18resolution using several methods. The most accurate estimate is based on streamflow data from the world’slargest 921 rivers, supplemented with estimates of discharge from unmonitored areas based on the ratios ofrunoff and drainage area between the unmonitored and monitored regions. Simulations using a river transportmodel (RTM) forced by a runoff field were used to derive the river mouth outflow from the farthest downstreamgauge records. Separate estimates are also made using RTM simulations forced by three different runoff fields:1) based on observed streamflow and a water balance model, and from estimates of precipitation P minusevaporation E computed as residuals from the atmospheric moisture budget using atmospheric reanalyses from2) the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR)and 3) the European Centre for Medium-Range Weather Forecasts (ECMWF). Compared with previous estimates,improvements are made in extending observed discharge downstream to the river mouth, in accounting for theunmonitored streamflow, in discharging runoff at correct locations, and in providing an annual cycle of continentaldischarge. The use of river mouth outflow increases the global continental discharge by ;19% compared withunadjusted streamflow from the farthest downstream stations. The river-based estimate of global continentaldischarge presented here is 37 288 6 662 km3 yr21, which is ;7.6% of global P or 35% of terrestrial P. Whilethis number is comparable to earlier estimates, its partitioning into individual oceans and its latitudinal distributiondiffer from earlier studies. The peak discharges into the Arctic, the Pacific, and global oceans occur in June,versus May for the Atlantic and August for the Indian Oceans. Snow accumulation and melt are shown to havelarge effects on the annual cycle of discharge into all ocean basins except for the Indian Ocean and the Med-iterranean and Black Seas. The discharge and its latitudinal distribution implied by the observation-based runoffand the ECMWF reanalysis-based P–E agree well with the river-based estimates, whereas the discharge impliedby the NCEP–NCAR reanalysis-based P–E has a negative bias.

1. Introduction

In a steady state, precipitation P exceeds evaporationE (or evapotranspiration) over land and the residual wa-ter runs off and results in a continental freshwater dis-charge into the oceans. Within the oceans, surface netfreshwater fluxes into the atmosphere are balanced bythis discharge from land along with transports withinthe oceans. The excess of E over P over the oceansresults in atmospheric moisture that is transported ontoland and precipitated out, thereby completing the land–ocean water cycle. While E and P vary spatially, thereturn of terrestrial runoff into the oceans is mostly con-centrated at the mouths of the world’s major rivers, thusproviding significant freshwater inflow locally and

* The National Center for Atmospheric Research is sponsored bythe National Science Foundation.

Corresponding author address: A. Dai, National Center for At-mospheric Research, P.O. Box 3000, Boulder, CO 80307.E-mail: [email protected]

thereby forcing the oceans regionally through changesin density (Carton 1991; Nakamura 1996).

To study the freshwater budgets within the oceans,therefore, estimates of continental freshwater dischargeinto the ocean basins at each latitude are needed. Baum-gartner and Reichel (1975, BR75 hereafter) derivedglobal maps of annual runoff and made estimates ofannual freshwater discharge largely based on streamflowdata from the early 1960s analyzed by Marcinek (1964).Despite various limitations of the BR75 discharge data(e.g., rather limited station coverage, areal integrationover 58 latitude zones, no seasonal values), these esti-mates are still widely used in evaluations of ocean andclimate models (Pardaens et al. 2002) and in estimatingoceanic freshwater transport (Wijffels et al. 1992; Wijff-els 2001), mainly because there have been few updatedglobal estimates of continental discharge. The primarypurpose of this study is to remedy this situation.

Correct simulations of the world river system and itsrouting of terrestrial runoff into the oceans has alsobecome increasingly important in global climate system

DECEMBER 2002 661D A I A N D T R E N B E R T H

models (Miller et al. 1994). For evaluating climate mod-els, a few streamflow datasets have been compiled (e.g.,Dumenil et al. 1993), although many studies includedonly 50 or so of the world’s largest rivers. Other ap-proaches (e.g., Hagemann and Gates 2001) utilize dailyprecipitation and parameterized evaporation to computerunoff that is then fed into a hydrological dischargemodel. This approach makes no use of actual measure-ments of discharge. Perry et al. (1996) gave an updatedestimate of annual-mean river discharge into the oceansby compiling published, gauge-data-based estimates for981 rivers. However, although they used the farthestdownstream gauges, the distances to the river mouthsranged from 100 to 1250 km and they made no ad-justments for this. They estimated the contributionsfrom ungauged small rivers by fitting a power law tothe dataset and extrapolating.

For years hydrologists have been improving terrestrialrunoff fields. One such example is the work by Feketeet al. (2000, 2002), who used a global river dischargedataset from the Global Runoff Data Centre (GRDC)coupled with a simulated river network and a waterbalance model to derive a set of monthly global mapsof runoff at 0.58 resolution. This merging approach of-fers the best estimate of the geography of terrestrialrunoff, with the magnitude of runoff constrained by theobserved discharge, while at the same time preservingthe spatial distribution of the water balance. For oceanicwater budget analyses, however, only the discharges atcoastal river mouths are of interest. To estimate conti-nental discharge using the runoff fields, a river transportmodel that routes the terrestrial runoff into the correctriver mouths is needed. Alternatively, estimates can bemade from gauge records of streamflow (e.g., Grabs etal. 1996, 2000), but then the issue of ungauged streamshas to be addressed and extrapolation from the gaugingstation to the river mouth is needed. To our knowledge,neither of these approaches has been done thoroughlyin estimating continental discharge, and much improvedestimates are the main purpose of this study.

In a steady state, P–E is a good proxy of runoff overland (BR75). Short-term P–E may, however, differ fromrunoff because of changes in storage and, in particular,snow accumulation and melt and infiltration of waterinto the ground. On a day-to-day basis, runoff greatlydepends upon the frequency, sequence, and intensity ofprecipitation and not just amount, as these factors alterthe extent to which water can infiltrate and be stored inthe soil. Changes in soil moisture can be important, andit is only on annual and longer timescales that conditionsmay approximate a steady state.

Another problem is human interference with the nat-ural water flows. The mining of groundwater effectivelychanges the subterranean water storage, although it isoffset to some extent by the filling of surface reservoirs.The biggest human influence comes from irrigation andwater usage, although this should be reflected in the P–E used in this study, which was derived from atmo-

spheric moisture budget analyses. For the MississippiRiver basin, for instance, Milly and Dunne (2001) es-timate precipitation as 835 mm yr21 and total evapo-ration as 649 mm yr21, of which consumptive use con-tributes 12 mm yr21 (1.8%); surface reservoir fillingprovides a loss of 1 mm yr21; and groundwater miningadds 2 mm yr21. Overall, the changes in storage overland, such as in lakes and reservoirs, and through melt-ing of glaciers are manifested as changes in sea level.Estimates of glacier contributions are 0.2 to 0.4 mmyr21 and terrestrial storage 21.1 to 0.4 mm yr21 (Churchet al. 2001). Note that 1 mm yr21 globally is equivalentto a volume of 357 km3 yr21 and is approximately themean annual flow of the river Amur in Russia (Table2). Nevertheless, these values are small compared withregional P–E values.

Substantial problems still exist in analyzed fields ofprecipitation and evaporation based upon four-dimen-sional data assimilation (Trenberth and Guillemot 1996,1998; Trenberth et al. 2001b; Maurer et al. 2000). Infact problems exist in all precipitation products (e.g.,Yang et al. 2001; Adler et al. 2001; Nijssen et al. 2001),and evaporation is not measured but can only be esti-mated from bulk formulas. Potential evapotranspirationis measured from pans, but is not a measure of actualevaporation, and over the United States, estimatedtrends in actual evaporation and pan evaporation areopposite (Milly and Dunne 2001) because increasedclouds reduce the latter, while increased soil moisturefrom increased precipitation (associated with the cloudincrease) increases the availability of moisture for evap-oration. Therefore, we believe that the P–E used here,which was computed as a residual of the atmosphericmoisture budget, is potentially much more promising.

Relatively new P–E estimates were derived from theatmospheric moisture budget based on the National Cen-ters for Environmental Prediction–National Center forAtmospheric Research (NCEP–NCAR, NCEP hereaf-ter) reanalyses (Kalnay et al. 1996) by Trenberth andGuillemot (1996, 1998) and the European Centre forMedium-Range Weather Forecasts (ECMWF) ERA-15reanalyses (Gibson et al. 1997) by Trenberth et al.(2001a). New P–E estimates will also be produced forthe ERA-40 reanalyses that are under way. One test forthe P–E fields is to compute the implied continentaldischarge and compare with that derived from gaugedata. Unfortunately, the results using model-based P andE are not good (Oki 1999) because these are purelymodel-predicted fields. Although not problem free, P–E estimates as a residual of the atmospheric moisturebudget are better partly because atmospheric wind andmoisture fields were calibrated every 6 h by atmosphericsounding and satellite observations in the reanalyses.They depend critically on atmospheric low-level diver-gence fields, which have considerable uncertainties, butsmall-scale errors naturally cancel as averages are takenover larger areas, and the global mean is guaranteed tobalance. If the reanalysis data are evaluated to be re-


TABLE 1. Datasets used in this study. All are monthly.

