Hydrodynamic and water quality 3D modelling of the Nam Theun … · Hydrodynamic and water quality...

Hydroécol. Appl. (2016) Tome 19, pp. 87–118© EDF, 2014DOI: 10.1051/hydro/2014009

http://www.hydroecologie.org

Hydrodynamic and water quality 3D modellingof the Nam Theun 2 Reservoir (Lao PDR): predictionsand results of scenarios related to reservoirmanagement, hydrometeorology and nutrient input

Modélisation 3D de l’hydrodynamique et de la qualité d’eaudu réservoir de Nam Theun 2 : prédictions et résultatsde scénarios liés à la gestion du réservoir,à l’hydrométéorologie et à l’apport de nutriments

V. Chanudet(1), J. Smits(2), J. Van Beek(2), P. Boderie(3), F. Guérin(4,5,6),

D. Serça(7), C. Deshmukh(7), S. Descloux(1)

(1) Électricité de France, Hydro Engineering Centre, Sustainable Development Dpt, Savoie Technolac,73373 Le Bourget du Lac, [email protected]

(2) Deltares, Marine and Coastal Systems, PO Box 177, 2600 MH Delft, The Netherlands(3) Deltares, Inland Water Systems, PO Box 177, 2600 MH Delft, The Netherlands(4) Université de Toulouse, UPS (OMP), LMTG, 14 avenue Edouard Belin, 31400 Toulouse, France(5) IRD, LMTG, 14 avenue Edouard Belin, 31400 Toulouse, France(6) Departamento de Geoquimica, Universidade Federal Fluminense, Niteroi-RJ, Brasil(7) Laboratoire d’Aérologie, Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, 31400 Toulouse, France

Abstract – A 3D water quality model has been applied to predict medium term evolution ofthe water quality in the Nam Theun 2 Reservoir and also to quantify the effect of various sce-narios. 15-year simulations show that the oxygen concentration will continue to increase inthe water column although the hypolimnion will remain anoxic in some areas of the Reservoir.In parallel, the concentration of reduced compounds will decrease with time. The significanceof the hydrodynamics in water quality evolution is pointed out with two scenarios in which nat-ural or human forcings have been modified. The comparison of simulations made for yearswith contrasted hydrometeorological conditions shows that and duration of major hydrom-eteorological related events (rainfall, flood and air temperature drop) have a major influenceon the seasonal evolution of water quality in the whole Reservoir. Simulations also show thatthe physico-chemical quality of the water released downstream of the power house wouldhave been different if the commissioning had been carried out immediately after the impound-ment. Finally, the model has been used to quantify the impact of an increase of the NO3

- andPO4

3- incoming flux consecutive to potential changes in the watershed land use. The fluxeshave been multiplied by a factor two separately (2 scenarios) and together. While the addi-tional load of NO3

- has almost no impact on physico-chemistry and phytoplankton activity, the

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88 V. Chanudet et al.

increase of PO43- leads to a larger increase of chlorophyll-a close to the Intake than close

to the dam.

Key words – water quality modelling, Nam Theun 2 Reservoir, simulation, medium termevolution

Résumé – Un modèle numérique 3D a été utilisé pour prédire l’évolution à moyen termede la qualité d’eau du réservoir de Nam Theun 2 et également pour quantifier l’effet de dif-férents scénarios. Les simulations sur 15 ans montrent que la concentration en oxygènedans la colonne d’eau continuera à augmenter bien que l’hypolimnion restera anoxiquedans certaines parties du réservoir. En parallèle, la concentration des composés réduits vadiminuer avec le temps. L’importance de l’hydrodynamique dans l’évolution de la qualitéd’eau est mise en avant dans deux scénarios dans lesquels les forçages naturel ou anthro-pique ont été modifiés. La comparaison de simulations réalisées pour des années présen-tant des conditions hydrométéorologiques contrastées a permis de mettre en évidence quela date et la durée des événements hydrométéorologiques majeurs (précipitations, crue,chute de température) ont une influence importante sur l’évolution saisonnière de la qualitéd’eau dans l’ensemble du réservoir. Les simulations montrent également que la qualitéphysico-chimique de l’eau restituée à l’aval de l’usine aurait été différente si la mise en ser-vice avait eu lieu immédiatement après la mise en eau. Enfin, le modèle a été utilisé pourquantifier l’impact de l’augmentation des flux entrants de NO3

- et PO43- suite à d’éven-

tuelles modifications des usages sur le bassin versant. Alors que l’augmentation de lacharge en NO3

- n’a pratiquement aucun impact sur la physico-chimie et sur l’activité phyto-planctonique, l’augmentation du flux de PO4

3- induit une augmentation de la chlorophyllea, hausse plus marquée à proximité de la prise d’eau que près du barrage.

Mots clés – modélisation de la qualité d’eau, réservoir de Nam Theun 2, simulation,évolution à moyen termes

1 INTRODUCTION

Water quality in reservoirs dependson many parameters and processes,including hydrodynamics. Interactionsare so numerous (Stumm & Morgan,1996) that it is difficult to predict orassess precisely the effect on waterquality of a modification of forcing data(from change in the watershed to glo-bal change). A possible approach toovercome this difficulty is numericalmodelling. Since the 1970s manywater quality and ecological process-based models have been developed(Jorgensen et al., 1996) and the stud-ies of water quality and ecosystem

problems with numerical models areincreasing rapidly (Cerco & Cole, 1993;Jorgensen, 1999; Gin et al., 2001;Arhonditsis & Brett, 2005; Chao et al.,2007). Some well-established three-dimensional water quality models havebeen successfully applied to simulatewater quality variables in rivers, lakes,estuaries and ocean environments(Cerco & Cole, 1995; Luyten et al.,1999; Antenucci et al., 2000; Wool et al.,2001; Hydroqual, 2004; Romero et al.,2004; Danish Hydraulic Institute, 2001;Deltares, 2013).

This type of model has been devel-oped for the Nam Theun 2 (NT2) Res-ervoir based on the Delft3D code

Hydrodynamic and water quality 3D modelling of the Nam Theun 2 Reservoir (Lao PDR) 89

(Chanudet et al., submitted). Correctmodelling of water quality requires highquality hydrodynamic parameters suchas thermal structure, current velocities,local residence time, or the vertical mix-ing due to density stratification. Theseparameters influence both the transport(advection and dispersion/diffusion)and the production or transformation ofchemical components.

The Delft3D-FLOW model wasdeveloped, calibrated and used to sim-ulate the hydrodynamics of the NamTheun 2 Reservoir (Chanudet et al.,2012). The same model has been usedin this study. For water quality, themodel is based on the 3D eutrophica-tion model Delft3D-ECO (Smits & vanBeek, 2013). The ECO model has beenextended for the NT2 Reservoir. The fullmodel development, set-up and calibra-tion are described in Chanudet et al.(submitted). For the main parameters(dissolved oxygen (DO), Secchi depth,nutrients (NH4

+, NO3-, PO4

3- and Ptot),SO4

2-, chlorophyll a, Dissolved OrganicCarbon (DOC), Fetot, CO2 and CH4),resulting simulations are in good agree-ment with measurements. Differencesbetween simulated and observed waterquality mainly concern the timing andthe magnitude of concentration peaksand dips. Parts of these differences aredue to inaccuracies in model forcing, innutrient loads in particular, and to sto-chastic patchiness of observed reser-voir water quality.

