Spatial and Temporal Patterns of Amazon Rainfall

388 Royal Swedish Academy of Sciences 2001 Ambio Vol. 30 No. 7, Nov. 2001http://www.ambio.kva.se

INTRODUCTIONThe current efforts for ecologic-economic zoning within theAmazon region, as for instance carried out for the Brazilian partunder the patronage of the MMA/PPG7/SPRN project (1), re-quire information on the spatial variation of climatological char-acteristics as one of the criteria.

Until recently the only climatological data available were thoseof the fully equipped meteorological stations in the region thathave at least 30 years of continuous recording (50 stations, ofwhich 30 in the Brazilian part).

These stations are located by-and-large along the major riv-ers and are therefore not necessarily representative for the up-per catchments. A much larger network of simple pluviometricstations, about 800 in total, has been operational in the Brazil-ian part of the Amazon region since the 1970s. The network isfairly evenly distributed over all subcatchment areas. It is there-fore a valuable source of additional information, especially onthe absence or strength of a dry season, which is decisive forplanning of agricultural settlement, avoidance of forest fires, as-sessment of biomass and biodiversity, and ecological-economiczoning in general.

MATERIALS AND METHODSThe study area was demarcated basedon hydrographic, vegetational and le-gal considerations. It concerns the ar-eas of the Amazon river basin down-stream of the Andean Cordillera, theAraguaia-Tocantins basin, the minorcatchments directly bordering the At-lantic ocean, as well as the river sys-tems of the Guyanas and southernVenezuela. The vegetational criterionis the Amazon humid tropical forestbiome, including its internal edaphicsavannahs. Legally, it concerns the 9countries of the Amazon CooperationTreaty, and the 9 states of BrazilianAmazonia Legal. The total area isabout 6 x 10 km2, which is equiva-lent to the size of Europe, excludingRussia.

The digitized topographic basemap of Amazonia Legal, scale1:3 000 000, of the Brazilian Geo-graphic and Statistics Institute (2),was used as cartographic base. Thewestern and northern edges weretaken from a map of the World Me-teorological Organisation (WMO)(3). Areas above 500 m altitude areindicated on the maps with a shading,on the basis of various cartographicsources. The location and data of fullmeteorological stations of long-termfunctioning in the Brazilian Amazonwere taken from various publicationsof the National Meteorological Insti-tute INMET (4). The non-Brazilianstations were taken from the WMOdatabase (3). The total number of sta-tions is about 50, or 1 per 120 000km2.

The network of simple pluvio-metric recording sites within theBrazilian Amazonabout 800, ofwhich about 700 within the forestzonewas copied from draft maps ofthe Rede Bsica Hidrometeoro-lgica Nacional, which is maintainedby the National Agency of ElectricEnergy (ANEEL, formerly DNAEE)on the basis of field recordings car-ried out by institutions such as CPRM(Geological Service) and IDESP

Wim Sombroek

Spatial and Temporal Patterns ofAmazon RainfallConsequences for the Planning of Agricultural Occupation andthe Protection of Primary Forests

Report

The spatial and temporal pattern of annual rainfall and thestrength of the dry season within the Amazon region arepoorly known. Existing rainfall maps are based on the datafrom full-scale, long-term meteorological stations, operatedby national organizations linked to the World Meteor-ological Organisation, such as INMET in Brazil. Stationswith 30 or more years of uninterrupted and reliable re-cordings are very few, considering the size of the region,and most of them are located along the major rivers. It hasbeen suggested that rainfall conditions away from theserivers are substantially different. An analysis has beenmade of the records of a network of simple pluviometricsites in the Brazilian part of the region as maintained bythe National Agency for Electric Energy (ANEEL) since1970. The latter data sets were used to draw more detailedmaps on annual rainfall, and on the strength of the dryseason in particular; average number of consecutivemonths with less than 100 mm, 50 mm, and 10 mm,respectively. Also, some data were obtained on the spatialexpression of El Nio events within the region. Sub-regional differences are large, and it is argued that theyare important for the success or failure of agriculturalsettlements; for the hazard of large-scale fire damage ofthe still existing primary forest vegetation; for thefunctioning of this land cover as stock and sink of CO2, andfor the likelihood that secondary forests on abandonedagricultural lands will have less biomass. The effects ofpast El Nio rainfall anomalies on the biodiversity of thenatural savannahs within the forest region are discussed.

Base map: IBGE 1988

Map digitized by International SoilReference and Information Centre,Wageningen, The Netherlandsfor PPG 7/GTZ, Manaus, Brazil.