Variables Type and coverageResolution

(8) Period Source and reference

Streamflow Station, land 1–1001 yr NCAR; Bodo (2001)Runoff Composite, land 0.5 Climatology GRDC–UNH; Fekete et al. (2000)E–P NCEP–NCAR reanalysis, global 2.8 1979–95 NCAR; Trenberth and Guillemot (1998)E–P ECMWF reanalysis, global 2.8 1979–93 NCAR; Trenberth et al. (2001a)P Gauge plus satellite plus reanaly-

sis, global2.5 1979–98 CMAP; Xie and Arkin (1997)

P Gauge plus satellite, global 2.5 1979–99 GPCP; Huffman et al. (1997)T Station data, land 0.5 1961–90 CRU; New et al. (1999)River basin at-

tributesSimulated river network, STN-30p 0.5 UNH; Vorosmarty et al. (2000)

liable for estimating continental discharge, then they canbe applied1 to study the interannual to decadal variationsof continental discharge because the reanalysis data are,or will be, available for the last 40–50 yr, and are likelyto be updated regularly. This could become a valuableapplication of the reanalysis data given that many rivershave no or only short records of streamflow and thatthere has been a widespread decline in the number ofriver gauges during the last few decades (Shiklomanovet al. 2002).

In this study, we analyze station records of streamflowfor 921 of the world’s largest rivers to provide newestimates of long-term mean annual and monthly con-tinental freshwater discharge into the oceans througheach 18 latitude 3 18 longitude coastal box. We makeuse of the composite runoff fields of Fekete et al. (2000,2002), a state-of-the-art database of the world river net-work, and a river transport model (RTM) to translateinformation from the farthest downstream station to theriver mouth, and to estimate contributions from un-monitored river basins. We also derive continental dis-charges using RTM simulations forced with various es-timates of terrestrial runoff, including P–E from thereanalyses from NCEP–NCAR and ECMWF. Hence wecompare the runoff-implied continental discharges withthe gauge-based estimate.

Our goal is to update and improve the continentaldischarge estimates of BR75 and provide an evaluationof the P–E fields from the reanalyses, and at the sametime provide useful data (e.g., river mouth flow rates)for climate model evaluations. The continental dis-charge can then be applied to produce detailed estimatesof mean meridional freshwater transport by the oceansthat can also be used in climate and ocean model eval-uations.

2. Datasets

We used a number of observational and reanalysisdata sets in this study, as listed in Table 1 and described

1 On annual and seasonal timescales, changes in soil moisture mustbe considred.

below. The P–E data were remapped onto the 0.58 gridfor driving the RTM.

Gauge records of streamflow rates are the basic datafor estimating continental discharge. We used monthlystreamflow datasets compiled by Bodo (2001) and oth-ers that are archived at NCAR (ds552.1, ds553.2,ds550.1; available online at http://dss.ucar.edu/catalogs/ranges/range550.html), together with the R-ArcticNETv2.0, a dataset created by C. J. Vorosmarty [Universityof New Hampshire (UNH)] et al. (online at http://www.R-ArcticNET.sr.unh.edu) that contains streamflowdata for the Arctic drainage basins. These datasets werecreated using various sources of streamflow data, in-cluding those collected by the United Nations Educa-tional Scientific and Cultural Organization (UNESCO),the United States Geological Survey (USGS), UNH,Russia’s State Hydrological Institute, the GRDC in Ger-many (only an earlier version of its collection), andmany national archives such as those for South Amer-ican nations. Data quality controls were applied duringthe compilation, primarily for errata and inconsistenciesin metadata. Undoubtedly, errors and discrepancies re-main, although the majority of the streamflow recordsare thought to have an accuracy to within 10%–20%(Fekete et al. 2000).

After eliminating duplicate stations (mainly throughlocations), there were 8878 gauge records of monthlymean river flow rates with varying lengths, ranging froma few years to over 100 yr. The latest year with data is2000, although most records end earlier (many in the1990s). For estimating continental discharge, we se-lected 921 near-coastal (i.e., the farthest downstream)stations (Fig. 1) from this merged global streamflowdataset using a procedure described in section 3. Mostof the selected stations are close to river mouths andhave 10 or more years of data, although a few of themare located far from the ocean [e.g., Niger (;1000 km),Mekong (;850 km), Irrawaddy (;800 km)] or haveonly a few years of data that may not be representativeof long-term means (cf. Table 2 and the appendix). Themean streamflow and drainage area data from thesecoastal stations were used to derive the river outflowinto ocean at the river mouth, as described in section


FIG. 1. Distribution of the farthest downstream stations for world largest 921 rivers included in thisstudy. Also shown are the world major river systems simulated by a river transport model.

3. Because of the varying time periods of records, themean river flow rates derived from these gauge recordsmay be inconsistent temporally among the rivers.

To derive estimates of continental discharge inde-pendent of those based on the streamflow records andto estimate the discharge from the land areas that werenot monitored by the gauges, we used the compositemonthly and annual runoff fields of long-term means ofFekete et al. (2000, 2002). Although only 663 gaugeswere used and biases are likely over regions with littlecalibration, their composite fields at 0.58 resolutionprobably represent one of the best estimates of terrestrialrunoff currently available.

We used the P–E fields derived from atmosphericmoisture budget analyses based on the NCEP reanalysesfor 1979–95 (Trenberth and Guillemot 1998) andECMWF reanalyses for 1979–93 (Trenberth et al. 2001a).These data are available online from NCAR’s ClimateAnalysis Section catalog (http://www.cgd.ucar.edu/cas/catalog/). We did not use the individual E and P fieldsfrom the reanalysis products.

We also used monthly and annual precipitation overland, derived by averaging the Climate Prediction Cen-ter Merged Analysis of Precipitation (CMAP) and theGlobal Precipitation Climatology Project (GPCP) cli-matology (Table 1). These products are based mainlyon rain gauge measurements over land and, therefore,depend on the distribution of gauges. In many openareas this is adequate, but it is typically inadequate inmountainous areas, where gradients of precipitation arevery large. Much more detailed estimates of precipi-tation by the Parameter-elevation Regressions on In-dependent Slopes Model (PRISM) at resolution of 2.59for the United States by Daly et al. (1994) have beencompared with GPCP results by Nijssen et al. (2001),

who found substantial discrepancies, usually underes-timates of precipitation, over mountains in GPCP. Nev-ertheless, the CMAP- and GPCP-averaged precipitationwas used to compute the river-basin-integrated precip-itation and to help evaluate the P–E products.

For estimating snow effects on the annual cycle ofrunoff, we used a simple scheme that requires dailysurface air temperature. The latter was interpolated fromthe New et al. (1999) monthly climatology.

To perform areal averaging and integration and toobtain estimates of drainage area for individual riverbasins, a database for the world river network is needed.We used the simulated river database STN-30p (Vo-rosmarty et al. 2000), a global simulated topologicalnetwork at 309 spatial resolution. The STN-30p contains397 named large river basins and 5755 unnamed smallriver basins for the world river network. In this database,each 0.58 cell of land surface belongs to a river basin.The database includes, among others, the upstreamdrainage area for each cell and the exact location wherethe runoff generated in any given cell enters an oceanor sea.

3. Analysis procedure

a. Selection of coastal stations

Ignoring discharge of groundwater, the continentalfreshwater discharge is the sum of river outflows intothe oceans. The farthest downstream gauging station(i.e., the station with the largest drainage area) for eachriver that reaches the ocean is the obvious choice forour purpose. Selecting the most-downstream stations forall the ocean-reaching rivers, while excluding upstreamstations, from the 8878 stations in the merged stream-


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flow dataset is, however, not an easy task. We first cre-ated a subset of stations that are not far away from thecoastlines using the distance to ocean as the criterion.This subset of coastal stations still included upstreamstations for a number of rivers, while some of the largerivers (e.g., Niger, Mekong, Huanghe) were not includedbecause the farthest downstream gauging stations arequite distant from their mouths. We manually deletedthe upstream stations.

We assigned a class number (1–6) to each river basedon the downstream width (;2/3 weight) and total length(;1/3 weight) of the river shown on the Times Atlas ofthe World (1999 edition; scales, 1 : 1.0–5.5 million) thatqualitatively represents the size or magnitude of the riv-er. For example, the Amazon, Congo, Orinoco, and theother top 10 rivers were assigned class 6 (largest rivers),while class 5 (very large rivers) included Tocantins,Yukon, Niger, Rhine, and the other top 50 rivers. Ex-amples of class 4 (large rivers) include Colorado inNorth America, Murray in Australia, and Taz in Russia.The majority of world rivers are in class 3 (significantrivers, such as Trinity in Texas, Deseado in Argentina,Limpopo in Mozambique), class 2 (small rivers, e.g.,San Antonio in the United States, Neepan in Australia,Orne in France), and class 1 (smallest rivers on the map).