In this paper, Delft3D-FLOW andDelft3D-ECO models were consideredreliable enough to test the effect of fourscenarios on the hydrodynamics andthe water quality. In the first scenario,the same average annual hydrody-namic simulation has been repeated as

a basis for a 15-year water quality sim-ulation. The objective of this simulationis to assess the medium term evolutionof water quality within the reservoir. Ina second scenario, the impact of amodification of hydrometeorologicalforcing data (input discharge and mete-orology) is tested on both hydrody-namic and physico-chemical waterquality parameters. The third scenarioassesses the effect of the initial filling ofthe Reservoir before the commission-ing, leading to a fill and flush effect (norelease through the power house forabout 2 years). Finally, in the fourthscenario, the impact of a modificationof the phosphorous and/or nitrogenconcentrations in the incoming riverson physico-chemical water quality(including chlorophyll a) of the NT2Reservoir is studied.

2 MATERIAL AND METHODS

2.1 Study system

The project area is located in thecentre of Lao PDR (KhammouaneProvince) on two sub catchments of theMekong River. It is a trans-basinproject: the reservoir is located on theNakai Plateau in the Nam Theun water-shed, while electricity generation facili-ties and water release are in the XeBangfai watershed (Fig. 1).

Information about the Nam Theun 2systems (Reservoir and downstreamrivers), technical features and hydro-meteorological conditions can be foundin Descloux et al. (same issue). Theproject area is characterised by a sub-tropical monsoon climate with distinctwarm-wet (WW, approximately mid-June to mid-October), cool-dry (CD,


Fig. 1. Location of the NT2 site and monitoring stations (Descloux et al., same issue).Fig. 1. Cartographie du site de NT2 et des stations de suivi (Descloux et al., same issue).


approximately mid-October to mid-February) and warm-dry (WD, approxi-mately mid-February to June) seasons.

The model concerns only the Res-ervoir. Upstream or downstream riversand the Downstream Channel are notmodelled. This paper focuses mainlyon stations located close to Dam(RES1) and close to the Water Intake(RES9) for the implications regardingwater quality in downstream systems(rivers and Downstream Channel).Some data for a station located in theupstream zone of the Reservoir(RES8) are also presented.

2.2 Model descriptionand calibration

Water quality in the NT2 Reservoirwas simulated with Delt3D-ECO(Deltares, 2013). ECO is based on theopen source water quality modellingframework D-Water Quality (DEL-WAQ), that facilitates the selection ofsubstances and processes from aprocesses library, and has manyoptions for mass conservative numeri-cal integration (Blauw et al., 2009;Deltares, 2013, Smits & van Beek,2013). ECO dynamically simulates aset of substances and processes ona 3D computational grid composed of awater grid and a sediment grid that mayhave the same horizontal resolution.Additional modules have been specifi-cally developed and implemented inECO for the NT2 Reservoir (vegeta-tion module, inorganic speciation,water-atmosphere exchanges, iron)(Chanudet et al., submitted).

The state variables simulated byECO and associated processes, asapplied for the NT2 Reservoir, are

presented in Table I. Figure 2 shows themain interactions between variables inthe water and sediment modules.

The decomposition of detritalorganic matter is formulated as themineralization (first-order kinetics) andconversion of six fractions withdecreasing degradation rates, describ-ing the entire organic matter cycle insurface water and bed sediment.The decomposition of organic matterrequires the consumption of electronacceptors: oxygen, nitrate, iron oxyhy-droxide, sulphate and carbon dioxide,which are consumed by bacteria in aspecific order.

Phytoplankton is simulated withBLOOM (Los & Wijsman., 2007; Blauwet al., 2009; Los, 2009; Smits &van Beek, 2013). In BLOOM the algaespecies compete within the constraintsfor available nutrients (N,P,Si), availablelight (energy), the maximum growth rateand the maximum mortality rate (bothtemperature functions). Linear program-ming is used as an optimization tech-nique to determine the algae speciescomposition that is best adapted to pre-vailing environmental conditions.

The computational grid for the waterbody in the NT2 Reservoir model is ahorizontally 8 x 8 aggregated version ofthe hydrodynamic grid model (Chanudetet al., 2012). It results in a grid cell sizein the horizontal plane of approximately1200 m x 1200 m. In the vertical direc-tion, layer thickness is 3.75 m for thetop water layer, less than 2.5 m for thebottom water layer, 2.5 m for the inter-mediate water layers, and increasingwith depth 0.5, 4.5, 10, 20 and 65 cmfor the sediment layers. At the deepestpoint of the Reservoir grid, the depth is28.75 m. The sediment/soil bed is 1 m


deep. The total number of layers in thewater column amounts to 12. Thenumerical solver applied in the ECOmodel of the NT2 Reservoir concernsan explicit upwind scheme in the hori-zontal direction, combined with animplicit in time scheme with central dis-cretization of advection in the verticaldirection. The computational time stepis 10 minutes. The flow and dispersionfields for the various scenarios withDelft3D-ECO were simulated with theNT2 Delft3D-FLOW hydrodynamicmodel (Chanudet et al., 2012).

As boundary conditions, the hydro-dynamic model requires tributariesdischarges and temperature. Forcingdata are the meteorological parameters

Table I. State variables and processes included in tTable I. Variables d’état et processus inclus dans le

State variable

dissolved oxygen (DO),detrital organic (6 fractions, POC1-5, DOC)and inorganic carbon (including carbondioxide and methane),detrital organic (6 fractions, PON1-5, DON)and inorganic nitrogen (nitrate (NO3),ammonium (NH4)),detrital organic (6 fractions, POP1-5, DOP)and inorganic phosphorus (dissolved andadsorbed phosphate (PO4, AAP), vivianite-P(VIVP), apatite-P (APATP)),detrital organic (6 fractions, POS1-5, DOS)and inorganic sulphur (sulphate (SO4) anddissolved sulphide (SUD)),phytoplanktonic biomass (C/N/P/Si/S) forfive species,biomass of nine types of vegetation,dissolved silicate, opal silicate,oxidized and reduced dissolved andparticulate iron,Apatite (APATP) and vivianite (VIVP)inorganic matter.

(air temperature, wind speed and direc-tion, solar radiations, humidity andatmospheric pressure) and the Secchidepth (Chanudet et al., 2012, Desclouxet al., same issue). For the water qualitymodel, all the simulated substances(Tab. I) need to be defined in the tribu-taries as boundary conditions. The var-iables measured (mean and standarddeviation between April 2008 andDecember 2010) and included in themodel for calibration/validation are(Chanudet et al., submitted): total sus-pended solids (8.4 ± 5.5 mg.L-1), dis-solved oxygen (7.2 ± 1.1 mg.L-1), NH4

+

(0.05 ± 0.03 mg N.L-1), NO3- (0.08 ±

0.04 mg N.L-1), total P (0.03 ± 0.01 mgP.L-1), SO4

2- (0.31 ± 0.20 mg S.L-1), Cl-

he Nam Theun Reservoir water quality model.modèle qualité d’eau du réservoir de Nam Theun.