0 100 200 300 Km

Other

Outside study area

Ocean and inland water

Altitude above 500 m

RiverInternational boundaryState boundaryLowland boundaryRoad

Meteostation

Full meteostation, includedin 30 year normals of WMO/FAO

Full meteostation, not includedin WMO/FAO database

Pluviometric station of ANEEL

Pluviometric station of ANEELwith short and/or less reliable data

Annual rainfall (mm)< 1200

120016001400160016001800180020002000220020002400

240028002800320032003600

3200400036004000

40006400< 6400

July, October:Central month of dry season

389Ambio Vol. 30 No. 7, Nov. 2001 Royal Swedish Academy of Sciences 2001http://www.ambio.kva.se

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(Socio-economic Development Institute of Par State). The Hy-drologic Resources Department of ANEEL kindly provided allcomputerized pluviometric recording data, on a monthly basis,for all of the Brazilian Amazon. Such pluviometric informationnetworks also exist for the Amazon parts of neighboring coun-tries, but they are of much lower density, except for the Guy-anas, and their data were not available to the author.

There are 2 types of fully equipped meteorological stations.Those qualifying for the 30-year averaged normals criterionof WMO (full periods 19301960 and 19601990) are indicatedwith large black dots within circles, while those that do not haverecordings of sufficient length, gauge, or reliability to be usedstatistically are indicated with large black dots without circles.The recordings of the simple pluviometric sites are of varyingquality. Some are quite long-term, having started around 1965and continued uninterruptedly till 1998. Others started recentlyor have been abandoned 10 to 15 years ago. Those sites withless than 10 years of recording, and/or low reliabilitygaps inthe monthly data or obvious anomaliesare indicated on themaps with small open dots, the others with small black dots. Thepoor accessibility of higher-altitude and broken landscape situ-ations of the Guyana and Brazilian Shields, and the occurrenceof Indian Reserveswhich often coincide with high and inac-cessible terrainhave resulted in several subareas of poor or in-existent rainfall recording sites. Examples are the whole fron-tier zone of the Guyanas with northern Par, and the interfluvezone between the Araguaia and the Middle Xingu rivers.

Interpolation lines of annual and monthly rainfall character-istics were drawn manually, taking into account the variation in

reliability as described above. To a degree, the hydrographic pat-tern was taken into account, once it became apparent that awayfrom the main rivers and their broad floodplain the annual rain-fall is higher and the dry season less pronounced than near theserivers, in agreement with a hypothesis of Molion and Dallarosa(5). This Manaus rule or brisa fluvial is defined as the effectof local river breeze circulation away from the waterbodies, dueto early-daytime preferential ascending cloud formation over theforest-covered land area. It apparently holds true for the stretchesSolimes Meio Amazonas Baixo Amazonas, but is replacedby a contrasting rule for the very wide water-bodies at the mouthof the river system, where the tropical Atlantic ocean causes highannual rainfall but a pronounced dry season. This is calledMaraj rule or brisa do mar (6). In the southeastern part ofthe region the parts of higher altitude (above 500 m) appearedto have somewhat higher annual rainfall and lower dry-seasonstrength. This phenomenon was subsequently used in the fine-tuning of interpolation lines.

Interpolation lines for the non-Brazilian parts of the region arevery tentative. They are based on the WMO (3) map on annualrainfall, while for the seasonal characteristics use was made ofthe Length-of-Growing-Period (LGP) approach of FAO (7) andits database on monthly rainfalls. Also a French study (8) on thegeography of the rainfall in the Guyanas and southern Ven-ezuela was taken into account.

Computer-driven kriging of the interpolation lines was not at-tempted, because both the reliability categories of the recordingstations and sites, and the influence of large waterbodies ororographic features are as yet too vague to be put in formal

Number of months

Number of conseq. months withless than 100 mm rainfall

Map no. 2


weight classes. However, gridding of the pattern that results fromthe present manual interpolation is probably a relatively simpleexercise for ecosystem modelling specialists. Translation of themap pattern into the Kppen or Thornthwaite climatic schemesis feasible as well.

Four different digital maps were prepared (Maps 14):Map #1: annual rainfall pattern, with isohyets of the WMO sys-tem: 4000, 3600, 3200, 2800, 2400, 2000, 1600, 1400, 1200 and1000 mm, and 2 extra isohyets in the lower range: 2200 and 1800mm.Map #2: the number of consecutive dry months, averaged overthe years of observation, with 100 mm monthly rainfall as cri-terion.Map #3: number of consecutive dry months, averaged over theyears of observation, with 50 mm monthly rainfall as criterion.Map #4: number of consecutive dry months, averaged over theyears of observation, with 10 mm as criterion, also implying alow atmospheric humidity during the dry months.In all 4 cases the central month of the dry season is indicated,but without line boundaries.