Although the merged streamflow dataset is among themost comprehensive available, it was unknown whetherit included all the major rivers of the world. Further-more, we wanted to ensure that we selected all the sig-nificant (classes 3–6) ocean-reaching rivers from themerged dataset. For these purposes, we looked throughthe atlas for all named ocean-reaching rivers on all con-tinents and large islands. If the name of a river fromthe atlas was not in the subset but found in the mergeddataset, then the farthest downstream station for thatriver was added to the subset of coastal stations.

Through this tedious process, we found out that themerged streamflow dataset included all the class 6 and5 rivers and most class 4 rivers (except for Cuanza inAngola, and Fuchunjiang and Yalujiang in China). How-ever, a large percentage of the class 3 (;1/3) and class2 (;1/2) rivers were not in the merged streamflow da-taset. Apparently, there have either been no gaugingstations or the data have not been released to the inter-national community for these rivers. This is particularlytrue in Indonesia and other south Asian countries.

For some big rivers, there are significant branch riversentering the main stream below the farthest downstreamstations. For example, Obidos, the farthest downstreamstation available for the Amazon, is ;750 km away fromthe mouth. Below Obidos, two very large (Tapajos andXingu), two large (Jari and Paru), and several class 3and 2 rivers enter the Amazon. Although these riversdo not reach the ocean by themselves, they are includedin our subset of ocean-reaching rivers if station data areavailable. Another example is the Benoue (no. 52 in theappendix), which flows into the Niger (no. 27 in Table2) well below the last gauging station at Gaya in Niger.


The search of a dataset from GRDC containing long-term mean streamflow data for 198 rivers, used byGRDC to estimate freshwater fluxes into world oceans(Grabs et al. 1996), resulted in an addition of four riversto our subset of ocean-reaching rivers.

Finally, we selected 921 ocean-reaching rivers (in-cluding those entering the Mediterranean and BlackSeas), whose station locations are shown in Fig. 1 to-gether with a simulated network of the world’s majorriver systems (by the river transport model describedbelow). While the coasts in North and South Americasand Europe are well monitored, there are large riversystems not monitored in tropical Africa and south Asia.Australia is a relatively dry continent and there is anabsence of rivers along the southern coastline, so thenetwork there appears to be sufficient.

b. Estimating continental discharge

1) DISCHARGE BASED ON STATION DATA

To estimate continental discharge into each ocean ba-sin at each latitude from the station data of streamflow,one can sum up the river outflow within each latitudeband for each ocean basin. Before this can be done,however, we need to estimate the streamflow at the rivermouth from the coastal station data and the contributionsfrom rivers (or drainage areas) that are not monitoredby the stations.

Although most of the selected coastal stations areclose to river mouth, it is rare that a gauging station isright on the coast. For many rivers (e.g., Niger, Mekong,Amazon), the farthest downstream stations available arequite distant from the coast and thus streamflow mea-surements are not representative of the actual outflowinto ocean. Although including the contributions sep-arately from the rivers that enter downstream partlysolves the problem (e.g., for Amazon), many down-stream rivers are not monitored.

We used an RTM to estimate river mouth outflowfrom the upstream station data. The RTM was developedby M. L. Branstetter and J. S. Famiglietti and is de-scribed by Branstetter (2001) and Branstetter and Fa-miglietti (1999). The RTM is used in the NCAR Com-munity Climate System Model (CCSM; Blackmon et al.2001) for routing surface runoff into the oceans. Usinga linear advection scheme at 0.58 resolution, the RTMroutes water from one cell to its downstream neigh-boring cell by considering the mass balance of hori-zontal water inflows and outflows (Branstetter 2001):

dS y5 F 2 F 1 R, F 5 S, (1)O in out outdt d

where S is the storage of stream water within the cell(m3), Fout is the water flux leaving the cell in the down-stream direction (m3 s21), SFin is the sum of inflows ofwater from adjacent upstream cells (m3 s21), R is therunoff generated within the cell (m3 s21), y is the ef-

fective water flow velocity [50.35 m s21, followingMiller et al. (1994)], and d is the distance between cen-ters of the cell and its downstream neighboring cell (m).

The water flow direction is one of eight directionschosen as the downstream direction for each cell, basedon the steepest downhill slope. Note that stream channellosses to groundwater recharge, reservoirs, and irriga-tion are not included in this version of the model. Testsshowed that the choice of y only affects the time forthe model to reach an equilibrium under a constant run-off field. This time is about 6 months with y 5 0.35 ms21 (for the Amazon) starting from empty river channels.We used the river flow rates after 200 model days inannual simulations, and used the data of the second yearin monthly simulations.

To simulate the annual-mean streamflow of the worldriver systems, we forced the RTM with the annual runofffrom Fekete et al. (2000, 2002). In general, this simu-lation, which was run with daily time steps, producedcomparable river flows at the coastal stations of mostmajor rivers (see Table 2 and the appendix). We usedthe ratio of the simulated streamflow at the river mouthto that at the farthest downstream station to scale up(on monthly and annual timescales) the observed stationflow as the river mouth outflow (vol in the tables) forthe world’s top 200 rivers listed in Table 2 and theappendix. In some cases (e.g., no. 23 Uruguay and no.29 Essequibo in Table 2), where this simulated-flowratio was unreasonable (,0.9, or .2.0 except for Ni-ger), we instead used the ratio of drainage areas (basedon STN-30p) at the river mouth to that at the stationfor the scaling. The estimated river mouth outflow wasthen used to compute continental discharge. All branchrivers above the mouth of the main river channel wereexcluded in this calculation. These excluded branch riv-ers are no. 12 Tapajos, no. 18 Xingu, no. 52 Benoue,no. 60 Chindwin, no. 89 Red, no. 105 Jari, and a fewsmaller rivers. For other smaller rivers, not listed inTable 1 and the appendix, station streamflow was treatedas river mouth outflow and used in estimating conti-nental discharge. The use of the river mouth outflowincreases the estimates of the global continental dis-charge by 18.7% and the total monitored drainage areaby 20.4%.

The total drainage area monitored by the 921 ocean-reaching rivers (at river mouths) is 79.5 3 106 km2,which accounts for about 68% of global nonice, non-desert land areas [ø117 3 106 km2; Dai and Fung(1993)]. Therefore, approximately one-third of theworld’s actively drained land area is not monitored bythe available gauging stations. Earlier studies (e.g., Mar-cinek 1964) estimated runoff from these unmonitoredareas on the basis of comparable regions (i.e., nearbyregions having runoff data). Fekete et al. (2000) showedthat simply using the ratio of global actively drainedarea to the monitored area to scale up the global con-tinental discharge estimate resulted in a number (28 700km3 yr21) that was too low compared with other esti-


TABLE 3. The ratio of mean runoff between unmonitored and monitored river basins for each receiving ocean basin and the global ocean.The monitored basins used in this calculation consist of the world’s largest 243 rivers with a total drainage area of 69.8 3 106 km2, or about59.7% of the total area of actively drained landmass. The global ocean includes the Mediterranean and Black Seas. Also shown (Au/Am) isthe ratio of unmonitored to monitored (by the 921-river network) drainage areas.

Runoff ratio

Jan Apr Jul Oct Annual

Area ratio

Au/Am

Arctic Ocean 1.514 0.918 1.169 1.634 1.051 0.331Atlantic Ocean 0.664 0.885 1.167 1.347 0.931 0.300Indian Ocean 1.158 1.427 0.293 0.336 0.525 1.323Mediterranean and Black Seas 1.599 2.790 0.556 0.534 1.191 0.735Pacific Ocean 4.301 2.350 1.691 2.529 2.300 0.535Global ocean 1.146 1.000 0.938 1.267 1.022 0.507

mates. Perry et al. (1996) used a power law size dis-tribution found for large rivers to estimate the total con-tribution from all small rivers.

Here we made use of the ratio of runoff (based onFekete et al. 2000) between unmonitored and monitoredareas to account for the contribution of the unmonitoredareas to continental discharge using the following for-mula:

R( j) 5 R (j)[1 1 r( j)A (j)/A (j)],0 u m (2)

where R( j) is the continental discharge for 18 latitudezone j (for individual ocean basins), and R0(j) is thecontribution from the monitored areas only (i.e., the sumof river mouth outflow for all rivers with data withinlatitude zone j); Au(j) and Am(j) are the unmonitored andmonitored drainage areas, respectively, whose runoffenters ocean in latitude zone j (i.e., Au and Am may beoutside latitude zone j), and r( j) is the ratio of meanrunoff over Au and Am. Equation (2) was first appliedto individual ocean basins. To create the discharge fromindividual 18 coastal boxes, the contribution from Au

was further partitioned into the individual coastal boxeswithin the 18 latitude zone j for each ocean basin basedon the allocation of Au on the coasts. For display pur-poses, the 18 discharge was aggregated onto a 48 latitude3 58 longitude grid. Note that there are inconsistenciesin Table 2 and the appendix between the two drainageareas listed, as the station drainage area comes from thestation datasets while the river mouth drainage areacomes from STN-30p; presumably the latter should begreater.