Process

exchange of dissolved oxygen, carbondioxide and methane with the atmosphere(reaeration),consumption of electron acceptors (oxygen,nitrate, iron(III), sulphate), methanogenesis,oxidation and ebullition of methane,speciation of dissolved inorganic carbon,decomposition of detrital organic matter,nitrification, denitrification,adsorption, precipitation of phosphate,oxidation, precipitation, speciation ofsulphide,growth and mortality of phytoplankton,extinction of light,mortality and re-growth of vegetationbiomass,dissolution of opal silicate,oxidation, reduction, precipitation,speciation of iron,net settling of particulate components,mass transport in the sediment.


(1.6 ± 0.8 mg Cl.L-1), total Fe (0.43 ±0.24 mg Fe.L-1), total organic carbon(1.43 ± 0.30 mg C.L-1) and dissolvedorganic carbon (1.23 ± 0.30 mg C.L-1),dissolved CO2 (3.1 ± 0.5 mg C.L-1), dis-solved CH4 (0.01 ± 0.01 mg C.L-1).PO4

3- concentration was always belowthe detection limit (0.01 mg P.L-1) anda constant value of 0.005 mg P.L-1 waschosen. Estimated constant valueshave been also used for dissolved sili-cate (4.4 mg Si.L-1). Other substancesincluding organic fractions (N, P and S),

Fig. 2. Schematic overview of the state variables atoplankton, vegetation (initial cover and re-growth)mitted).Fig. 2. Vue d’ensemble schématique des variablessus affectant le phytoplancton, la végétation (couvsont pas inclus (Chanudet et al., submitted).

adsorbed phosphorus, vivianite, apatite,opaline silicate, dissolved and particu-late sulphide have been allocated theconcentration of 0.0 mg.L-1.

The calibration and validation of theNT2 model was carried out with data(DO, Secchi depth, nutrients (NH4

+,NO3

-, PO43- and Ptot), SO4

2-, chloro-phyll a, dissolved organic carbon(DOC), Fetot,CO2, CH4 and phytoplank-ton biomass) acquired between April2008 and January 2011 (Chanudetet al., same issue).

nd processes included in ECO. Processes for phy-and silicate are not included (Chanudet et al., sub-

d’états et processus inclus dans ECO. Les proces-erture initiale et re-croissance) et les silicates ne


2.3 Definition of the scenarios

2.3.1 Medium term evolutionof physico-chemical water qualityin the NT2 Reservoir

For this simulation, hydrodynamicandmeteorological forcingdataandriverloading as determined for the periodJune 2002 to June 2003 were repeatedeach year for 13 years (Tab. II).

The initial conditions for waterquality were taken from simulationresults of the calibration phase in June2011. Details about NT2 Reservoirhydrodynamics during this period (dis-charges, temperature...) can be foundin Chanudet et al. (2012).

2.3.2 Influence of the hydro-meteorology on physico-chemicalwater quality in the NT2 Reservoir

Three different simulations wereconducted by using the hydro-meteoro-logical conditions of June 2002 – June2003 (simulation 1), June 2003 – June2004 (simulation 2) and June 2005 –June 2006 (simulation 3) (Fig. 3). Thefirst simulation, considered as the refer-ence one, corresponded to a hydrologyclose to an average year (1986-2008).The periods chosen for the second(June 2003 – June 2004) and third(June 2005 – June 2006) simulationswere respectively among the driest andthe wettest ones observed during thetwo last decades. The average inflowswere 134 m3.s-1 and 440 m3.s-1 in2003-2004 and 2005-2006, respec-tively (279 m3.s-1 in 2002-2003 for thereference simulation) (Fig. 3A). Theinitial water level (1st of June) was

525.5 m for the 3 simulations. The ini-tial water temperature was obtainedfrom the result of previous simulations:26.9, 27.0 and 28.6 °C for surface watertemperature for the three simulationsrespectively and 19.1, 19.1 and 17.0 °Cfor bottom temperature.

During the WW season, the maindifferences between simulations con-cerned inflows and water level. Thehighest discharges in simulations 1and 3 were above 2500 m3.s-1 whilevalues did not exceed 800 m3.s-1 forsimulation 2. In average, input dis-charge during the WW season were646, 261 and 1015 m3.s-1 for the simu-lations 1, 2 and 3 respectively. As aconsequence, the Reservoir reachedthe full supply level (538 m) only forsimulations 1 and 3 (Fig. 3D). Forthese two simulations, spillages wereobserved: 813 m3.s-1 and 2112 m3.s-1

maximum in September for simulations1 and 3, respectively (Fig. 3C). Duringthe “dry” year (simulation 2), the high-est water level was only 532.3 m andno spillage occurred. During the CDand WD seasons, input discharges sig-nificantly reduced and were similarbetween the simulations: 59, 58 and94 m3.s-1, respectively. Some differ-ences regarding air temperatures canbe observed, in particular s and theduration of periods of the lowesttemperatures during the CD season(Fig. 3E).

In order to have a relevant assess-ment of the impact of the hydrology onwater quality, simulations did not startfrom the impoundment (April 2008) but3 years after (June 2011). As of thisdate the water quality in Reservoir hasbecome more stable (Tab. II).


Tab

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2.3.3 Effect of the fill and flush strategyon physico-chemical water quality inthe NT2 Reservoir

Between the first filling (April 2008)and the beginning of significantreleases through the power house(March 2010), most of the water cominginto the Reservoir was released at theNakai Dam (riparian release or spillway)

Fig. 3. Tributaries (A), dam (B) and turbine (C) discthe reference simulation (continuous line), the simu(“dry” year, broken line).Fig. 3. Débits des entrants (A), au barrage (B) et depour la simulation de référence (ligne continue), lala simulation 3 (année « sèche », ligne discontinue)

into the Nam Theun River. The Reser-voir was close to the full supply level (orclose to this level) for many months(simulation 4, Fig. 4). During thesemonths, the Reservoir was flushedmore than three times (Chanudet et al.,same issue). To assess the effect onwater quality of this time-lag betweenthe impoundment and the end of 2009,a hypothetical case has been studied

harges, water level (D) and air temperature (E) forlation 2 (“wet” year, dotted line) and simulation 3

s turbines (C), cote (D) et températures de l’air (E)simulation 2 (année « humide », ligne pointillée) et.


during which the normal Reservoirexploitation started on June 2008(simulation 5). As a consequence,average discharge through the turbinesincreased significantly from 10.0 to126.7 m3.s-1 for the simulated periodbetween simulations 4 and 5 (Fig. 4C).On the contrary discharge at Nakai Damdecreased in simulation 5 as comparedto the real case (simulation 4) from204.3 to 78.8 m3.s-1 (Fig. 4B). Thewater level decreased from November2008 to May 2009 in simulation 5 asobserved in normal operation periodsafter commissioning while it remainedstable in simulation 4 (Fig. 4D).

