the water balance and stable isotope hydrology of lake edward,...

J. Great Lakes Res. 32:77–90Internat. Assoc. Great Lakes Res., 2006

The Water Balance and Stable Isotope Hydrology ofLake Edward, Uganda-Congo

James M. Russell1,* and Thomas C. Johnson2

1Department of Geological SciencesBrown University

Box 1846Providence, Rhode Island 02912

2Large Lakes ObservatoryUniversity of Minnesota Duluth

10 University Drive, RLBDuluth, Minnesota 55812

ABSTRACT. Lake Edward, Uganda-Congo, is one of the least studied of the great lakes of East Africa,and little is known of its physical hydrology. Stable isotope data and modeling and previously publishedestimates of Lake Edward’s water balance are used to constrain the physical hydrology of the lake, andparticularly the relative proportion of surface outflow to evaporative water losses. Stable isotope calcula-tions suggest that Lake Edward loses roughly 50% of its water income by evaporation, while reviews ofpublished hydrologic data together with our calculations suggest that evaporation comprises 54% ofwater losses. The similarity of these two sets of calculations lends credence to their validity, and providesa new water budget for the lake. Our results have important implications for the chemistry and hydrocli-matic sensitivity of Lake Edward.

INDEX WORDS: Lake Edward, East Africa, rift lake, stable isotope, hydrology.

INTRODUCTION

The Great Lakes of East Africa are rich sourcesof information about past variations of the Africanmonsoons. The potential for these lakes to recordpast variations in monsoon intensity is partly due totheir hydrologic sensitivity that is driven by hydro-logic budgets for the lakes in which water losses aredominated by evaporation (Spigel and Coulter1996). Lake Edward, located on the equator at theborder between Uganda and the Democratic Repub-lic of the Congo, has received perhaps the least at-tention of the East African Great Lakes, despitepaleoclimatic studies that have revealed a rich andvaried paleoclimatic history for the lake (e.g., Rus-sell et al. 2003, Laerdal et al. 2002). A thoroughunderstanding of the modern hydrology of Lake Ed-ward is critical to interpreting paleoclimate datausing Lake Edward’s sedimentary record; however,estimates of Lake Edward’s hydrologic budget arefew and often contradictory (Lehman 2002).

*Corresponding author. E-mail: [email protected]

77

This paper seeks to refine calculations of themodern hydrologic balance of Lake Edward usingpast measurements and stable isotope data. First,we summarize previous estimates and measure-ments of Lake Edward’s hydrology. We then pre-sent new stable isotopic data for the lake andwatershed that help to constrain the water balanceof the lake. Our data and literature review suggestthat evaporation and outflow each account forroughly 50% of water loss from Lake Edward, esti-mates that are similar to some previous studies andhelp us to better understand the lake’s physical andchemical structure.

Background:Regional Geology and Hydrology

Lake Edward (0°–0°40′S, 29° 20′–29° 50′E, 912m a.s.l.) is situated in a Cenozoic half-graben in theWestern Arm of the East African Rift Valley (Fig.1). The lake is presently open, draining northwardto Lake Albert via the Semliki River. Lake Edward

78 Russell and Johnson

is bounded by the Lubero border fault to the westand the Kigezi highlands to the east, the Ruwenzorimountains to the north, and the Virunga volcanoesto the south (Fig. 2). These four regions, togetherwith the Lake George catchment to the northeast,comprise five major catchment areas that providerunoff to Lake Edward.

Lake George is drained by the Kazinga Channel,which flows sluggishly for 60 km to Lake Edward.The Ruwenzori Mountains to the north of Lake Ed-ward rise from the rift floor to heights of over 5,000m and are currently glaciated. Principal inflowsfrom the Ruwenzoris to Lake Edward are from theNyamugasani and Lubilia rivers (Fig. 2), while con-siderable additional inflow from the Ruwenzoris isdelivered to Lake Edward via Lake George. Moun-tains to the west of Lake Edward along the Luberoborder fault rise steeply from the lake to heights of2,500 to 3,000 meters within 15 km of the lakeshore, and are drained by numerous short, steeprivers. The Kigezi highlands to the east rise moregently to form a low divide between Lakes Edwardand Victoria. Principal inflows from the Kigezihighlands to Lake Edward are from the Ishasha,Ntungwe, Nchwera, and Nyamweru rivers. TheVirunga volcanoes to the south divide Lakes Ed-ward and Kivu and are very important to the hy-drology and chemistry of Lake Edward (Kilhamand Hecky 1973, Lehman 2002). Principal inflows

to Lake Edward from the Virunga region are theLula, Rwindi, and Ruchuru rivers.