In this calculation, the coastal outflow location forthe runoff generated over each 0.58 land cell was firstderived based on STN-30p, and the runoff over Au isput into the ocean at the correct location. Here, r( j) wascomputed using monthly and annual runoff fields fromFekete et al. (2000). It was found necessary to smoothr( j) over 48 latitude zones, because for narrower zonesAm can be zero or small. These r( j) values were thenused in monthly and annual calculations of R( j) at 18latitude resolution. Typical values of r( j), together withAu(j)/Am(j), averaged over the drainage area of eachocean basin, are shown in Table 3. The value of r( j)varies from 0.29 for the Indian Ocean in July, owing to

very dry unmonitored regions in the summer monsoon,to 4.30 for the Pacific Ocean in January, where theunmonitored wet regions of Indonesia come into play.For the global land areas, however, this ratio is close toone, which supports the notion that the unmonitoredlandmass as a whole is unlikely to be significantly wetterthan the monitored land areas (Fekete et al. 2000). Table3 also shows that the drainage areas into the Atlanticand Arctic oceans are well monitored by the 921 rivernetwork, while the monitoring network is poor for theIndian Ocean [partly due to large unmonitored arid areaswhere r( j) is small and thus has little effect on R( j);cf. Fig. 1]. Globally, the unmonitored areas (includingarid regions) account for about half of the monitoredareas, or one-third of the total drainage areas.

Implicit in our approach is that we are dealing withthe actual flows into the oceans, not the natural flowsthat would occur without dams and other interferenceby humans. This aspect does causes complications be-cause comparisons with climate models that do not in-clude or allow for effects of human interference shouldexpect discrepancies. The gauge records reflect the ac-tual flow, regulated or not, and as this may well havechanged over time, the more recent records may differfrom those of previous decades. Moisture from irriga-tion can be evaporated, which should be reflected inmeasurements of atmospheric moisture and thus in ourP–E estimates.

Compared with earlier studies, our method for esti-mating the discharge from the unmonitored areas hassome unique features that should improve our station-data-based estimate of continental runoff. For example,it makes use of runoff information over the monitoredand unmonitored areas. It also integrates the runoff fromthe unmonitored areas and puts it into oceans at correctlocations with the help of the river database. Most earlierstudies merely scale up the global total discharge de-rived from the monitored areas and fail to do this geo-graphically.

2) DISCHARGE BASED ON RUNOFF FIELDS

Continental discharge can also be estimated based onrunoff fields. BR75 used the runoff field (in mm) derived


by Marcinek (1964) to estimate continental dischargefor each 58 latitude zone using simple areal integration.However, as many large rivers (e.g., Mississippi, Parana,Nile, Niger, and most Russian rivers) flow mostly north–south, simple zonal integration of runoff does not pro-vide correct estimates of continental discharge for a giv-en latitude zone.

A substantial advance can be made by using the RTMto route the excess water (runoff ) within each 0.58 cellinto the ocean and then estimate continental dischargeby totaling the simulated coastal river outflows withineach 18 latitude zone. Maps of coastal discharges at 18and 48 latitude 3 58 longitude resolution were obtainedfrom the RTM simulations. Three different runoff fieldswere used: (i) the composite runoff fields of Fekete etal. (2000), and runoff estimated using the multiyearmean monthly and annual P–E fields from the (ii) NCEPand (iii) ECMWF reanalyses (Table 1). Daily runoff wasinterpolated from monthly mean fields and used to drivethe RTM. These discharge estimates provide additionalchecks for the gauge-based estimate, as well as an eval-uation of the reanalysis P–E fields.

Fekete et al. (2000) simulated snow accumulation andmelt using some simple empirical equations in produc-ing the composite runoff. We adopted a simple schemefrom Haxeltine and Prentice (1996) to simulate the ef-fects of snow on runoff in using the monthly P–E fromthe reanalyses for driving the RTM. The climatologicaldaily surface air temperature (td in 8C), interpolated fromthe monthly climatology of New et al. (1999), is com-pared with a constant ts. When td , ts, then positive P–E accumulates as snowpack and runoff is zero; if td .ts, then snow melts at a rate sm (mm day21) 5 k (td 2ts), where k is a constant in millimeters per (day 8C).Haxeltine and Prentice (1996) used 22.08C for ts and0.7 for k. We tested a number of values for these twoparameters in order to produce the best match betweenthe simulated and observed annual cycle of continentaldischarge and selected 0.08C for ts, which is physicallyreasonable, and 0.7 for k. In reality, of course, temper-atures vary considerably within a month, so that evenif the monthly mean is less than 08C, several days couldbe above that threshold, but the simplification we useis consistent with the other shortcomings in approxi-mating runoff by mean P–E, rather than employinghourly, or at least daily, data to properly deal with issuesdiscussed in the introduction.

4. Comparison among runoff estimates

To help understand discharge differences discussedbelow, we first compare annual means of the Fekete etal. runoff and the P–E derived from the NCEP andECMWF reanalyses. Figure 2 shows the annual-meanrunoff from Fekete et al. (2000), the P–E fields, and themean precipitation (average of CMAP and GPCP) (leftcolumn), together with the implied E (i.e., the precip-itation minus the runoff or P–E) and the difference be-

tween the NCEP and ECMWF P–E (right column).First, both the NCEP and ECMWF P–E fields shownegative values over many regions of all continents,especially in central and western Asia and northern Af-rica. While it is physically possible to have negative P–E over land (e.g., due to water flowing into the region,groundwater mining, and evaporation from reservoirsand lakes), negative P–E cannot be used as runoff. Someof these regions indicate deficiencies in P–E arisingfrom insufficient atmospheric observations (especiallyin Africa). We set the negative annual P–E to zero inforcing the RTM. The negative P–E over many coastalareas, such as eastern Africa, appears to result fromextensions of oceanic P–E patterns. This problem maystem from the relatively low horizontal resolution of thereanalysis P–E. On the other hand, some regions, suchas southern California, are heavily irrigated using, forinstance, water from aqueducts. As a result, negativeP–E values are plausible.

Second, the implied E for both the P–E fields and,to a smaller extent, the Fekete et al. runoff, has negativevalues over a number of land areas. Some of these neg-ative values of E suggest that the Fekete et al. runoffand the P–E are too large since the CMAP and GPCPprecipitation climatologies are generally reliable overnonmountainous land areas and they cover similar dataperiods (Table 1). These areas with negative E (espe-cially nonmountainous areas), together with those landareas with negative P–E for the reanalyses cases, con-tribute to errors in estimating continental freshwater dis-charge.

Third, substantial differences exist on regional scalesover both land and ocean between the P–E fields derivedfrom the NCEP and ECMWF reanalyses, even thoughthe large-scale patterns are comparable. While the dif-ference patterns are noisy, the NCEP P–E is generallylower than the ECMWF P–E over the southern sub-tropical (208–408S) oceans, the eastern Pacific and theAtlantic around 38–128N, and much of the western NorthAtlantic. Over the continents, the NCEP P–E also hasa dry bias compared with the ECMWF P–E. On theother hand, the ECMWF ERA-15 reanalysis data havea major problem over Africa, namely a spurious south-ward shift of the intertropical convergence zone (ITCZ)after 1987 (Stendel and Arpe 1997; Trenberth et al.2001b).

The implied E fields are spatially correlated with theprecipitation field, with the correlation coefficient rranging from 0.66 to 0.79 over land and slightly lowerover ocean for the reanalysis cases. This is expectedsince precipitation provides the main water source forevaporation over most land areas.

5. Freshwater discharge from the world’s majorrivers

Table 2 and the appendix present the annual-meanriver flow rate and drainage area at the farthest down-


FIG. 2. Annual runoff from Fekete et al. (2000), P–E derived from the NCEP and ECMWF reanalyses, mean precipitation of CMAP andGPCP (left column), and the implied E (i.e., P–runoff or (P–E)] and the NCEP 2 ECMWF difference of P–E (right column).

stream station and the river mouth for the world’s largest200 rivers. The river mouth flow was estimated usingthe method described in section 3b. The drainage areaat the river mouth was estimated using the river database

STN-30p. The river flow simulated at the station by theRTM forced with the annual runoff field of Fekete etal. (2000) is also shown next to the observed volume.

Many of the world’s largest river systems, with the


TABLE 4. Comparison of estimates of mean annual continental freshwater discharge into the individual and global oceans (km 3 yr21).