Fig. 4. Tributaries (A), dam (B) and turbine (C) discsimulation 4 (continuous line) and for simulation 5 (

Fig. 4. Débits des entrants (A), au barrage (B) et dede référence 4 (ligne continue) et la simulation 5 (sa

2.3.4 Influence of a modification of thenutrient load in the tributaries on waterquality in the NT2 Reservoir

Nutrients (N and P) concentrationshave remained consistently low in theNT2 Reservoir and its tributaries sinceimpoundment (Chanudet et al., sameissue). The average NO3

- concentra-tion in the incoming rivers was 0.08 ±0.04 mg N.L-1 between 2008 and 2013and the PO4

3- concentration, mostlybelow the 0.01 mg P.L-1 (detectionlevel), was estimated around 0.005 mgP.L-1. Nutrients in the Reservoir haveseveral origins: (i) release due to the

harges and water level (D) used for the referencewithout fill and flush, dotted line).

s turbines (C) et cote (D) utilisés pour la simulationns « fill and flush », ligne pointillée).


degradation of the organic matterpresent in the vegetation and the soilsbefore impoundment, (ii) input fromthe rivers into the Reservoir and(iii) recycling within the water column.The contribution of these sourcesamong the overall nutrient load in theReservoir is likely to change with time(intra- and inter-annual). To assessthese changes, three simulationshave been done in addition to the ref-erence simulation (simulation 6):– Increase in NO3

- concentration inthe river inputs by a factor 2 (simu-lation 7);

– Increase in PO43- concentration in

the river inputs by a factor 2 (simu-lation 8);

– Increase in NO3- and PO4

3-

concentrations in the river inputs bya factor 2 (simulation 9).

Fig. 5. Surface (continuous line) and bottom (dottRES9 for the simulation used for medium term predFig. 5. Température en surface (ligne continue) etpour la simulation utilisée pour les prédictions à mo

These simulations were conductedto mimic the potential increase of thenutrient load with a possible futuredevelopment of the local population(input from sewage), or a change in thewatershed land use for instance.

The same hydrodynamics was usedfor these simulations. As for the hydro-dynamics, simulations were conductedafter a 3-year calibration period, i.e. inJune 2011 (3 years after the beginningof the impoundment) (Tab. II).

3 RESULTS AND DISCUSSION

3.1 Medium term prediction

Simulated surface and bottomwater temperatures (Fig. 5) show thatfor the annual hydro-meteorological

ed line) water temperatures at RES1, RES8 andictions.au fond (ligne pointillée) à RES1, RES8 et RES9yen terme.


conditions chosen for the mediumterms predictions, the Reservoir over-turns in December – January. This isdue to the decrease in air temperature(not shown here) during the CD sea-son. The high discharges during thewet season also induce an increase inbottom temperature but it does notreach values calculated in surface. ForRES9, between June and February,the thermal stratification is weak orabsent.

Fig. 6. 15-year simulations of bottom (dotted line) asolved oxygen, NO3

-, NH4+ and PO4

3- at RES1 (lefside).Fig. 6. Simulation sur 15 ans des concentrations auen oxygène dissous, NO3

-, NH4+ et PO4

3- à RES1 (

The evolution of the DO concentra-tions in the Reservoir for the 15 yearsfollowing the impoundment (Fig. 6)shows differences in both horizontaland vertical directions. At the seasonalscale, similar patterns are reproducedeach year because the same annualhydrodynamic conditions have beenapplied for each year. At the annualscale, the bottom DO concentrationand the duration of the oxic periodsduring destratification increase slowly

nd surface (continuous line) concentrations of dis-t hand side), RES8 (middle) and RES9 (right hand

fond (ligne pointillée) et en surface (ligne continue)gauche), RES8 (milieu) et RES9 (droite).


at RES1. For RES8, bottom DO con-centration increases rapidly probablybecause of the low depth (about 13 min the model grid at full supply level)which favours vertical mixing (weakstratification). At RES9, bottom DOconcentration increases more rapidlydue to the vertical mixing of the watercolumn in this area as a result of thecurrent induced in the HeadraceChannel. Surface DO concentrationsimprove rapidly for the three stationsand almost no differences are noticea-ble 4-5 years after the impoundment,except for short periods during annualReservoir overturn when the surfaceDO concentration at RES1 can bebelow 5 mg.L-1 (58%), even after15 years. The medium term increase inDO concentration can probably beexplained by the decrease of minerali-sation flux (i.e. organic matter oxic deg-radation) due to the diminution of theavailable organic matter as evidencedby its decrease in water and sedimentas the initial stock is progressively con-sumed (Fig. 7). The organic matterexport (river outflow) also decreaseswith time and after a period, the carboninput by the rivers becomes larger thanthe output by the rivers (Fig. 8). How-ever, the reduction of the output is lessobvious as in the sediment becausepart of the exported organic mattercomes from the internal primary pro-duction from inorganic carbon.

From 2010, the improvement of DOconcentration induces an increase ofthe NO3

- concentrations in the wholewater column at the expense of NH4

+

which declines rapidly. Then, becauseof the reduction of nitrogen releasefrom the sediment and dead vegetationinto the water, both NO3

- and NH4+

concentrations tend to decrease grad-ually: - 49% (p < 0.01) and – 4% (p =0.842) for NO3

- surface and bottomaverage annual concentrations and –42% (p < 0.01) and – 73% (p < 0.001)for NH4

+ (relative difference between2012 and 2020) for instance at RES1.Seasonal near bottom NO3

- peaks cor-respond with the destratification peri-ods during which oxygen from thesurface enhances nitrification. WWseason NO3

- concentration peaks nearthe surface arise from high river loads.The better oxygenation conditions, theadditional mixing turbulence and theshorter residence time at RES9 inducea lower NH4

+ concentration and ahomogenous vertical profile for NO3

-.At RES8, the shorter residence timeand the weaker stratification (as com-pared to RES1) also favour NO3

- at theexpense of NH4

+ like at RES9. What-ever the site studied, the contribution ofriver inputs gradually becomes moreimportant as the bottom load reduces.As for DO, the global reduction of Nspecies (organic and inorganic) inthe water can be explained by thedecrease and the stabilization of theconcentrations in the sediments asthe less refractory organic matter ismineralized (Fig. 7). As a consequenceof this decrease, the nitrogen input(tributaries) becomes higher that theriver exportation after some years(Fig. 8). The nitrogen excess probablyescapes the system through the deni-trification process.

Contrary to NO3-, a slight increase in

PO43- bottom concentration is observed

atRES1(Fig.6).Onepossibility toexplainthis fact is that contrary to nitrogen thatcan escape the system through nitrifica-tion-denitrification processes, P tends to


accumulate in the sediment due to thesettling of adsorbed inorganic and detri-tal organic phosphorus (Fig. 7). AtRES1, the residence time is highbecause there is no significant outflowat the dam and PO4

3- tends to accumu-late at the bottom where it is not con-sumed. At RES9, where the residencetime is much lower under normal oper-ation conditions, PO4

3- bottom concen-tration remains stable with time. In spiteof this increase at the bottom, PO4

3-sur-face concentrations are most of the time(except sometimes at RES1) at the

Fig. 7. Medium-term evolution of the organic (dottednitrogen and phosphorus concentrations in the wanon bio-available inorganic P fraction is also consiconsists here only in CO2, CO3

2-, HCO3-, H2CO3 an

Fig. 7. Évolution à moyen terme des concentratitotales (ligne continue) de carbone, d’azote et de phRES1 (la fraction non bio-disponible de P est égaments, le C inorganique consiste ici seulement en C

algal uptake threshold imposed on themodel (0.005 mg P.L-1) because of theconsumption of phosphate by algae.