The set of 3 maps (Maps 24) on the characteristics of thedry season is rather uncommon. Many studies on the Amazonclimate provide maplets with isohyets of selected dry months,for instance July and December, but examples of the integrationof all dry months providing quantitative information on thelength, strength and regularity of the whole dry season are veryfew. The Length-of-Growing-Period map of FAO (7) at scale1:5 000 000 shows, understandably, the wet season instead of thedry one.

A recent NGO study on the risks of forest fires (9) recognizesthe importance of the dry season, but does not show it in mapform, though it refers to an attempt on gridded input in a NASA-CASA ecosystem production model (10). Characterizing a drymonth as having less than 45 mm rain, it used only the normal-ized 19611990 records of the recognized WMO meteorologi-cal stations for the whole of Brazil. An early attempt was madeby the present author (11), applying the 100 mm month1 and50 mm month1 criteria to the recordings of the 25 meteostationsavailable in 1959 (12).

Attention was given to the occurrence of years with excep-tionally low rainfall, which would give indications of local in-fluence of the El Nio phenomenon. However, the relativelyshort length of many of the pluviometric records did not allowfor delineation of spatial drought patterns with any degree of ac-curacy on an additional map.

RESULTSThe use of the computerized pluviometric database of ANEELyields a more detailed pattern of annual rainfall (Map #1) forthe Brazilian Amazon, when compared to published maps (3, 13,14) and a number of page-size maps in review articles (1518).The annual rainfall away from the main rivers is higher. The spa-tial pattern is complex, not only in the water-divided areas whereorographic unevenness may have an influence, but also in ho-mogeneously low altitude areas. An example is the configura-tion for Maraj island in the mouth of the Amazon river. A farmore intricate pattern was found than assumed thus far: less rain-

Number of months

Number of conseq. monthswith less than 50 mm rainfall

Map no. 3


fall in its western interior part without a dry season, contrastingwith high rainfall on its coasts, but partly with a pronounced dryseason. Areas with low annual rainfall (< 1800 mm) occur inRoraima, the upper Trombetas-Jar area of northern Par, south-west of Altamira on the Trans-Amazonica, and the Aailndiaarea on the frontier Par-Maranho. Together they constitute asort of dry belt or corridor. However, these are also the areaswith relatively low-density field information, and many of theirrecording sites have moreover become nonoperational.

The bulge of dryness in the Paragominas-Aailndia area, withits high degree of deforestation since the early 1960s, is note-worthy. During the combined forestry- and soil survey of FAOin 1960/61, when the primary forest coverage was still complete,both the high timber volumes per ha and the low base satura-tion status of the soils in the Paragominas area suggested thatin those days the dry season was little pronounced. Only 200km south of Paragominas town the timber volumes dropped be-low 100 m3 ha1 and the base saturation of the soils reached levelsabove 35% (19), indicating a strong seasonal drought. It is pos-sible that the strength of the dry season and its duration havebecome more evident because of the large-scale deforestation.However, records of the Paragominas meteostationstarted onlyin 1965 and out of operation since 1995 because of lack of mu-nicipal interestdo not show such a negative trend. Any ten-dency to decreasing rainfall in the region after large-scale de-forestation may be verified easier in northern Mato Grosso orcentral Rondnia where pluviometric records are of longer term.An indirect indication of a negative effect of large-scale defor-estation on the amount and regularity of subregional rainfall isgiven by the 2-lobed extension of drier conditions in theBarcelos-Rio Branco area in the south of Roraima State. Theabundance of edaphic white-sand savannahs there would implyreduced evapotranspiration, hence less cloud formation in the dryseason.

Screening of the 800 individual pluviometric records; analy-sis of the data of the full Brazilian meteostationsthe first onesdating from 1910as well as scanning of the oscillations in thedischarge regime of the Rio Negro-Rio Solimes system atManaus, measured since 1902 (20, 21) give the following indi-cations on El Nio occurrences since 1900:

weak droughts in: 1903 (Manaus), 1911 (Manaus), 1930(Parentins), 1933 (Manaus), 193940 (Barcelos), 194142(Taperinha), 195152 (Porto Velho, Taperinha), 1955 (Manaus),1963 (Porto Velho, Taperinha), 198081 (Tapau, So Felix doAraguaia), 198788 (Aailandia), 199596 (Apui);

moderate droughts in: 1909 (Sena Madureira, Manaus),191516 (Sena Madureira, Taperinha, Manaus), 1936 (Manicor,Manaus), 195758 (Itacoatiara, Parentins), 196768 ( Coar,Humait, Manaus), 197172 (Boa Vista, Porto Velho, Tarauac),1976 (Rio Branco), 199192 (Prainha, Humait, Coar,Barcelos);

strong droughts in: 1906 (Manaus), 192527 (Fonte Boa,Erunep, So Gabriel de Cachoeira), 1948 (Manaus), 1983 (eve-rywhere), 19971998 (everywhere).