Lake Edward: Bathymetry, Morphology,Limnology, and Climatology

Lake Edward has a surface area of 2,325 km2 andmaximum depth of 117 meters, located within 5 kmof the western shore (Fig. 3; Lehman 2002). LakeEdward has an oxycline commonly found at about40 m depth and is thought to be oligomictic (Beadle1981, Hecky and Degens 1973). Although strati-fied, the temperature difference between surfaceand deep waters is only about 1°C, with an averageannual surface temperature of about 26.5°C (Ver-beke 1957, Beadle 1966). Chemically, Lake Edwardis a Na-Mg-HCO3 system with a salinity of approx-imately 0.8 g/L and a pH averaging 8.9 (Talling andTalling 1965).

Rainfall in the Lake Edward region falls in tworainy seasons coinciding with the passing of the In-tertropical Convergence Zone, from October to De-cember and March to May (Viner and Smith 1973,Nicholson 1996). Rainfall in the region, andthroughout East Africa, varies strongly with altitude(Viner and Smith 1973, Nicholson 1996). Thus, thehighlands surrounding Lake Edward receive consid-erably more rainfall than the lake itself, which ex-periences a more arid climate than much of theregion (Hurst 1927, Viner and Smith 1973).

FIG. 1. Map of the East African Great Lakes region. Dashed lines in the left-hand figureindicate the position of the eastern and western arms of the rift. Shaded gray regions indicatemajor water bodies. A close-up at right of the equatorial lakes region shows political bound-aries (dashed lines) bisecting Lake Edward.

The Hydrology of Lake Edward, Uganda 79

Previous Work and Data Sources

The first estimates of the hydrologic budget ofLake Edward were made by Hurst (1925, 1927) aspart of a survey of the Nile River headwaters.Hurst’s work contains single-sample river gaugedata and runoff estimates for several rivers in LakeEdward’s catchment as well as Lake Edward’s out-flow. Viner and Smith (1973) provided a hydrologic

budget for Lake George based upon 5 years of de-tailed hydrologic and climatic monitoring. Theirdata include daily to monthly river gauge data, theonly such data available for the Edward basin. Datafrom these authors, supplemented by other esti-mates of Lake Edward’s hydrologic and limnologiccharacteristics (Worthington 1932, Damas 1937,Verbeke 1957, Hydromet 1982) form the basis for

FIG. 2. Map of the Lake Edward region showing catchment areas, major rivers, highelevation areas, swamps, and water sampling sites.


our study. Lehman (2002) combined hydroclimaticdata with his own hydrologic calculations and anenergy balance model into the first physical, hydro-logic, and chemical model of Lake Edward.Lehman’s calculations suggest Lake Edward’s out-flow exceeds annual water losses by evaporation bya factor of nearly five, an estimate that differs con-siderably from previous researchers.

Bathymetric and morphometric measurements forLake Edward were calculated by Laerdal (2000)and Lehman (2002) (Table 1). Catchment areas forLakes Edward and George were measured from De-fense Mapping Association maps L-4, L-5, and M-5(Fig. 2). Our estimate for the catchment area ofLakes Edward and George differs slightly from pre-vious studies (Lehman 2002). Based upon the DMAmaps, it appears that Lehman (2002) underesti-mated the catchments of the Ntungwe andRusangwe rivers by about 90% and 116%, respec-tively. We are uncertain as to why these discrepan-cies exist, but we note that our revised catchmentfor the Rusangwe River matches that of Viner andSmith (1973), who explored the area extensively.Our revised catchment area increases the proportionof low-elevation areas to the east that drain intoLakes Edward and George, which could affect sur-face runoff into the lake.

The isotopic composition of Lake Edward, in-flowing rivers and springs, and occasional rainfallsamples were sampled and analyzed between 1996and 2003. 20-mL samples from rivers were taken at

road crossings within 15 km of Lake Edward’sshore, and lake waters were sampled from openwater at least 5 km from shore. Water samples werecollected and stored in high-density polyethylenevials prior to analysis. Analyses were conducted ona Finnegan Delta S mass spectrometer at the Uni-versity of Arizona; results are expressed in deltanotation with respect to the SMOW standard. Ana-lytical error was 0.1 ‰ for δ18O and 1.0 ‰ for δD.