3.2 Influence of the hydro-meteorology on water qualityin the NT2 Reservoir

3.2.1 Hydrodynamics

The results of the hydrodynamicsimulations for the 3 years show similar

line) and total (continuous line) species of carbon,ter (black) and the sediments (grey) at RES1 (thedered in the total P in the sediments, inorganic Cd CH4).

ons des espèces organiques (ligne pointillée) etosphore dans l’eau (noir) et les sédiments (gris) àlement considérée dans le P total dans les sédi-O2, CO3

2-, HCO3-, H2CO3 et CH4).


seasonal temperature patterns at theannual scale close to the Dam (RES1,Fig. 9A). During the CD season, thewhole water column is destratified.Then the stratification starts (WD sea-son) and no clear difference betweenthe years is calculated. Bottom watertemperature differs by about 2 °Cbetween simulations. The most signifi-cant difference occurs during the WWseason. In August 2005 (simulation 3)the flow-induced destratification is totalfor some days (no difference betweensurface and bottom temperatures)while in 2003 (simulation 1) the flow-induced destratification is only partial(about 4 °C between surface and bot-tom temperatures). Close to the waterintake (RES9, Fig. 9B), the seasonalevolution is the same as described pre-viously but the difference between sur-face and bottom temperature is lower.The water column is (almost) totallydestratified more often and for longerperiods than at RES1.

Fig. 8. Medium-term evolution of the differences bepower house) minus tributary inputs of total carbon,Fig. 8. Évolution à moyen terme des différencesusine) moins les entrées par les affluents en carbon

3.2.2 Physico chemical water qualityfor simulation 2: “dry” year

During the WW season, the aver-age DO concentration at RES1 andRES9 tends to decrease between thereference simulation 1 and simulation 2(Fig. 10, Fig. 11) but the differences arenot significant (p = 0.101 and p = 0.329for surface and bottom concentration atRES1 and p = 0.125 and p = 0.350 atRES9). During the CD and WD sea-sons, differences are also erratic andlimited except in December when DOconcentration increases from 1.6 mg.L-1

(reference) to 8.8 mg.L-1 (simulation 2).During this period, the average DOconcentration remains unchanged atthe surface (p = 0.275) and at the bot-tom (p = 0.41) at RES1 as compared tosimulation 1.

Like for DO, significant changes inNH4

+ concentrations can be observed.During the WW season, surface concen-trations tend to increase as compared to

tween river outputs (downstream the dam and thetotal nitrogen and total phosphorus.

entre les sorties par les rivières (aval barrage ete total, azote total et phosphore total.


simulation 1 (+59%) while bottom con-centrations remain almost unchanged(+8%, p = 0.086) at RES1 (Fig. 10).During the CD and WD seasons, evenif the overall trend is an increase ofNH4

+ concentrations as compared tothe reference simulation (+49% and+10% for surface and bottom concen-trations), the evolution is highly varia-ble. For NO3

- at RES1 (Fig. 10), thegeneral trend is a decrease as com-pared to the reference: -8 and -31% forsurface and bottom concentrations,

Fig. 9. Water temperature at RES1 (A) and RES9 (turbine) (C) and to outflow at the dam only (D) for thlation 2 (“wet” year, dotted line) and simulation 3 (“dFig. 9. Température de l’eau à RES1 (A) et RES9aux débits sortants totaux (barrage + turbines) (C)tion de référence (ligne continue), la simulation 2 (a(année « sèche », ligne discontinue).

respectively. On the opposite, NO3- con-

centrations at RES9 increase (Fig. 11):+22% and +36% at the surface and thebottom, respectively. PO4

3- simulationsare not shown. During the WW season,no differences are observed at RES1:0% and +3% (p = 0.742) for surface andbottom concentrations. At RES9, sur-face concentration remains constant(+0%) as compared to the referencesimulation while bottom concentrationdecreases (-41%, p < 0.001). During theCD and WD seasons, no clear trend can

B), daily water renewal due to total outflows (dam+e reference simulation (continuous line), the simu-ry” year, broken line).(B), taux de renouvellement quotidien de l’eau dûet aux débits au barrage seuls (D) pour la simula-nnée « humide », ligne pointillée) et la simulation 3


be observed: -1% and +3% for sur-face and bottom average concentra-tions. No clear patterns can beobserved for chlorophyll a concentra-tion (Figs. 10 and 11). The dynamicsof the phytoplankton, mainly influ-enced by the PO4

3- concentration in

Fig. 10. Surface (left hand side) and bottom (right haparameters in the Reservoir (RES1) for the refere(“wet” year, dotted line) and simulation 3 (“dry” yearFig. 10. Concentrations de quelques paramètres p(droite) du réservoir (RES1) pour la simulation de r« humide », ligne pointillée) et la simulation 3 (anné

the NT2 Reservoir, is complex andresponds rapidly to the slightestchanges in hydrodynamics.

The behaviour of total iron is close tothat of NH4

+ (Chanudet et al., sameissue). An accumulation is observedduring the WW season as compared to

nd side) concentrations of some physico-chemicalnce simulation (continuous line), the simulation 2, broken line).hysico-chimiques en surface (gauche) et au fondéférence 1 (ligne continue), la simulation 2 (annéee « sèche », ligne discontinue).


the reference (+168% and +41% forsurface and bottom concentrations atRES9 for instance). During the CDand WD seasons, the increase is stillsignificant, especially at RES9 (+42%and +21% for surface and bottomconcentrations).

Fig. 11. Surface (left hand side) and bottom (right haparameters in the Reservoir (RES9) for the refere(“wet” year, dotted line) and simulation 3 (“dry” yearFig. 11. Concentration de quelques paramètres p(droite) du réservoir (RES9) pour la simulation de« humide », ligne pointillée) et la simulation 3 (anné

3.2.3 Physico-chemical water qualityfor simulation 3: “wet” year

DO concentrations at the surfaceduring the WW season are close to thatcalculated for the reference year(Figs. 10 and 11). Close to the Dam

nd side) concentrations of some physico-chemicalnce simulation (continuous line), the simulation 2, broken line).hysico-chimiques en surface (gauche) et au fondréférence (ligne continue), la simulation 2 (annéee « sèche », ligne discontinue).


(RES1), average bottom concentra-tions are 24% lower than for the refer-ence simulation. For CD and WD sea-sons, changes are lower as comparedto the reference year (-14% and +3%for surface and bottom concentrationsin average). Like for the “dry” year (sim-ulation 2), the most significant differ-ence occurs in December.