The ANEEL data set provides also some information on the spa-tial variation in the strength of the El Nio phenomenon withinthe Amazon region. The central-northern zone apparently expe-riences stronger El Nio effects than the southern zone. In thenorth, the dry season has a gradual start and an irregular length(wobbly character); in some years there are one or more com-pletely dry months outside the long-term average period, whichhas its center in January. In the south, the dry season starts quiteregularly and abruptly, with the center in JulyAugust (south-west) or September (southeast). The dry season in the easternpart of the Brazilian Amazon, near the ocean coast, is transitionalbetween the 2 systems, with its center in October.

CONSEQUENCES FOR ECOLOGIC-ECONOMIC ZONINGThe new set of rainfall maps can be used in various ways. Intheir present vectorized form they have applications at regional,state and municipal levels. In a gridded and digital form, theycan be helpful in modelling biosphere processes for the regionas a whole, in different scenarios of human occupation and cli-mate change. Gridding with 30' x 30' latitude/longitude for glo-bal models, or at 5' x 5' pixels for Amazon-level models (10)may want to combine the data of Maps #2, #3 and #4 through acomposite legend.

Number of months

Number of conseq. months withless than 10 mm rainfall


The map on annual rainfall (Map #1) is relevant for hydro-logical studies as it provides information, together with the ex-isting ANEEL set of fluviometric data, on the minimum, aver-age and maximum water flows of subcatchment areas. These areof use for the planning of road constructional work, for river traf-fic, groundwater recharge, aqueous pollution control, urban-center water supply and sanitation discharge, local hydropowergeneration, etc.

For agricultural and ecological studies, the dry-season Maps(#2, #3, and #4) are more important. Viability and sustainabilityof agricultural settlements, forest biomass growth, risks of for-est fires, chances of forest recuperation, atmospheric carbon se-questration, supplementary soil moisture storage, and El Nioeffects in ecological tension areas will be discussed here.

Agricultural SettlementsBuilding of rural access roads and their maintenance is muchless problematic when there is a distinct dry season. The sameholds for infrastructural facilities at the settlement centers,whether such centers are oriented on agricultural produce, or en-gaged in mining, logging or nontimber forest product gathering.Human and animal health is favored by a dry period, restrictingthe spreading of endemic diseases and the multiplication of theirvectors. Even more significant for the viability and sustainabilityof agricultural settlements are the dry-season effects on landclearing practices and crop growth. In the absence of a dry sea-son (the 0 and 1 dry-months zones under the less than 100 mmmonth1 criterion; Map #2) the burning of primary or secondaryforest vegetation after complete slashing remains incomplete.This makes the newly cleared land surface less suitable for theplanting of food or cash crops or the implantation of pastures.Crops such as upland rice, maize and beans need a dry spell forripening and hardening of their grains, and the prevention of rot-ting after harvesting. The yield of such crops is anyhow boundto be lower because the near year-round cloud cover implieslower daylight intensities, hence lower photosynthesis. This isalso reflected in the 365+ isoline of length-of-growing-periodmaps of FAO (7, 22). The above restrictions hold even more forlarge-scale soybean growing. This crop is sensitive to attacks bya number of diseases and pests in its vegetative stage if near-surface air humidity remains high during a good part of the day,such as in the edaphic savannah area of Humait. The use ofheavy machinery at the required high degree of mechanizationfor the crop to become commercially viable, is only feasiblewhen the land surface is reliably dry at both planting and har-vesting times. Only zones 1, 2 and 3 of Map #4 (which uses the10 mm month1 criterion) are therefore suitable for mechanizedsoybean cultivation. FAO recognizes a growing period of only120 to 300 days as suitable to very suitable for the crop (7).

Suitable food crops in the continuously moist regions are cas-sava, bananas, and certain tree-fruit crops (23, 24). The healthof cattle, for household use or meat commercialization, is prob-lematic in near-constant rainy conditions because of abundanceof ecto-parasites and blood-sucking insects (25), as well as nu-trient imbalances in the soils (26) and long distance to marketsand supply centers. It is noteworthy that the zones 0 and 12 ofMap #2 coincide with the near absence of sustained agriculturalsettlements of substantial size. Whether government-organizedor spontaneous, many recent settlements in these zones havebeen abandoned. This is not clearly indicated on Remote Sens-ing based maps of deforestation areas, because they do not nor-mally distinguish between active settlement and frequency of oldregrowth.

There are indications, from the denseness of patches of terra-preta-do-indio soil (11) and early historical records, that the pre-Colombian Amerindian populations also preferred dry-seasonareasunless soil and river-strategic conditions were particu-larly favorable.