Arctic Atlantic IndianMediterranean

Seac Pacific Totald

Largest 921 riversa 3658 19 168 4532 838 9092 37 288Fekete runoff 3263 18 594 5393 1173 10 420 38 843ECMWF P–E 3967 20 585 4989 1144 7741 38 426NCEP–NCAR P–E 4358 16 823 3162 909 7388 32 640Fekete et al. (2000) 2947 18 357 4802 1169 11 127 38 402Korzun et al. (1977)b 5220 20 760 6150 14 800 46 930Baumgartner and Reichel (1975) 2600 19 300 5600 12 200 37 713Oki (1999) 4500 21 500 4000 10 000 40 000

a Largest 921 rivers are scaled up by accounting for the unmonitored areas and the runoff ratio at 48 lat resolution.b Korzun et al. (1977) include groundwater runoff (2200 km3 yr21 globally) and iceberg runoff (2700 km3 yr21 globally).c Mediterranean Sea includes the Mediterranean and Black Seas.d Total excludes discharge into inland (besides Black) seas and from Antarctica, which puts ;2613 km3 yr21 freshwater into the ocean

(Jacobs et al. 1992).

exception of the Yenisey, Lena, Ob, and Amur, are lo-cated at low latitudes, where precipitation rates are high-est. River discharge in general increases with drainagearea, but the river flow to drainage area ratio varies fromabout 1 km3 yr21 per 1000 km2 in the Tropics (equiv-alent to P–E of 1 m yr21) to less than 0.1 km3 yr21 per1000 km2 in high latitudes and dry areas.

The total global freshwater discharge, excluding thatfrom Antarctica, is about 37 288 6 662 km3 yr21 (Table4), which is ;7.6% of global precipitation or 35% ofterrestrial precipitation (excluding Antactica and Green-land) based the averaged precipitation of CMAP andGPCP. Since a time series of global discharge is noteasy to derive because of a changing number of gauges,this uncertainty range was estimated as the square rootof the sum of the variance of long-term mean annualflows at the farthest downstream stations of all the 921rivers multiplied by 1.187, the global-mean factor forconverting the farthest downstream station flows to theriver mouth outflows. Hence it is based on the assump-tion that the covariance of streamflow among large riv-ers is small. The world’s 50 largest rivers (Table 2)account for ;57% of the global discharge, while theirtotal drainage area is ;43% of the global activelydrained land areas (i.e., excluding glaciers and deserts).Adding the next 150 largest rivers (the appendix) in-creases these to 67% and 65%, respectively, while add-ing the next 721 rivers in our dataset of coastal stationschanges the numbers only moderately (to 73% and 68%,respectively). This suggests that an increasingly largenumber (in the thousands) of smaller rivers are neededto improve the coverage of the station network for mon-itoring global freshwater discharge.

Table 2 and the appendix show that the farthest-down-stream station data underestimate—by 10% to over100%—the river discharge and drainage area at the rivermouth in many cases (e.g., Amazon, Orinoco, Missis-sippi, Parana, Mekong, Irrawaddy, St Lawrence, andNiger). Most earlier estimates of streamflow and drain-age area for the world’s largest rivers used unadjusteddata from the available farthest downstream station

(e.g., Probst and Tardy 1987; Perry et al. 1996; Grabset al. 1996, 2000). These unadjusted estimates not onlyunderestimate the true river outflow, but also contributeto the large range among various estimates of flow ratefor world’s major rivers because the distance from thefarthest downstream station to the river mouth variesamong different studies from hundreds to thousands ofkilometers (Perry et al. 1996). For example, Perry et al.(1996) listed the Amazon with a mean flow rate of 60886 465 (std dev) km3 yr21 based on 11 different sources,while it is 6000 km3 yr21 in BR75. Although these arehigher than our mean flow rate (5330 km3 yr21) at sta-tion Obidos, they are about 10% lower than our estimateof Amazon mouth flow (6642 km2 yr21).

When forced with the annual runoff from Fekete etal. (2000), the RTM simulates the station flow ratesreasonably well for most major rivers (Fig. 3a, also seeTable 2 and the appendix). This suggests that both theFekete et al. runoff and the RTM routing scheme arelikely to be correct over most regions. However, prob-lems are evident over a few river basins such as theNile (no. 83) and Zambeze (no. 36), where the RTMgreatly overestimates the river channel flow. For theNile, river channel water losses due to evaporation andhuman withdrawal contribute to the smaller-than-sim-ulated flow rate. The drainage areas derived from theSTN-30p are close to those given in the station datasets(Fig. 3b), except for a few cases (e.g., Zambeze) wheredrainage areas from the streamflow datasets are likelyto be incorrect, as the STN-30p provides one of mostreliable estimates for the total drainage areas of world’slargest rivers.

When the RTM is forced by the P–E fields derivedfrom the NCEP and ECMWF reanalyses, the simulatedstation flow rate generally agrees with the observed atmost of the major rivers (Fig. 4). Substantial differencesexist, however, for the world’s largest rivers. For ex-ample, the simulated flow rate is 3063 and 3833 km3

yr21 for the Amazon at Obidos in the NCEP andECMWF cases, respectively, where the observed rate is5330 km3 yr21. In general, the Fekete et al. runoff re-


FIG. 3. (a) Observed (x axis) vs RTM-simulated annual river flowand (b) the drainage area from the streamflow datasets and from thesimulated river network STN-30p at the farthest downstream stationsfor the world largest 200 rivers listed in Table 1 and the appendix.The annual runoff field from Fekete et al. (2000) was used in theRTM simulation. The r is the correlation coefficient of the data points.Note that both of the coordinates are on a logarithmic scale.

FIG. 4. Same as in Fig. 3a but with the RTM forced by the P–Efields derived from the (a) NCEP and (b) ECMWF reanalyses.

sulted in better simulated station flow rates, especiallyfor the world’s largest rivers.

The third column in Table 2 and the appendix liststhe standard deviation (std dev) of year-to-year varia-tions of river flow rate at the farthest downstream sta-tion. It can be seen that for the large rivers the std devis ;10%–30% of the mean flow. This ratio increasesto over 100% for smaller rivers (the appendix), sug-gesting that streamflow rates of small rivers are muchmore variable from year to year than those of largerivers. This result arises because river flow rates areareal integrals of surface runoff and an integral over alarger basin is usually less variable than one over asmaller region.

The streamflow of many rivers has a large annualcycle. Figure 5 compares the mean annual cycle of dis-charge (thick, solid curve) with those of basin-integrated

precipitation (thin, solid curve), runoff, and P–E for the10 largest rivers. Note that the RTM is not used in thesebasinwide summation and therefore the differences be-tween the river discharge and the basin-integrated valuesillustrate the effects of snow and the time delay of watertraveling downstream to the river mouth. The Amazonhas the highest flow from May to June, while its basin-integrated precipitation, runoff, and P–E peak in earlyspring. This lag reflects the time needed for surfacerunoff to travel to the river mouth. For the Changjiang,Brahmaputra/Ganges, Mississippi, Mekong, and smallerrivers, this time lag is shorter (about 1 month or less),suggesting that the seasonal changes in surface runoffoccur in the area not far away from the river mouth.The Congo and Parana have relatively small annual cy-cles, even though precipitation and runoff (for Paranaonly) exhibit large seasonal variations. There are severalreasons for this to happen. First, evaporation positivelycorrelates with precipitation and thus reduces the sea-sonal variation of surface runoff, which is the case forthe Congo. Second, for large rivers like the Parana,


FIG. 5. Mean annual cycle of river discharge, river-basin-integrated precipitation (read on the right or-dinate), runoff, and reanalysis P–E for world largest 10 rivers. Note that the RTM was not used for thisplot.

which runs through different climate zones, regionalvariations of surface runoff within the river basin, com-bined with the time lag due to river channel transport,tend to compensate and thus reduce the seasonal vari-ation of downstream river flow. More fundamentally,the relative uniformity of river flows throughout the yearin spite of large precipitation variations is a sign of largeand diverse lags and obstructions, such as lakes, thathelp smooth out and regulate the flow.

The big Russian rivers (Yenisey, Lena, Ob, etc.) havelarge peak flows in June, which result from spring(April–May) snowmelt as precipitation does not peakuntil July–August in these regions (Fig. 5). In fact, thedrainage-area-integrated discharge and precipitation for

the entire Arctic region (cf. Fig. 12; also see Grabs etal. 2000) are very similar to those for Yenisey and Lena(Fig. 5). Early spring snowmelt over the Mississippibasin also appears to be the main reason for the Aprilpeak discharge from the Mississippi, whose integratedprecipitation peaks in May–June.

At low latitudes, the basin-integrated P–E generallyagrees with the Fekete et al. runoff, indicating that themonthly P–E fields are reasonable proxies of monthlyrunoff provided the areas are large enough. One excep-tion is the ECMWF P–E over the Congo River basin,where it follows the precipitation annual cycle ratherthan the runoff (Fig. 5). This bias is reflected in the highvalues of P–E in tropical Africa (Fig. 2) and is mainly


FIG. 6. Annual discharge rate (103 m3 s21) from each 48 lat by 58 lon coastal box. The numbers are the total discharge (in 103 m3 s21)from the coasts behind the solid lines. Blank coastal boxes have zero discharge.

due to the higher-than-observed precipitation in theECMWF data resulting from the spurious ITCZ shiftover Africa noted earlier. As noted, spring snowmeltresults in peak river flows in late spring or early summerat northern mid- and high latitudes. It also disrupts thegeneral agreement between surface runoff and the P–Eseen at low latitudes. This suggests that on a monthlytimescale the P–E fields at mid- and high latitudes can-not be used as a proxy of runoff without including theeffects of snow accumulation and melt.