Higher NO3- concentrations during

the WW season are simulated at thebottom (Fig. 10) at RES1. At RES9(Fig. 11), lower surface concentrationsare observed as compared to the refer-ence simulation (-13%, p < 0.01). ForCD and WD seasons, mean changesare not significant as compared to thereference year (+4% and -1% for sur-face and bottom concentrations). Asregard to PO4

3-, almost no changeoccurs at the surface as compared tothe reference year. During the WW sea-son, the bottom concentration decreases(p < 0.01) for all stations (-16% and -11%for RES1 and RES9). No significantchange can be observed as regard tochlorophyll a concentration.

During the WD season, iron con-centrations are close to that measuredduring the reference year. A decreaseis nevertheless observed at RES1(- 23% at the bottom). During the CDand WD seasons, no clear trend can beobserved. On average, concentrationsare lower than for the reference simula-tion: -27% and -9% for surface and bot-tom concentrations.

3.2.4 Effect of the hydrodynamics onphysico-chemical water quality

The impact of the hydro-meteoro-logical conditions on water quality var-ies spatially between stations close to

the Nakai Dam (RES1) and close to thewater intake (RES9). At RES1, fromAugust for the “wet” year and fromSeptember for the reference simula-tion, floods induce the input of largequantities of riverine water from thetributaries. The spillage at the damenhances water renewal for the twosimulations in this period (Fig. 9C- D).Consequently, several phenomena areobserved at this station for the “wet”year (simulation 3): (i) DO increases atthe bottom while reduced compounds(NH4

+, Fe) decrease to get closer tothe concentration measured in thetributaries, (ii) an homogenisation isobserved between surface and bottomwater for all the parameters includingtemperature (Fig. 9A). The opposite isobserved for the “dry” year (simulation2). The absence of spillage precludesany significant water renewal close tothe dam. After the spillage (October),water quality evolves similarly for thethree simulations: decrease of DO con-centration at the bottom and increaseof reduced compounds concentration.From December, the differences inthe Reservoir hydrodynamics do notdepend on the hydrology (same dis-charges for the three simulations) butrather on the meteorology. For thethree simulations, the destratification,as evidenced by surface and bottomtemperatures (Fig. 9A), induces ahomogenisation of the water column. Itoccurs mid-December for the “dry” and“wet” years and about two weeks laterfor the reference simulation. From Feb-ruary, water quality parameters stabi-lize and become similar for the threesimulations.

At RES9 (close to the water intake),the impact of the floods between


August and October are less significantthan close to the dam. This is probablydue to the fact that the total waterrenewal (outputs from the dam and thePower House) is not drastically differ-ent between the simulations (Fig. 9C).While this renewal is mainly due tospillage for simulations 3 and 1, itis almost exclusively supported bythe discharge through the turbines(Fig. 9C-D) coupled with a lower Res-ervoir volume (Fig. 3D) for the “dry”year. Moreover, at this station, the arti-ficial mixing induced by the waterintake also explains most of the differ-ences as compared to RES1. This mix-ing reduces the difference betweensurface and bottom temperatures(Fig. 9B) and promotes the verticalexchange of chemical compounds(Chanudet et al., same issue). FromDecember, the same phenomenon asin RES1 (in spite of a less pronouncedstratification) explains the irregularevolution of water quality parametersfor the three simulations.

3.3 Effect of the fill and flushstrategy

3.3.1 Hydrodynamics

Close to the dam (RES1), the lowerdischarge and water renewal rate in thesimulation 5 (without fill and flush,power house commissioning in June2008) reduces turbulences and verticalexchanges during the WW season ascompared to the reference simulation 4(Fig. 4A). As a consequence, bottomtemperature is lower for this simulation(Fig. 12A). During the WD season, thethinner water layer in the simulation 5

(Fig. 4D) does not favour an increaseof the bottom temperature (Fig. 12A).

Close to the water intake (RES9),the commissioning of the power housein June 2008 (simulation 5) induces ahigher water renewal rate (Fig. 12C-D)and turbulence as compared to the ref-erence simulation 4. It favours a bettervertical homogenisation throughout theyear (Chanudet et al., same issue) asevidenced by a higher bottom temper-ature (Fig. 12B) compared to the refer-ence simulation.

3.3.2 Physico-chemical water quality

At RES1, the DO bottom concentra-tion decreases from June 2008 ascompared to the reference simulation(2.95 mgO2.L-1 in average betweenJune and October 2008 for simulation 4and 1.33 mgO2.L-1 for the sameperiod for simulation 5). In the sametime, the concentration of reducedcompounds such as NH4

+ (from 0.09to 1.17 mgN.L-1) or Fetot (from 2.39 to2.96 mgFe.L-1) increases at the bot-tom. The same observation can bedone during the 2009 WW season(May-September) for reduced com-pounds. In both cases, the higherresidence time close to the dam inabsence of fill and flush as comparedto the reference simulation (Fig. 4D)probably explains it. Between October2008 and May 2009, the discharges atthe dam are similar for the two simula-tions and the water renewal is thus notdrastically modified between simula-tions in spite of the lower Reservoir vol-ume in simulation 5. During this dryperiod, the differences between thetwo simulations are low. The smaller


water depth for simulation 5 (Fig. 4B)may however explain why surfacewater quality degrades slightly (lessDO and more reduced compounds) inApril-May 2009 as compared to the ref-erence simulation: vertical mixing ofsubstances released from the sedi-ment is probably enhanced. Both Fetotsurface and bottom concentrationsincrease gradually during the wholesimulated period.

At RES9, the effects of fill and flushare more significant (Fig. 13) than in

Fig. 12. Surface and bottom temperatures at RES1outflows at the dam (C) and through the turbine (D)for simulation 5 (without fill and flush, dotted line).Fig. 12. Température en surface et au fond à RES1dien de l’eau dû aux débits sortants au barrage (C)de référence 4 (ligne continue) et la simulation 5 (sa

other parts of the Reservoir. DO con-centration increases at the bottom(from 1.00 to 1.84 mgO2.L-1 for simula-tions 4 and 5, respectively in averageover the whole period), and to a lowerextent, at the surface (from 5.94 to6.22 mgO2.L-1). NH4

+ concentrationalso reduces between simulations 4and 5 from 0.32 to 0.16 mgN.L-1 at thebottom. The average surface concentra-tion remains unchanged (0.01 mgN.L-1).Both surface and bottom NO3

- concen-trations increase as compared to the

(A) and RES9 (B) and daily water renewal due tofor the reference simulation 4 (continuous line) and

(A) et RES9 (B) et taux de renouvellement quoti-et aux débits turbinés seuls (D) pour la simulationns « fill and flush », ligne pointillée).


reference simulation. The evolution ofFetot is not as clear as the evolution ofNH4

+. Nevertheless, the average bot-tom concentration decreases signifi-cantly from simulation 4 (9.93 mgFe.L-1)to simulation 5 (4.42 mgFe.L-1).These observations can probably beexplained by: (i) the vertical mixingand (ii) the water renewal rates.Close to the water intake (RES9), tur-bulences induced a quasi-permanentvertical mixing as shown from tem-peratures (Fig. 12B). For simulation 5,

Fig. 13. Surface (left hand side) and bottom (right haparameters in the reservoir (RES9) for the reference(without fill and flush, dotted line).Fig. 13. Concentration de quelques paramètres p(droite) du réservoir (RES9) pour la simulation de ré« fill and flush », ligne pointillée).