The largest areas with active deforestation for agricultural pur-poses and permanence of rural settlements coincides with Zone5 of the 100 mm month1 Map (#2), as confirmed by Landsat-TM imagery, by the reas antrpicas of the 1995 IBGE mapon Amaznia Legal (2) and the network of feeder roads on thepresent maps. These are apparently the areas where one or twocrops per year, and several perennials, have secure yields.

Among the commercial perennials, only oil palm thrives inthe permanently humid zones, as proven by several successfulplantations near Belm. The lack or neglect of processing facili-ties has thus far been an impediment for sustainable oil palmenterprises in government-led efforts in Amazonas state (Tef;Distrito Agropecuario north of Manaus). Rubber plantationshave been a failure nearly everywhere in the region, because oftheir sensitivity to the Dothidella ulei fungus. A FAO/UNDPcommitted study by the Agronomic Institute of Campinas (27)concluded that only areas with a distinct dry season and a low-temperature spell would be safe niches for rubber plantations.The southern higher-altitude areas, indicated with a shading onthe present maps, would therefore be of potential interest for rub-ber plantations.

Timber Volume, Biomass Productivity,and Carbon StorageEarly FAO forest and soil inventories (11, 28), the more recentRADAMBRASIL multidisciplinary inventories (29) and theirspatial generalization (30), all indicate that the highest gross tim-ber volumes and thus aboveground biomass values, > 200 m3

ha1 implying > 100 tonnes C ha1, occur in areas where the to-tal annual rainfall is about 2000 mm and there is a dry seasonof 2 to 3 months (50 mm criterion) and 4 to 5 months (100 mmcriterion), respectively. These are also the areas preferred foragricultural settlement, especially where the soil conditions arefavorable, as is the case for the diabase- or limestone-deriveddusky red soils near Altamira and the high base status soils inRondnia. Safeguarding the remaining forest patches in such ar-eas, as permanent ecological reference sites, should be given highpriority in the hot spot approach to tropical forest conserva-tion.

Forest growth apparently benefits from a short dry spell, pos-sibly because of smaller damage from diseases and pests. Prob-ably more important is the lower interception of sunlight, an es-sential plant growth factor, in comparison with the near-continu-ously cloudy areas. Modelling of the potential Net Primary Pro-ductivity of biomass (NPP), based on remotely sensed averagecloud cover as derived from NDVI-AVHRR data sets of NASA,gave NPP values of 10001200 g C m2 yr1 for the continuouslymoist zone vs. 12001400 g C m2 yr1 for other parts of the re-gion (10).

Timber volumes are lower and the chances for high-forest re-cuperation are poorer when soil conditions in the short-dry-sea-son zones are marginal (shallowness; compactness of the sub-soil, swell-shrink behavior, or extreme sandiness). Under suchconditions anthropical disturbances, whether present-day or frompre-Columbian Indian population groups, will result in bambooforest (bambuzal of Acre and adjoining parts of Amazonas (31);creeper-and-vine forest areas (cipoal areas of the lower Xinguplanalto stretches (11, 32); Orbignia palm forest (babaual ar-eas in the middle Xingu area (32, 33), or even shrub savannah(campina areas near Santarm and Manaus). All are divergentvegetation types, with lower stocks of carbon and often alsolower biodiversity than the closed-canopy high forest.

Forest Fires and Forest RecuperationAs demonstrated by the 1997/98 extensive forest fires in Roraimastate, there is a large spatial variation in the risk of forest fires,which may spread from slash-and-burn sites or are caused bylightning. Fire risks are low if logging takes place very selec-


tively for rare precious timber or with careful planning for long-term sustainable yields of more tree species, say only 30 m3

ha1, once in 20 years or so (34). If higher volumes are cut andtowed away then the remaining off-fall can become a source ofeasily flammable dry matter, augmented by litter from semi-deciduous species where the dry season is pronounced. The re-sulting below-canopy fires often remain undetected by radar- orvisual-light-based Remote Sensing, but may cause considerabledamage to floral and faunal diversity (9).

The chances for accidental extensive forest fires are largestin the zones 5 and 6 of the 100 mm Map (#2), and the zones 3and 4 of the 50 mm Map (#3). Variation in soil conditions, es-pecially the moisture storage capacity, plays a modifying role.

It is generally assumed that it will take 300 to 1000 years be-fore a completely burned-down humid tropical forest regains itsoriginal structure and species composition (35). However, recu-peration to a high, closed or open-canopy forest is far from sure,as demonstrated by several field studies (36). As soon as thereis a long dry season and soil physical conditions are poor, recu-peration becomes unlikely. Zones 1, 2 and 3 of Map #4 (the 10mm month1 criterion) with their very pronounced dry seasonare areas where the primary forest, already semideciduous, is arelict from former, more humid climatic conditions. It will neverregrow to its present status if subjected to large-scale slash-and-burn practices for ranching or soybean cultivation. There are,however, several large Indian Reserves in these zones that mayprovide protection from massive deforestation.