6. Latitudinal distribution of discharge into theoceans

In this section, we consider the continental dischargefrom the ocean perspective, with a view toward theiruse in oceanic freshwater transport and budgets. We startfrom the total discharge into the global ocean and thendiscuss individual ocean basins separately, with a focuson the latitudinal distribution.

a. Global ocean

Figure 6 shows four estimates of annual freshwaterdischarge rates from individual 48 latitude 3 58 longi-tude coastal boxes. Also shown are the total discharge(in 103 m3 s21) from the coasts behind the solid lines.2

As described in section 3b, these estimates were derivedusing long-term mean streamflow data from 921 riversthrough Eq. (2), the composite annual runoff from Fek-ete et al. (2000), and the multiyear-averaged P–E fieldsfrom the reanalyses (see Table 1).

As indicated by the scale of the color bar, coastaldischarge rates vary from ,10 to 215 000 (at the Am-azon mouth) m3 s21 per 48 3 58 box. Most of the globaldischarge comes from the world’s major rivers, whosemouths are indicated by the boxes in warm colors inFig. 6a. The eastern coasts of the Americas, primarily

2 Note that the sum of these numbers differs slightly from thatlisted in Table 4 due to the use of a different (scale dependent)drainage-basin mask for the coastal integration.


FIG. 7. Estimates of annual mean continental freshwater discharge into the global oceans for each 18 latzone (right ordinate, lower stepwise lines and the inset) and the cumulated discharge starting from 908N(upper curves). Each line pattern represents an estimate based either on the largest 921 rivers (thin solidline) or on a runoff field (dashed lines), which was used to force a river transport model to derive thedischarge. Also shown is an estimate from Baumgartner and Reichel (1975, thick, solid line).

in the Tropics, provide ;40% of global discharge (Fig.6a). Discharge from east Asia into the North Pacificaccounts for another ;15% of the total. However, lessthan 10% of the global discharge comes from the west-ern coast of South America.

While the coastal boxes show the approximate lo-cations of the discharge, it is difficult to compare thedischarges among the four cases for each grid box inFig. 6. The integrated coastal discharge provides an eas-ier measure for comparison. It can be seen that the Fek-ete et al. runoff implies discharges comparable to theriver-based estimates from most of the coastlines exceptfor those from east Asia (too low), Indonesia, the Bayof Bengal, and the tropical west Pacific islands (toohigh) (Fig. 6b). The ECMWF P–E implied dischargesare also similar to the river-based estimates, except forthe coasts of east Asia, western Australia, and Africa(Fig. 6d). The NCEP case reproduces the discharge intothe Arctic, South Atlantic, and South Pacific Oceans,but has large deficiencies over eastern Africa, Europe,and east Asia (Fig. 6c). In general, both the ECMWFand NCEP cases underestimate discharges from north-ern Africa and east Asia, largely due to the negative

values of P–E (which was set to zero in the RTM sim-ulations) over these regions (cf. Fig. 2).

Figure 7 shows the annual-mean continental fresh-water discharge into the global oceans for each 18 lat-itude zone (stepwise lines) and the discharge accumu-lated starting from 908N (upper curves). For compari-son, the estimate by BR75 is also shown (thick, solidline). As expected, the continental discharge is domi-nated by the peak outflows from the world’s largestrivers such as the Amazon (;0.21 Sv at 0.758S, 1 Sv5 106 m3 s21), Congo (0.041 Sv at 5.758S), Orinoco(0.036 Sv at 9.258N), Changjiang (0.030 Sv at 32.258N),Brahmaputra/Ganges (0.033 Sv at 24.258N), Mississippi(0.019 Sv at 30.258N), and Parana (0.018 at 34.758S)(note that the peaks in Fig. 7 may exceed these numbersbecause of contributions from small rivers within thesame latitude band). The northern mid- to high latitudes(458–758N) encompass the largest landmass and manylarge rivers, including the Yenisey, Lena, Ob, and Amurin Russia; Mackenzie and St. Lawrence in Canada; andYukon in Alaska. Many of the Russian and Canadianrivers run from south to north and enter the ArcticOcean. Collectively, these rivers provide a large fresh-


FIG. 8. Annual discharge into the global ocean smoothed using a 58 lat running mean from four differentcases, compared with that of Baumgartner and Reichel (1975).

water discharge into the Arctic, North Atlantic, andNorth Pacific Oceans (cf. Fig. 2), thereby affecting theoceanic water budget and circulation, both locally andglobally, especially through the thermohaline circula-tion. Since the northern mid- and high latitudes are ex-pected to have the largest temperature and precipitationincreases in the next 100–200 yr due to increases inCO2 and other greenhouse gases (e.g., Dai et al.2001a,b), it is vital to establish an observed baseline.

The RTM reproduces the peak outflows from theworld’s largest rivers when forced with the Fekete et al.runoff and the reanalysis P–E fields, although the mag-nitude of the peaks differ somewhat from the estimatesbased on observations (see the insert in Fig. 7, also seeTable 2). These differences are also shown by the ac-cumulated discharges. Note that the accumulated dis-charge, a common measure used in previous studies(e.g., Wijffels 2001), integrates the errors from 908Nsouthward, and the differences at southern latitudes donot reflect the actual errors at those latitudes becausecontributions are small south of 108S.

The accumulated discharge for the NCEP P–E caseis considerably lower than the others, whereas the BR75case agrees remarkably well with our estimates basedon the streamflow data, Fekete et al. runoff and ECMWFP–E. However, the latitudinal distribution from BR75at 58 resolution is too smooth and quite unrealistic. Even

after smoothing the 18 discharge data using 58 latituderunning-mean, large differences still exist between theBR75 and our estimates, whereas the agreement amongour four different estimates improves (Fig. 8).

The total continental discharge into the oceans is alsoof interest for the global water cycle. This flux is oftenestimated by totaling the discharge from the world’smajor rivers and then adjusting it to account for thecontribution from the unmonitored smaller rivers. Asdiscussed in section 3b, our method for estimating thedischarge from the unmonitored areas makes our river-based estimate of continental discharge much more re-liable than in earlier studies (e.g., BR75; Perry et al.1996).

Table 4 compares our estimates of total freshwaterdischarge into the ocean basins with four earlier esti-mates. Discharge from groundwater, which was esti-mated to be around 5% of the total discharge by Lvovich(1970), is included only in the estimates using the P–E data and from Korzun et al. (1977). Small surfacefreshwater discharge from Antarctica (51987 km3 yr21

according to BR75) and changes in land storage (e.g.,effects of melting glaciers, discussed in section 1) arealso not included. Our 921-river-based estimate of theglobal discharge is 37 288 6 662 km3 yr21 (1 km3 yr21

5 31.69 m3 s21), which is very close to that of BR75(37 713) and Perry et al. (1996) (37 768). However,


FIG. 9. Estimates of annual-mean continental freshwater discharge into the Atlantic Ocean for each18 lat zone (right ordinate, lower stepwise lines and the inset) and the cumulated discharge startingfrom 908N (upper curves).

Perry et al. (1996) used streamflow rates from the far-thest downstream stations, which underestimate the trueriver discharge by an average of 18.7% in our analysis(see Table 2 and the appendix). This result implies thatthe power law size distribution used by Perry et al.(1996) substantially overestimates the total dischargefrom small rivers with an annual flow rate ,250 m3

s21.The global discharge implied by the Fekete et al.

runoff and ECMWF P–E is only slightly higher thanthat based on streamflow data, while the NCEP P–Eresults in lower global discharge (Table 4), consistentwith the dry bias (cf. Fig. 2). This result suggests thatthe ECMWF ERA-15 P–E is likely to be more reliableover global land than the NCEP P–E and it may beconsidered as a proxy for terrestrial runoff for estimatingcontinental discharge, although problems exist in Africaand South America.

Although the various estimates of total continentaldischarge listed in Table 4 are in good agreement, exceptfor Korzun et al. (1977), their partitioning of the totaldischarge into individual ocean basins differs substan-tially. For example, compared with the 921-river-basedestimate, the discharge implied by the Fekete et al. run-off is lower for the Arctic and Atlantic Oceans and

higher for the Pacific and Indian Oceans and the Med-iterranean and Black Seas, whereas the ECMWF P–Eimplies higher discharge into all but the Pacific basin.Table 4 shows that both of the reanalysis P–E fieldsunderestimate the discharge into the Pacific Ocean sig-nificantly—by 15% for ECMWF and 19% for NCEPcompared with the river-based estimate. These biasesresult primarily from the smaller P–E than the Feketeet al. runoff over China and Southeast Asia (Fig. 2).