The Hydrology and Water Balanceof Lake Edward

The fundamental equation for the hydrology ofLake Edward assumes the lake is in a steady statewith respect to its volume:

Evaporation + Outflow = Direct precipitation + Catchment inputs (1)

Previous estimates of the magnitudes of each ofthese terms will be discussed below.

Direct Precipitation

Hulme (1998) estimates precipitation in the LakeEdward region at 1.217 m/yr, similar to the esti-mates of Lehman (2002) of 1.214 m/yr, as well asthe 1.1 m/yr estimated by Hurst (1927). However,the highland regions surrounding Lake Edward re-ceive far more rainfall than the lowlands and thelake itself (Viner and Smith 1973). The estimatesabove are based upon weighted averages of rainfallstations in all of southwestern Uganda, including

FIG. 3. Bathymetric Map of Lake Edward, withdepth contours in meters. The position of craterlakes within the basin are also shown.

TABLE 1. Morphometric and Catchment infor-mation for Lakes Edward and George, Uganda-Congo.

Lake EdwardSurface Elevation 912 m a.s.l.Surface area 2,325 km2

Volume 767 × 108 m3

Max Depth 117 mCatchment Area (less lake) 15,840 km2

Ruwenzori Catchment Area 1,231 km2

Western Escarpment Area 1,136 km2

Eastern Rivers 5,680 km2

Southern Rivers 7,793 km2

(including Ishasha)

Lake GeorgeSurface area 250 km2

Catchment 9,976 km2


several stations in the highlands surrounding LakeEdward, and therefore probably overestimate directprecipitation to the lake’s surface (Nicholson 1996).Viner and Smith estimate direct precipitation ontothe surface of Lake George averages 0.82 m/yr.Rainfall stations nearest to the surface elevation ofLake Edward, Kasese and Kabale, receive 0.87 and0.99 m/yr, respectively (National Climate DataCenter archive). We have averaged these three val-ues and estimate that direct precipitation to LakeEdward is 0.9 m/yr.

Catchment Inputs

Catchment inputs include river inputs, surfacerunoff, and groundwater inputs. Groundwater, al-though it may be important to the chemical balanceof Lake Edward, is assumed to be negligible in thehydrologic budget (Lehman 2002). Catchment in-puts comprise the largest source of water to LakeEdward (Lehman 2002), yet they are by far themost difficult to estimate due to a nearly completelack of river gauge data from the Lake Edwardbasin.

The available catchment-normalized surfacerunoff data from Lake George demonstrate the het-erogeneity of the region’s hydrology (Table 2;Viner and Smith 1973). All of the rivers draininginto Lake George measured by Viner and Smith(1973), except the Mpanga and Kyambura, drainthe Ruwenzori Mountains and have very high sur-face runoff rates, ranging from 0.514 to 1.54 m/yr.However, when the less steep, low-elevation east-

ern part of Lake George’s drainage basin is takeninto account, the average runoff for the entireGeorge basin is only about 0.2 m/yr. This is likelydue to the steeper elevation gradients of the Ruwen-zoris, which yield higher runoff, as well as higheraverage annual rainfall at higher elevations withinthe lake’s catchments.

The only river that flows into Lake Edward thathas annual gauge data is the Nyamugasani River,which drains the Ruwenzori Mountains (Viner andSmith 1973). Lehman (2002) applied the runoff de-rived from the Nyamugasani catchment, 0.514m/yr, to the entire Lake Edward basin, and calcu-lated inputs to the lake totaling 8.85 × 109 m3/yr.Based on the example of Lake George it seemslikely that this is an overestimate, given that theslope, climate, and bedrock geology of the Ruwen-zori mountains is prone to high runoff as comparedto the Lake Edward catchment as a whole. In pointof fact, the slope of the Nyamugasani River is about6% over the river’s catchment, while the averageslope of the rivers draining into Lake Edward fromthe east is only 1.5%. The average slope of riversdraining into Lake Edward from the south is 3%,while rivers draining from the west have slopesequal to, or higher than, the Nyamugasani River. Ifwe assume that rivers draining from the Ruwen-zoris and the western mountains into Lake Edwardhave surface-area normalized runoff yields equal tothe Nyamugasani River, that rivers draining theeastern slope provide runoff equal to that of theMpanga River, and that the southern rivers providerunoff intermediate between these two areas, we

TABLE 2. River inputs to Lakes Edward and George from Hurst (1927), Viner and Smith(1973), and Lehman (2002).