Carbon Sequestration PotentialThe Amazon forest ecosystem is a large store of carbon. Untilrecently it was assumed that in mature forests the uptake of at-mospheric CO2 for daytime assimilation is balanced by an equal-strength CO2 emission through nighttime canopy respiration andheterotrophic soil respiration. Only young secondary forestwould act as a CO2 sink. However, studies in Rondnia(ABRACOS project in the Ji-Paran area (37)) and north ofManaus (BIONTE project (34)), using eddy-correlation fluxmeasuring techniques from towers in old-growth forest andpartly also tree-girth measurements, suggest otherwise for recentyears. Apparently the increased CO2 concentration in the atmos-phere stimulates tree growth, resulting in a net uptake, called se-questering or sink condition for CO2, by the forest ecosystem inmost years. The amount sequestered in the Rondnia case was13 tonnes C ha1 yr1, and in the Manaus case even 57 tonnesC ha1 yr1 (38). A net uptake is consistent with theoretical con-siderations and actual measurements in temperate regions on C3plantsto which nearly all tropical tree species belongand isknown as the CO2 fertilization effect. Apparently even in thehumid tropical forest environment, with its low chemical soil fer-tility, the atmospheric CO2 concentration has been in the mini-mum as a growth factor until some decades ago. Measured cu-mulative change in aboveground dry biomass in Amazonia since1977 points in the same direction (39). The spatial and tempo-ral consistency of this fertilization phenomenon will be furtherresearched by the new interdisciplinary research program LBA(40). It is a cooperative program of Brazilian institutions led byINPE (So Paulo) and INPA (Manaus), with participation ofAmerican and European institutional consortia funded by NASAand the European Community, respectively. Additional CO2 fluxmeasurement sites will be Caxiuan and Santarm, both in mid-Par state, and possibly Tef and So Gabriel do Rio Negro inAmazonas state.

It is debatable whether the extra growth will continue as higheraboveground biomass in the long run. It is more likely that agood part of the generated higher biomass will end up in the for-est soil, in the form of higher soil organic matter content of astable nature. Soil organic matter is already nowadays a very sig-nificant stock of carbon (4144). Increased above- and below-

ground tree growth implies earlier maturity and thus earlier se-nescence, hence higher tree fall intensity and associated higherinput of organic material over the rootable depth of the soil.

There are indications that the function of the Amazon primaryforest as a CO2 sequestering mechanism is interrupted during ElNio years. It has been suggested that there would be insuffi-cient soil moisture reserve in such years (45), thus moisturerather than CO2 concentration becoming the limiting growth fac-tor. In contrast, the Net Primary Productivity in those areas thatare normally without any dry season, such as west of Manaus,may benefit from an El Nio year because of the temporaryavailability of more sunlight for photosynthesis.

Soil Moisture StorageAt the beginning of the dry season, plant growth can continueunabatedly for some time because of a reserve of moisture inthe rootable soil. However, the plant-available moisture storagecapacity (PAM, moisture in the suction range of pF 2.04.2) ofthe various Amazon soils is poorly quantified. There is an acutescarcity of direct measurement of soil moisture suction charac-teristics on undisturbed samples and of their bulk densities. Thefew hard data carried for Brazilian soils are summarized byTomasella and Hodnett (46). About 30 of the profiles with pFdata are from the Amazon region itself, mainly from Par Stateand the neighborhood of Manaus, where Xanthic Ferralsols(Latosssolos amarelos) predominate. There are no soil profileswith pF data for the Brazilian or Guyana Shield areas, nor forthe vast area (700 000 km2) southeast and east of Manaus thathas fine sandy to silty sediments of the Late Pleistocene I for-mation and Plinthic Acrisol as main soil (47). Xanthic Ferralsolsand related soils are deep but are known to have low values ofPAM, viz. 510 vol.% in the central and lower parts of the pro-files, even if they are very clayey. Hydrologically, they act likesandy soils of temperate regions. Less weathered soils of theAmazon and Brazil-at-large, such as Orthic Acrisols, Luvisolsand Cambisols have substantially higher values, viz. 1015 vol.%and even more in topsoils when rich in humus, but these soilsare less deep.