the commissioning date is June 2008.From this date, the enhanced verticalexchanges favour the homogenisationof the water column. This phenomenonis particularly noticeable betweenNovember 2008 and February 2009. Inthis Reservoir area, high dischargesthrough the water intake imposed insimulation 5 (Fig. 4C) increase thewater renewal rate (Fig. 12D) and theinput of riverine water coming from thetributaries mainly located in the south-east of the Reservoir (Chanudet et al.,

nd side) concentrations of some physico-chemicalsimulation 4 (continuous line) and for simulation 5

hysico-chimiques en surface (gauche) et au fondférence 4 (ligne continue) et la simulation 5 (sans


2012). Moreover, a modification of awater quality parameter, such as DOfor instance, may modify the concen-tration of another one. For instance thehighest DO concentration for simula-tion 5, especially close to the bottom,may partly explain the highest NO3

-

concentration at the expense of NH4+,

due to an enhanced nitrification flux.If the commissioning of the power

house had occurred almost immedi-ately after the dam closure (in June2008, simulation 5), the physico-chem-ical parameters in the Reservoir closeto the dam (RES1) and in the NamTheun River downstream the damwould not have been drastically modi-fied. The aeration in the downstreamriver (hollow jet valve for the riparianrelease and turbulences in the rivers)(Chanudet et al., same issue) wouldhave probably compensated for thefaster decrease in DO concentrationand the slight increase in reduced com-pounds. Iron concentration would haveprobably increased but the loads wouldhave been reduced (Fig. 4B). Down-stream of the Power House, changeswould have been more significant. InMarch 2010 physico-chemical parame-ters have evolved as compared to thefirst 1.5 years (Chanudet et al., sameissue). From March 2010, bottomwater at RES9 was almost always oxicwith low concentrations of reducedcompounds ([NH4

+] < 0.1 mgN.L-1,Fetot < 2 mg.L-1). Water quality down-stream the power house reflected thisevolution as evidenced by the depthaveraged concentrations simulatedclose to the Water Intake (Fig. 14). Ifthe turbines had started in June 2008,the average DO concentration in thewater column at RES9 (between April

2009 and January 2010) would havebeen reduced from 2.7 to 1.9 mg.L-1

(-30%) as compared to the referencefor the same period (but not for thesame year). NO3

- concentration wouldhave also been reduced from 0.21 to0.10 mgN.L-1 while the NH4

+ concen-tration would have remained constant(0.09 mgN.L-1).

3.4 Influence of a modificationof the nutrient load in the tributarieson water quality in the NT2 Reservoir

3.4.1 Simulation 7: doubling NO3- load

The increase of the NO3- load in

the incoming river does not impact thesurface and bottom NH4

+ and PO43-

concentrations at RES1 (Fig. 15). TheNO3

- surface concentration in theReservoir increases only by 9%(+ 15 µgN.L-1 in average and + 36 µgN.L-

1 maximum) as compared to the refer-ence simulation (176 µgN.L-1 on aver-age). At the bottom, the increase of theNO3

- concentration is lower in absolutevalue: +5 µgN.L-1 on average or 13%of the concentration calculated for thereference simulation. The increase inNO3

- concentration in the Reservoirrepresents between 6% and 19% of theaverage concentration increase in thetributaries (+ 80 µgN.L-1). This meansthat in this part of the Reservoir, mostof total-N in the water column does notcome directly from the tributaries. Thediffusion from the sediment and therecycling in the water column are prob-ably the main N sources.

At RES8 (Fig. 16) and close to thewater intake (RES9), the trend is the


same, even if the NO3- gain is slightly

higher: 20% and 17% at the surface forRES8 and RES9 and 28% and 14% atthe bottom. The phytoplankton activityremains the same in the two simula-tions (no modification of the chlorophylla concentration) and the oxygen pro-duction is not enhanced as comparedto the reference (Tab. III). Moreover,the oxygen demand is not boostedsince the concentration of in situorganic matter is stable as well as thereduced compounds concentration(NH4

+). As a result, the DO concentra-tions in the two simulations are almostsimilar.

Five-year mass balance shows thatan increase by 100% of the NO3

- inputs

Fig. 14. Depth-averaged concentration of some phythe water intake (RES9) for the reference simulatiofor simulation 5 (without fill and flush, April – DecemFig. 14. Concentrations moyennées sur la vertica(RES9) pour la simulation de référence 4 (avril – dé5 (sans « fill and flush », avril – décembre 2009, lign

results globally in an increase by 18.2%of the total N input (+2.99 ktN) ascompared to the reference simulation.The output also increases by 8.6%(+2.91 ktN). It means that almost no Nis stocked in the Reservoir (water + sed-iment) within five years (+0.08 ktN). Nosignificant change occurs for P species.

3.4.2 Simulation 8: doubling PO43- load

Doubling the PO43- load in the tribu-

taries leads to a limited increase in sur-face PO4

3- concentration for someperiods (July, August and December)at RES1 (Fig. 15). On average, theincrease is 0.8 µgP.L-1, or 15% of the

sico-chemical parameters in the Reservoir close ton 4 (April – December 2010, continuous line) andber 2009, dotted line).

le dans le réservoir à proximité de la prise d’eaucembre 2010, ligne continues) et pour la simulatione pointillée).


average concentration in the referencesimulation (5.2 µgP.L-1). At RES8(Fig. 16) and RES9, no difference atthe surface is observed as compared tothe reference. At the bottom of RES8,the average increase (+ 1.5 µgP.L-1)represents 9% of the average reference

Fig. 15. Surface (left hand side) and bottom (right haparameters in the Reservoir (RES1) for the refebetween simulation 7 (increase NO3

- river input) antion 8 (increase PO4

3- river input) and the referencescale).Fig. 15. Concentration de quelques paramètres p(droite) du réservoir (RES1) pour la simulation de résimulation 7 (augmentation de l’apport en NO3

- paentre la simulation 8 (augmentation de l’apport ennue) sont également présentées (échelle de droite)

concentration (10.2 µgP.L-1) at RES8.At RES9, the average increase at thebottom is 1.0 µgP.L-1, 13% of the refer-ence bottom concentration. The sameprocesses as for NO3

- can explain thelow impact of the concentration in thetributaries. In situ production terms

nd side) concentrations of some physico-chemicalrence simulation 6 (dotted line). The differencesd the reference (broken line) and between simula-(continuous line) are also shown (right hand side

hysico-chimiques en surface (gauche) et au fondférence 6 (ligne pointillée). Les différences entre lar les rivières) et la référence (ligne discontinue) etPO4

3- par les rivières) et la référence (ligne conti-.


(diffusion from the sediments and inter-nal recycling) probably dominate inputsfrom the incoming rivers.