River Flow Catchment Runoff Annual InputRiver (m3/sec) (km2) (m/yr) (109 m3/yr)

Ruchuru, dry season 40.000Ishasha, dry season 8.000Ntungwe, dry season 7.000Nyamugasani 8.330 507 0.514 0.260Sebwe (George catchment) 2.040 83 0.777 0.060Rukoki/Kamulikwezi (George) 4.100 183 0.707 0.129Mubuku (George) 12.500 256 1.540 0.394Ruimi (George) 6.000 266 0.711 0.660Mpanga (George) 11.500 4,670 0.080 0.374Kyambura (Kazinga Channel inflow) 9.500 660 0.450 0.297

George basin (Viner and Smith, 1973) 61.800 9,976 0.196 1.948Edward basin, Lehman (2002) 280.000 15,840 0.514 8.850Edward basin, Hurst (1927) 141.000 15,840 0.280 4.435


calculate an average runoff for the Edward catch-ment of 0.25 m/yr, very similar to the value of 0.28suggested by Hurst (1927). Hurst’s value is inter-mediate between that of Lehman (2002) for LakeEdward and Viner and Smith (1973) for LakeGeorge, and seems reasonable given that the LakeEdward catchment contains a slightly higher pro-portion of steeply sloping terrain than the LakeGeorge catchment. Therefore, we assign a runoffvalue equivalent of Hurst’s estimate of 0.28 m/yr,or 4.435 × 109 m3/yr, to catchment inputs to LakeEdward excluding inputs from Lake George.

In addition to general catchment inputs, theKazinga Channel delivers 1.70 × 109 m3/yr to LakeEdward (Viner and Smith 1973), a value deter-mined at its exit from Lake George both by hydro-logic modeling and gauge data. This represents thecombined inputs of rivers and precipitation to LakeGeorge, less evaporation from Lake George’s sur-face (Viner and Smith 1973).

Outputs

Surface Evaporation

Published estimates for evaporation from LakeEdward vary widely (Table 3). The most commonmethods of estimating evaporation from a lake sur-face are energy balance and Penman’s (1948)method. The latter combines a formula for potentialevapotranspiration with energy balance and water-mass transfer. Both methods require numerousinput variables, including air vapor pressure, laketemperature, cloudiness, and surface radiation.

Input data for evaporation calculations includessurface pressure, dew point, cloud fraction, andwind-speed data from the Kasese weather station(Table 4), which lies between Lakes Edward andGeorge. Lake water temperature is derived frommean monthly measurements reported in Verbeke(1957), which are slightly cooler than more recentvalues reported from Lehman (2002). Insolation

TABLE 4. Meteorological input data used in evaporation calculations.

Top of Atmosphere Surface Surface Dew TempInsolation pressure Air Temp Point Cloud Lake Wind-speed

Month W/m2 mb °C °C Fraction °C m/s

Jan 416.2 903.6 23.36 19.01 0.413 25.9 2.41Feb 431.8 903.5 23.58 17.81 0.305 26.0 2.14Mar 438.2 903.4 23.63 19.02 0.481 26.1 2.48Apr 427.1 904.7 23.68 19.73 0.333 26.5 2.14May 406.2 905.7 23.57 19.66 0.257 27.1 2.00Jun 392.5 904.9 23.24 19.00 0.292 27.2 1.65Jul 397.0 905.6 22.81 18.15 0.318 25.8 1.66Aug 414.7 905.1 22.83 17.44 0.494 25.3 2.22Sep 429.8 905.1 22.73 18.72 0.353 25.8 2.68Oct 430.3 904.3 22.88 19.13 0.370 26.8 2.67Nov 418.2 903.8 23.01 19.40 0.517 27.2 2.33Dec 409.0 904.3 23.26 19.15 0.420 26.5 2.33

TABLE 3. Evaporation Estimates for Lakes Edward and George, fromHurst (1927), Viner and Smith (1973), Lehman (2002), and Penman andenergy balance calculations of this study.

Annual Water Loss, Author Method Rate, m/yr km3/yr

Lehman (2002) Mass Transfer 1.16 2.59Hurst (1927) comparison to Lake Victoria 1.20 2.79Viner and Smith (1973) Penman 1.83 4.24This Study Energy Balance 1.98 4.60This Study Penman 2.10 4.87


and surface air temperature were obtained from theNational Center for Environmental Prediction(NCEP) Electronic Reanalysis Atlas. Kasese datafor windspeed, surface pressure, and average airtemperature were checked against NCEP data, andlittle difference was observed.