Comparison of analytical data on disturbed samples with tex-turally equal, but mineralogically different temperate-zone soilswill give too high PAM values. Special equation calibration isrequired (48) to arrive at estimates that are in line with the fewexisting pF data. The overestimation problem is also pointed outin the recent NGO study (9) on fire and deforestation risks. Thestudy could not use the RADAMBRASIL data set of 1147analyzed soil profiles of the Brazilian Amazon (29) for compari-son with soil textural situations of temperate zones because noneof these profiles have pF or bulk density data. An additionalproblem is the relevance of the maximum soil moisture-suctionvalue for the humid tropical forest environment. The internation-ally used value of pF 4.2 (15 atm. suction) was established forthe wilting point of temperate-zone annual crops such as sun-flower and wheat. It is possible that tropical forest species havesmaller suction force, but this was never investigated, either inthe field or in laboratory growth chambers. With respect to maxi-mum photosynthesis and CO2 uptake, the leaf stomata shouldbe fully opened during daytime. Therefore, the more readilyavailable moisture (pF 3.5 minus pF 2.0 values), which may beonly two-thirds of the PAM value, should be the really impor-tant amount at modelling of the carbon sink function of the for-est.

The total moisture availability for uninterrupted plant growthis a combination of the PAM value per soil horizon and the to-tal depth of the soil that is rootable. Total rootable depth varieshighly over the Amazon region, from extremely deep (510 m)on the freely draining Xanthic Ferralsols (49, 50), to quite shal-low (50100 cm) for many of the Plinthic Acrisols southwestof Manaus with their relatively low timber volume (43). The


ABRACOS project monitored at weekly or half-weekly inter-vals for 2 to 4 years the moisture content till a depth of 3.6 munder forest and planted pasture, at 3 sites (north of Manaus;Ji-Paran in Rondnia, and Marab in southern Par State) (51).Soil moisture abstractions at all depths were much greater un-der forest than under pasture, and also deeper, well into theweathering substratum. Per soil horizon there was a muchsmaller abstraction in the Ferralsols than in the other 2 soils,which is consistent with its assumed lower PAM values (no pFcurves established).

It can be concluded that forests on Ferralsols, when a distinctdry season is present, need to root deeper to secure sufficientsoil moisture for uninterrupted growth than those on less weath-ered soils. In general, representativeness of analyzed soil pro-files for broad landscape-related soil units, rather than indiscrimi-nate statistical analysis of the total number of samples, shouldbe used to arrive at spatially improved estimates on effectivedepths and moisture storage capacities for forest growth, includ-ing validation by measurements on undisturbed samples (51).

El Nio and Ecotonal BiodiversityAmong the El Nio occurrences in the Amazon as listed above,the period 19251927 stands out. More than a full-year droughtoccurred in the Rio Negro catchment area, with many forest firesand extremely low river-water levels that impeded river travel.This event is well documented in a report by the Salesian bishopof the area to the Vatican (52).

Very strong El Nio events such as the 19251927 one willhave occurred earlier as well, perhaps at intervals of 70 years.There are reports (24) of very strong droughts in 1860 (Purusarea) and in 1774 (Rio Negro area). They must have had pro-nounced effects in ecotonal field situations at the edge of theforest biome, and in areas of ecological tension within the sys-tem. The latter would apply, for example, to the edaphically de-termined patches and stretches of savannahs and savannah-for-ests, which are particularly frequent in Amazonas state (campos,campinas, campinaranas, caatinga amaznica). Their present-day specific biodiversity, including endemism, and theirvegetational structureboth not well studied thus farwouldbe reimpacted in such exceptionally dry years (53). Physiologi-cal drought effects would be enhanced by accidental fires, orpurposely started ones by Amerindians or early settlers, result-ing in sharp boundaries with the surrounding forest.

From studies on charcoal occurrences in the So Carlos doRio Negro area (54) it was concluded that extensive fires musthave occurred 250 to 400 years ago. Still larger effects will havehad the very broad-periodicity prehistoric Mega El Niosrecognized by Meggers (55, 56) on the basis of pollen sequencesand river-level fluctuations in the northern part of the region(stone-grinding sites of Amerindians are occurring belowpresent-day low-water levels of rivers). She identified dry peri-ods around the years 400, 700, 1200 and 1500 BP. The occur-rence of 3 levels of broad low terraces of the Late PleistoceneIa-formation in western Amazonas (47) suggests that also be-fore 1500 BP there were times with lower rainfall and/or strongerdry seasons, succeeding an overall wetter period between 12 000and 5000 BP (17).

An example of sharp vegetation boundaries induced by fireis the large forest-encircled open savannah area in southeasternAmazonas state, below the southern dip of the Trans-Amaznicahighway. It is a surmised source area of the Tup groups of Am-erindian languages (57). These campos de Tenharim may haveundergone extensive anthropogenical vegetation degradation ac-companied by water and wind erosion during these early droughtperiods. Low dunes are still present, while water-erosion prod-ucts apparently have accumulated in the original downstream riv-erbeds, blocking their discharge function. Nowadays, thesedownstream stretches feature as winding and narrow, elongated

strips of shrub savannah vegetation on a white-sand soil surface(campinas de Manicor) (11, 43).