The average NH4+concentration is

increased by 3% at the surface and 8%at the bottom at RES1. The impact onNO3

- concentration is more significant

Fig. 16. Surface (left hand side) and bottom (right haparameters in the Reservoir (RES8) for the refebetween simulation 7 (increase NO3

- river input) antion 8 (increase PO4

3- river input) and the referencescale).Fig. 16. Concentration de quelques paramètres p(droite) du réservoir (RES8) pour la simulation de résimulation 7 (augmentation de l’apport en NO3

- paentre la simulation 8 (augmentation de l’apport ennue) sont également présentées (échelle de droite)

since concentrations are reduced by12% at the surface and 19% at the bot-tom. At RES8, the trend is the same:–15% at the surface and –15% at thebottom. These values are similar atRES9. This decrease of the NO3

- con-centration can probably be explained

nd side) concentrations of some physico-chemicalrence simulation 6 (dotted line). The differencesd the reference (broken line) and between simula-(continuous line) are also shown (right hand side

hysico-chimiques en surface (gauche) et au fondférence 6 (ligne pointillée). Les différences entre lar les rivières) et la référence (ligne discontinue) etPO4

3- par les rivières) et la référence (ligne conti-.


Tab

leIII

.F

ive-

year

mas

sba

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are

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Tab

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.Bila

nen

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.Que

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Bal

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by an enhanced consumption byphytoplankton due to the higher PO4

3-

incoming fluxes. At the whole systemscale, the five-year mass balanceshows that the impact on N species islow (Tab. III).

PO43- concentration remains low

since this compound is immediatelyconsumed by phytoplankton, andphosphorus is the main limiting nutri-ent. Five-year mass balance showsthat an increase by 100% of thePO4

3- inputs results globally in anincrease by 19.2% of the total P input(+0.33 ktP) as compared to the refer-ence simulation. P outputs increaseby 9.3% (0.09 ktP). The final stock(water + sediments) increases by 1%(+0.24 ktP) (Tab. III).

As compared to the reference simu-lation, the average concentration ofchlorophyll a increased by 0.64 µg.L-1

(+18% of the reference average con-centration) at RES1 (Fig. 15). At RES8(Fig. 16) and RES9, the fraction ofPO4

3- coming from the tributaries (ofthe total PO4

3- input) is higher thanclose to the dam because of the lowerresidence time close to the WaterIntake (Chanudet et al., same issue).As a consequence, the increase inPO4

3- load in the tributaries has ahigher impact on primary production forthese stations. On average, the chloro-phyll a concentrations at RES8 andRES9 increased by 2.17 and 1.32 µg.L-1

(+37% and +29% of the reference aver-age concentration), respectively. Massbalance shows that the primary produc-tion increases by 18% (+53.92 ktC)(Tab. III).

The impact on DO is not constantover time. At RES1, the higher photo-synthetic activity induces an increase

in surface DO concentration fromAugust to November. Then, except inDecember when the Reservoir over-turns, both surface and bottom DO con-centrations decrease as compared tothe reference simulation. This is proba-bly due to the enhanced oxygen con-sumption induced by the higher concen-tration of labile organic matter producedin the Reservoir by phytoplankton asevidenced by the higher chlorophyll aconcentration. At RES8, most of thetime, the impact is an increase of theDO concentration, especially at the sur-face and from June to December. At thewhole system scale (water + sedi-ments), the increase in oxygen produc-tion (+ 143.95 ktO) is balanced by anincrease in oxygen consumption bymineralisation (+ 78.71 ktO) (Tab. III).

3.4.3 Simulation 9: doubling NO3-

and PO43- load

The cumulated increases of NO3-

and PO43- concentrations in the tribu-

taries (simulation 9) produce the sameresults as seen for the increase ofPO4

3- only (simulation 8) (Tab. III). Itconfirms that the NT2 Reservoir isP limited, and is sensitive to any addi-tional P input.

4 CONCLUSION

Four different types of scenario simu-lations have been performed with thewater quality model of the Nam Theun 2Reservoir. These simulations show thecapabilities of the model to answer ques-tions related to natural or human forcingin such a system. The conclusions foreach scenario type are presented below.


1.Prediction of medium term evolutionconsidering constant hydrodynamicconditions. A 15-year simulationconfirms the observation made dur-ing the first five years afterimpoundment. The DO concentra-tion will probably continue toincrease in the water column asso-ciated with a decrease of the con-centrations of reduced compounds.However, even after 15 years, thesimulation shows that the bottomwater in the Reservoir close to thedam may remain anoxic most of thetime (assuming yearly repeatedhydrodynamics with a yearly Reser-voir mixing).

2.Assessment of the effects of the nat-ural inter-annual variability of hydro-meteorological conditions on waterquality in the Reservoir. At the annualscale, , the duration and the magni-tude of the wet season have an influ-ence on the water quality in thewhole Reservoir and spatial hetero-geneities are observed. The waterlevel at the moment of a flood eventalso affects the Reservoir water qual-ity. If the water level is low, a flood willrenew a larger fraction of the water inthe reservoir. In addition to differ-ences due to in and outflows, mete-orological parameters could alsodrastically modify water quality bychanging the stratification conditionsand thus the vertical exchanges.However, simulations show that dif-ferences are mainly perceptible dur-ing short periods of time and theannual evolution is similar in thesimulations.

3.Assessment of the effects of themanagement of the Reservoir onwater quality. Simulations have

been done to quantify the effect ofan earlier commissioning on waterquality in the Reservoir and in down-stream rivers. Close to the dam(RES1), physico-chemical waterquality would not have changeddrastically in spite of a lower waterrenewal rate due to the reduction ofdischarges at the dam as comparedto how it evolved in reality. At thewater intake (RES9), and conse-quently in the downstream system(NT2 Downstream Channel and XeBangfai River), some modificationsof the water quality would haveoccurred. Simulations show that ifthe commissioning had occurredimmediately after the impoundment,the physico-chemistry of the waterreleased downstream from thepower house would have beendegraded (only one year simulated)with a decrease of the DO concen-tration and an increase of reducedcompounds.

4.Assessment of the impacts of amodification of the watershed (pop-ulation, land use...) on water qualityin the Reservoir. The sensitivity ofthe model to an increase of thenutrients (NO3

- or/and PO43-) load

from the tributaries of the Reservoirhas been quantified. Doubling theNO3

- load in the rivers has almostno effect on water quality. On thecontrary, an additional input ofPO4

3- induces changes of the con-centrations of both N and P species.Such a modification of the nutrientbalance enhances photosyntheticactivity, because phosphorus is themain limiting nutrient for phyto-plankton. This increase is more sig-nificant close to the Intake where


the influence of the load from thetributaries is higher than close to thedam. The enhancement of primaryproduction affects DO through twoopposite ways: (i) a direct increasenear the surface due to the oxygenproduction during photosynthesisand (ii) a decrease near the bottomdue to the oxygen consumption forthe mineralization of the newly pro-duced organic matter.From the results of the scenario

simulations, it appears that the modelis an efficient tool for hydrodynamicand water quality studies in the NT2Reservoir.

ACKNOWLEDGEMENTS

This research has been conductedat the Aquatic Environment Laboratoryof Nam Theun 2 Power Company in LaoPDR whose Shareholders are Électric-ité de France, Lao Holding State Enter-prise and Electricity Generating PublicCompany Limited of Thailand.

We are also grateful to the NamTheun 2 Power Company for logisticalsupport and to all the members of theAquatic Environment Laboratory (AEL)of the Nam Theun 2 project for sam-pling and analysis.

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