For the Arctic Ocean, estimates range from 2600 km3

yr21 by BR75 to 5220 km3 yr21 by Korzun et al. (1977).Grabs et al. (2000) obtained an annual discharge of 2603km3 into the Arctic Ocean by totaling discharge datafrom the 35 farthest downstream stations, which accountfor 70% of the total Arctic drainage area. Assuming theunmonitored 30% drainage area has similar runoff rates,this would imply a total discharge of 3718 km3 yr21,which is comparable to our estimates (except for theNCEP case) (Table 4).

Figure 6 shows that the right amount of total dis-charge into a particular ocean basin may result fromdischarges coming off the wrong coasts. For example,the total discharge from the western and eastern coastsof the Indian Ocean is similar for the 921-river and


FIG. 10. Estimates of annual-mean continental freshwater discharge into the Indian Ocean for each18 lat zone (right ordinate, lower stepwise lines and the inset) and the cumulated discharge startingfrom 908N (upper curves).

NCEP cases, but the NCEP P–E implies too much runofffrom Australia and too little from Africa.

b. Individual oceans

1) ATLANTIC OCEAN

The latitudinal distribution of the zonal and accu-mulated discharge into the Atlantic Ocean (Fig. 9) re-veals that the Amazon accounts for more than one-thirdof the total discharge (;0.60 Sv). Other major riversinclude the Congo, Orinoco, Mississippi, Parana, andSt. Lawrence (;0.011 Sv at 48.758N). Most of the dis-charge comes from low-latitude America and Africa,although substantial freshwater fluxes also occur at 408–608N, mostly from North America. The Fekete et al.runoff and the two reanalysis P–E fields all slightlyunderestimate the river discharge at many latitudes with-in 158–558N. The NCEP P–E also underestimates thedischarge from rivers between 38 and 78N (such as no.39 Sanaga) and from the Congo, which adds to its neg-ative bias (ø20.08 Sv for the total accumulated dis-charge). The ECMWF P–E overestimates dischargefrom the Orinoco River (by ;0.007 Sv), which com-pensates for the negative bias accumulated to the north,indicating a dislocation of the atmospheric moisture

convergence. The ECMWF P–E also overestimates thedischarge from the Congo by ;0.05 Sv. This approx-imately equals the overall bias in the ECMWF estimateof the accumulated discharge compared with the river-based estimate. In general, the discharge implied by theFekete et al. runoff agrees well with the river-basedestimate in the Atlantic basin.

2) INDIAN OCEAN

The zonal and accumulated discharge into the IndianOcean (Fig. 10) reveals that the Brahmaputra/GangesRivers (0.033 Sv at 24.258N) account for about one-quarter of the total discharge. Other large rivers enteringthe Indian Ocean include Irrawaddy (0.012 Sv at17.758N), Zambeze (0.0037 Sv at 18.258S), Indus(0.0033 Sv at 24.008N), Godavari (0.0031 Sv at16.758N), and Mahanadi (0.0023 Sv at 20.258N). TheIndian subcontinent and the coast of the Bay of Bengalprovide the largest freshwater flux into the Indian Oceanaround 158–278N. Substantial discharge also occurs onthe eastern African coast between 68 and 248S. Thereis little discharge from arid to semiarid western Aus-tralia.

The NCEP P–E underestimates the outflow from the


FIG. 11. Estimates of annual-mean continental freshwater discharge into the Pacific Ocean for each 18lat zone (right ordinate, lower stepwise lines and the inset) and the cumulated discharge starting from 908N(upper curves).

Brahmaputra/Ganges by ;37% and the Zambeze by;50%, but overestimates discharge around 358S (thelatter bias is also seen in the ECMWF case). Hence, theNCEP-implied discharge into the Indian Ocean (;0.10Sv) is substantially lower than the river-based estimate(;0.14 Sv). However, the ECMWF case overestimatesoutflow from the Irrawaddy by more than 100%, whilethe outflow from Zambeze is underestimated. Whilecomparable with the river-based estimate at most lati-tudes, the discharge implied by the Fekete et al. runoffis also too high for the Irrawaddy.

3) PACIFIC OCEAN

The zonal and accumulated discharge into the PacificOcean (Fig. 11) features the Changjiang (;0.030 Sv at32.258N), Mekong (0.017 Sv around 12.08N), Amur(0.011 Sv at 51.758N), and Xijiang (0.0086 Sv at22.258N). The RTM underestimates the observed out-flow from the Xijiang by over 50% when forced withthe Fekete et al. runoff and the reanalysis P–E fields,while it puts higher outflows through nearby smallerrivers around 22.258N. The outflow from the Changjiangis also underestimated in the NCEP (by 28%) andECMWF (by 22%) cases. These negative biases are con-

sistent with the negative P–E (set to zero in the RTMsimulations) or smaller P–E values than the Fekete run-off over east Asia, the southwestern United States, andcentral America (Fig. 2).

Within 108S–108N, the RTM simulated discharge ismore continuous than the river-based estimate, partic-ularly for the Fekete et al. case, which results in a larger(by ;0.05 Sv) total accumulated discharge. The dif-ference within 108S–108N results primarily from the In-donesia–Malaysia–New Guinea region, where a largenumber of class 3 and smaller rivers are not monitored(cf. Fig. 1), making our river-based estimate less reliablefor this region. Further, the small islands and varyingcoastlines in this region are difficult to simulate accu-rately and require a resolution higher than 0.58, as usedby the RTM and the STN-30p river database. The latterwas used in estimating the contribution by the unmon-itored rivers in the river-based estimate. Therefore, sub-stantial uncertainties on the order of 0.05 Sv exist forthe discharge estimates from this region.

c. Seasonal variations

Figure 12 shows the mean annual cycle of total fresh-water discharge into the individual and global oceans,


FIG. 12. Mean annual cycle of freshwater discharge into individual and global oceans based on various estimates.The thin, solid line is for the estimate based on the 921 rivers without scaling to account for the unmonitored areas.

as estimated based on the 921 rivers with and withoutscaling for the contribution from the unmonitored areas,Fekete et al. runoff, and P–E fields from the NCEP andECMWF reanalyses. The inclusion of the unscaledgauge-based estimate shows that the contribution fromthe unmonitored areas does not significantly alter thephase of the mean annual cycle and is large, rangingfrom ;20% of the monitored discharge for the Atlanticto ;100% for the Pacific.

The discharge into the Arctic Ocean has a sharp peakin June arising from snowmelt in late spring, althoughthe Fekete et al. result has a lower maximum but withhigher values in most other months. Both the NCEP andECMWF P–E fields result in too much discharge in Julyand too little from January to March. We tested various

combinations of k and ts (see section 3b) to account forsnowmelt but all produced the peak discharge in July.This suggests that the simple scheme has limitations,probably arising from the use of only daily mean tem-peratures.

The total freshwater discharge into the Atlantic Oceanhas a similar peak in May for all but the ECMWF andthe unscaled cases (Fig. 12). For the Atlantic, theECMWF case has too much discharge in May and toolow discharge in August and September and the latterbias is even larger for the NCEP case. The May peakin all the cases results from the concurrence of highdischarge in May from the Amazon and Mississippi(Fig. 5). The discharge into the Indian Ocean peaks inAugust, mainly from the heavy Indian summer monsoon


FIG. 13. Mean annual cycle of freshwater discharge into individual and global oceans estimated using ECMWF andNCEP P–E fields with and without the effects of snow.

rainfall. For the Pacific Ocean, the discharge peaksaround June–July primarily because of heavy monsoonrainfall over east Asia during these months. Over theMediterranean and Black Seas, the discharge is low inthe warm season and high during the winter and springseasons. Globally, total freshwater discharge is highfrom May to September with a peak in June and a lullfrom October to April. This annual cycle results mainlyfrom the discharge from Northern Hemisphere land ar-eas. All the cases broadly reproduce the annual cyclesrevealed by the gauge data.

Figure 13 shows the annual cycles estimated with andwithout the snow scheme (i.e., all P–E was consideredas runoff ) for the reanalysis P–E cases to illustrate theoverall effects of snow accumulation and melt on con-

tinental discharge. It can be seen that without the snoweffects, the discharge into the Arctic Ocean would beradically different and would not have the sharp Junepeak, the peak discharge into the Atlantic around Maywould also be considerably lower, the discharge into thePacific would have a much lower peak in August insteadof June, and the global discharge would have little sea-sonal variation. On the other hand, snow has negligibleeffects on the discharge into the Indian Ocean and Med-iterranean and Black Seas.

7. Discussion

Our freshwater discharge estimates have varioussources of errors. For example, the 921-river-based dis-


charge contains uncertainties associated with 1) the es-timate of discharge from the unmonitored areas, 2) theadjustment for river mouth flow rates, 3) varying lengthand periods of streamflow records, and 4) varying hu-man influences on the natural streamflow. We also didnot include groundwater runoff and the runoff from Ant-arctica. For the reanalysis P–E fields, there are manydocumented regional biases such as those related tomoisture convergence and the atmospheric circulation(Trenberth and Guillemot 1998; Trenberth et al. 2001a),as well as known problems in ERA-15 over Africa (e.g.,the spurious ITCZ shift) and South America (Trenberthet al. 2001b). The Fekete et al. runoff has uncertaintiesarising from its use of limited streamflow data and awater balance model over many regions (Fekete et al.2000). The strength of the Fekete et al. (2000) runoffis in its patterns of the distribution, but the calibrationfor the total amount contains uncertainties over manyregions.