A large area of uninterrupted shrubby savannah or savannah-forest vegetation (campina and campinarana) occurs northwestof Barcelos in the Rio Negro region. On Landsat-TM imagesthere are clear traces of former longitudinal and barchane dunes,which must have developed on the coarse sandy soils concerned,during a dry period of the Late Pleistocene or Early Holocene.

CONCLUSIONS AND RECOMMENDATIONSInterpolation of the data of simple pluviometric recording sitesaway from the main rivers is helpful in assessing and mappingthe spatial variation in annual rainfall, as well as the length,strength and regularity of the dry season in the Amazon region.The dry season characteristics are important for the success orfailure of agricultural settlements, the variation in abovegroundbiomass, the risk of forest fires, and the chances for forest re-generation. They also provide information on the extent of theCO2 sink function of the primary forest, as well as the spatialvariation and strength of El Nio drought anomalies. All of thesefactors need to be taken in consideration with the current effortsto establish national and state-level policies for forest protectionin harmony with sustainable rural settlement.

Many of the pluviometric recording sites in the Brazilian partof the region have recently become nonoperational. For the ben-efit of agro- or forest-ecological and socioeconomic zoning pur-poses at subregional or municipal levels, it is recommended thatthis situation be remedied soonest, and that extra sites be estab-lished in the so-called dry corridor of the eastern part of the re-gion. At the same time, several fully automatic and satellite-linked complete meteorological stations should be establishedand maintained in remote water divide areas, to substantiate theinformation from the pluviometric network.

There is a great need to correlate rainfall patterns with dataon key soil physical parameters, with specific attention to meas-urement of total rootable depth, bulk density and net soil mois-ture storage capacity, in actual field situations and on undisturbedsamples at laboratory analysis. These properties need to be de-termined also for the deeper subsoil and the weathering substra-tum, particularly in areas where there is a long dry season. Theresulting Amazon soil moisture storage map should be linkedwith the 3 dry-season maps (Maps 24), to arrive at quantita-tive estimates of the occurrence of a CO2 sink function of theAmazon old-growth forests, spatially from per-humid to semi-deciduous and temporally in and between El Nio years. Oneof the outcomes may be that the sink function is more substan-tial and continuous in the zone with little or no dry season, likemost of the state of Amazonas and the adjoining parts of Peruand Colombia, even though this zone does not have the highesttimber volume at present. They would then qualify even morefor protection through the Ecological Corridors idea of the PPG7program.

Quantifying the spatial and temporal carbon sink function ofthe forest also requires that the network of CO2 flux monitoringof the ongoing LBA program be extended to areas beyond theaxial zone Manaus-Santarm-Belm, to incorporate differentrainfall and soil conditions in western Amazonia and in the Bra-zilian Shield area.

The origin and the specific biodiversity values of the manyintraforest natural savannahs merit additional study, also in re-lation to past and present El Nio droughts and early Amer-indian land use-activities.


Wim Sombroek holds a PhD in agricultural and environ-mental sciences from Wageningen University. From 1959 to1979 he participated in tropical soil survey and landevaluation projects in Latin America (Brazil, Uruguay) andAfrica (Nigeria, Kenya). From 1979 till 1991 he was directorof the International Soil Reference and Information Centre(ISRIC) in Wageningen and during those years he alsoserved as Secretary-General of the International Society ofSoil Science (ISSS), a scientific associate of ICSU. In 1991,he became Director of the Land and Water Division of theFood and Agriculture of the United Nations (FAO) and wasits focal point on Climate Change matters. He retired fromthe FAO in 1996 and then accepted an invitation from GTZto become a team member/consultant in a Manaus-basedproject on natural resources policies, within the PilotProgramme for the Protection of the Brazilian TropicalForests of the G7 countries and Brazil (PP/G7). His address:ISRIC, P.O. Box 353, 6700 AJ Wageningen, The Netherlands.E-mail: [email protected]

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58. The author wishes to thank the ANEEL organisation in Brasilia (Eng. Vinicius de Se Benevides; Eng. Valdemar Guimares) for providing all digitized monthly data ofits pluviometric stations in the Brazilian Amazon region. He also wishes to acknowl-edge the critical reading of an earlier draft text, with valuable suggestions, by Dr.Therezinha Xavier Bastos of EMBRAPA in Belm, Prof. Jan Sevink of the Universityof Amsterdam / Tropenbos-Araracuara, and Ir Johannes van Leeuwen of INPA, Manaus.

59. A CD-Rom with the digital version of the four maps may be obtained from the GTZoffice at IPAAM, Manaus

60. First submitted 14 March 2000. Accepted for publication after revision 7 Feb. 2001.

Spatial and Temporal Patterns of Amazon Rainfall

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