Nevertheless, we believe that the estimates based onthe 921-river streamflow data, the Fekete et al. runoff,and the ECMWF P–E are likely to be close to the truth.In particular, we think the 921-river-based estimate isthe most reliable, except perhaps for the Mediterraneanand Black Seas where the estimate seems to be a bittoo low (Table 4). This conclusion follows because the921 rivers cover over two-thirds of world’s activelydrained land areas and the errors in computing the dis-charge from the unmonitored areas result primarily fromthe uncertainties in the Fekete et al. runoff through r( j)in Eq. (2); that is, these errors are also in the Fekete etal. runoff-based estimate. The freshwater discharges im-plied by the reanalysis-derived P–E fields generallyagree with the 921-river-based estimate, although largeregional biases exist (cf. Fig. 2), especially for the NCEPP–E. This result suggests that the P–E products couldbe used as proxies of runoff over most land areas and,with some tuning and information about variability ofgroundwater, may be applied to study the interannualto decadal variations in continental freshwater dis-charge. The NCEP reanalysis is available for the last50 years or so and an improved ECMWF reanalysis(ERA-40) for a similar period will soon become avail-able. For such applications, however, improvements inaccounting for the effects of snow accumulation andmelt on seasonal to interannual timescales are needed.Furthermore, changes in human influences on the nat-ural streamflow and evaporation over the last severaldecades will also be important considerations.

We have shown that comparisons of basinwide dis-charges (Table 4) have only limited value. It is obviouslya more challenging task to derive correct estimates ofdischarge from all major rivers than just for the totaldischarge. For detailed evaluation of climate models andfor estimating meridional transport of freshwater in theoceans, discharge from individual rivers (e.g., Table 2and the appendix) and spatial distributions of discharge(Fig. 6) are needed. Because the freshwater fluxes from

the large rivers are substantial, use of the unrealisticdischarge distribution from BR75 in estimating oceanicmeridional freshwater transport (e.g., Wijffels et al.1992; Wijffels 2001) is likely to induce significant er-rors. Using the BR75 discharge for evaluating climatemodels (e.g., Pardaens et al. 2002) is also not a goodchoice.

8. Summary

We have created and compared several estimates ofcontinental freshwater discharge into the oceans. Thefirst is built on discharge data from 921 ocean-reachingrivers selected from several comprehensive streamflowdatasets. The drainage area of the 921 rivers is 79.5 3106 km2, or about 68% of global non-ice, nondesertland areas. We estimated the river mouth outflow fromthe world’s large rivers by adjusting the streamflow rateat the farthest downstream station using the ratio ofsimulated flow rates (or drainage areas in some cases)at the river mouth and the station. The discharge fromthe unmonitored areas was estimated based on the ratiosof runoff and drainage area between the unmonitoredand monitored areas at each latitude. A river transportmodel, the composite runoff field from Fekete et al.(2000), and a simulated global river database, STN-30p,were used in this analysis. Long-term mean annual andmonthly freshwater discharge at each latitude into in-dividual and global oceans were derived based on theadjusted river outflow and the estimated contributionfrom unmonitored areas.

Second, we have separately computed annual andmonthly continental discharge at each latitude into theoceans by forcing the RTM with the Fekete et al. runoffand the P–E fields derived from the NCEP and ECMWFreanalyses with a adjustment for snow effects. Theseimplied discharges were compared with that derivedfrom the streamflow data. The main results are sum-marized as follows.

1) The use of river mouth outflow increases the globalcontinental discharge by ;18.7% and the total drain-age area by ;20.4% compared with estimates usingthe unadjusted data from the farthest downstreamstations (cf. Fig. 1). This result suggests that usingunadjusted streamflow data from the farthest down-stream stations (e.g., Perry et al. 1996; Grabs et al.1996, 2000) substantially underestimates global con-tinental freshwater discharge.

2) Globally, the annual runoff rate over the unmoni-tored areas is comparable to that over the monitoredareas, although their ratio varies from 0.29 for theIndian Ocean drainage basin in July to 4.30 for thePacific Ocean drainage basin in January (Table 3).

3) Our 921-river-based estimate of global continental


freshwater discharge (excluding Antarctica3) is37 288 6 662 km3 yr21, or 1.18 6 0.02 Sv, whichis ;7.6% of global P and 35% of terrestrial P. Al-though this value is comparable to earlier estimates,large differences exist among the discharges into theindividual ocean basins. The estimates of global dis-charge based on the Fekete et al. runoff and ECMWFP–E are slightly higher than the river-based estimate,while the NCEP P–E implies lower discharge (Table4). In general, the reanalysis P–E fields underesti-mate discharge from east Asia and northern Africa(Fig. 6). About 57% of the global discharge comesfrom the world’s 50 largest rivers (Table 2).

4) When forced with the Fekete et al. runoff and re-analysis P–E fields, the RTM simulates the stationstreamflow rates reasonably well for world’s majorrivers (Figs. 3, 4, and 6). This is especially true forthe Fekete et al. runoff case (Table 2 and the ap-pendix) and suggests that river transport models at0.58 resolution, such as the one used in the NCARCCSM, can realistically simulate the world river sys-tem and its routing of terrestrial runoff into theoceans.

5) The continental discharges into the oceans withineach 18 latitude band implied by the Fekete et al.runoff and reanalysis P–E fields agree reasonablywell with the river-based estimates, which we regardas the closest to the truth. This is particularly truefor the Fekete et al. runoff and ECMWF P–E casesand for the global oceans and the Atlantic Ocean(Figs. 7 and 9). In general, the NCEP P–E under-estimates continental discharge at many latitudes forall the ocean basins except for the Arctic Ocean.

6) The latitudinal distribution of accumulative dis-charge into the global oceans estimated based on the921 rivers is similar to that from BR75, although thedischarge at individual latitudes differs greatly. TheBR75 estimate is unrealistically smooth (Figs. 7 and8) even compared with our 58 smoothed discharge.Our continental discharge has realistic latitudinaldistributions that are needed for reliable estimates ofmeridional transport of freshwater in the oceans. Ear-lier estimates (e.g., Wijffels et al. 1992; Wijffels2001) may contain significant errors as a result ofusing the unrealistic latitudinal distribution of con-tinental discharge from BR75.

7) Discharges from most of the world’s largest rivershave large annual cycles. For example, the Amazonpeaks in May–June, Orinoco peaks in August,Changjiang peaks around July; whereas large Rus-sian rivers (e.g., Yenisey, Lena, Ob,) have a sharppeak in June arising from snowmelt (Fig. 5). Bas-inwide-integrated precipitation usually does not have

3 Jacobs et al. (1992) gave a total freshwater flux of 2613 km3 yr21

from Antarctica into the ocean, of which 2016 km3 yr21 is fromicebergs, 544 km3 yr21 from ice shelf melting, and 53 km3 yr21 fromsurface and underground runoff.

the same seasonal phase as for river discharge, whichillustrates the important effects of snow accumula-tion and melt and river transport. The total dischargeinto the Arctic, the Pacific, and the global oceanspeaks in June, whereas the peak is in May for theAtlantic Ocean and in August for the Indian Ocean(Fig. 12). Snow accumulation and melt have largeeffects on the annual cycle of discharge into the Arc-tic, Atlantic, Pacific, and global oceans, but littleinfluence on the discharge into the Indian Ocean andthe Mediterranean and Black Seas.

The long-term mean values of river runoff and con-tinental discharge reported here is available for freedownload from NCAR’s Climate Analysis Section cat-alog (http://www.cgd.ucar.edu/cas/catalog/).

Acknowledgments. We thank W. G. Large and G. B.Bonan for helpful discussions, S. Sorooshian, D. P. Let-tenmaier, and two reviewers for helpful comments, S.Levis for help with the RTM codes, M. L. Branstetterand J. S. Famiglietti for providing the RTM, J. Caronfor help with the reanalysis datasets, and B. M. Fekete,C. J. Vorosmarty, B. A. Bodo, and others for makingthe streamflow, runoff, and STN-30p datasets available.This research was sponsored by grants from NOAA’sOffice of Global Programs and jointly by NOAA–NASAunder NOAA Grant NA17GP1376.

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161

162

163

164

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166

167

168

169

170

Lim

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126

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63

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62

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63

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610

15 8 26 13 13 2 1 18 12 8

14 13 13 12 12 11 11 11 11 10

420 62 83 58 808 73 49 72 73 121

342 50 84 56 629 65 51 66 67 129

3 26 23 80 72 7 4 76 16 47

33.0

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175

176

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56 47 58 109

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51 20 4 67 67 23 41 30 30 32

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Estimates of Freshwater Discharge from Continents ... · this discharge from land along with...

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