Energy Balance

The energy balance method for estimating evapo-ration assumes that heat inputs from net radiationare balanced by latent heat loss and sensible heattransfer. Equations for our energy balance calcula-tions are discussed extensively in Yin and Nichol-son (1998) and will not be repeated here. Briefly,top of atmosphere solar radiation calculated for 0°latitude is modified by cloud cover and lake albedobefore entering the lake as incoming radiation. Thenet longwave flux from the lake is determined as afunction of lake temperature, humidity, cloud cover,and water emissivity. The difference between thesetwo terms is the net radiation income to the lake.Calculated radiation income in Lake Edward variesfrom 140 to 190 W/m2.

The ratio of the energy loss from conduction tothat from evaporation is referred to as the Bowenratio, which compares humidity differences in airwith a saturated lake surface:

B = (Ca*(TL – Ta))/(L*(e0 – ea)) (2)

where Ca is the specific heat of dry air, TL is thelake surface temperature, Ta is surface air tempera-ture, L is the latent heat of vaporization, e0 is thesaturation vapor pressure, and ea is the measured airvapor pressure. Monthly Bowen ratio values forLake Edward vary between 0.1 and 0.16. Solutionof the Bowen ratio allows for the calculation ofevaporation by converting latent heat loss to evapo-rated water using the latent heat of evaporation atmeasured lake temperatures.

Values calculated for monthly evaporation varyfrom 0.131 to 0.191 m/month (Table 5). Our esti-mate of annual evaporation exceeds previous esti-mates for Lake Edward, but is similar to Penmanand water-balance-based estimates for Lake George(Viner and Smith 1973).

Penman Evaporation

The Penman (1948) approach has been used innumerous studies (Winter et al. 1995, Turner et al.1996, Yin and Nicholson 1998). Here we rely on a

form of the equation discussed in Jensen (1974)that has windspeed coefficients modified for use inlarge lakes:

Evap = {(s/(s + ∆))*(Qn – Qx) + (∆/(∆ + s))[(15.36*(0.5 + 0.01U))*(e0 – ea)]} / L (3)

where s is a parameter determined from the slope ofthe saturated vapor pressure-temperature curve atthe mean air temperature, ∆ is the psychrometricconstant, Qn is net solar radiation, Qx is change inheat stored in the water body, U is windspeed at 2m height above the water body, e0 is saturatedvapor pressure, ea is the measured vapor pressure atair temperature and humidity, and L is the latentheat of vaporization.

Both Penman and energy-balance derived evapo-ration estimates exceed previous estimates of evap-oration rates from Lake Edward (Table 5) (Hurst1927, Lehman 2002). Hurst’s estimate is basedupon extrapolation of evaporation estimates fromLake Victoria to Lake Edward; however, subse-quent estimates have shown that evaporation ratesfor Lake Victoria exceed Hurst’s estimate by atleast 30% (Yin and Nicholson 1998). Lehman(2002) estimated evaporation using mass transfercalculations, and used diel temperature variationsfor Lake Edward calculated from his physicalmodel of the lake. While this approach should yieldbetter estimates of evaporation than our calcula-tions above, the diurnal temperature fluctuations ofLake Edward are not known. Moreover, evapora-

TABLE 5. Monthly evaporation estimates forLake Edward calculated from using both Penmanand energy balance methods.

EnergyPenman, Balance,

Month m/month m/month

Jan 0.1695 0.1609Feb 0.1836 0.1795Mar 0.1788 0.1564Apr 0.1718 0.1812May 0.1886 0.1913Jun 0.1726 0.1701Jul 0.1496 0.1689Aug 0.1642 0.1380Sep 0.1865 0.1727Oct 0.2004 0.1770Nov 0.1921 0.1313Dec 0.1796 0.1555Annual 2.1373 1.9828


tion calculations using mass transfer equations areproblematic when shore-based climatic data areused (Winter et al. 1995). Penman and energy bal-ance formulations are less problematic in this re-gard due to the prominence of the radiation terms inthose equations.

Monthly Penman estimates for evaporation ratesexceed most previously published values (Table 5),but are again in rough agreement with both our en-ergy budget calculation and Penman estimates forLake George (Viner and Smith 1973). Due to thepotential problems with Penman-based estimationof evaporation (Winter et al. 1995, Nicholson andYin 1998) the energy-budget derived estimate isused in water balance calculations below.

Outflow

The final term in our hydrologic budget for LakeEdward is outflow via the Semliki River. Annual-ized discharge estimates for the Semliki varywidely (Table 6), ranging from 3.3 × 109 m3/yr(Worthington 1932) to 10.8 × 109 m3/yr (Lehman2002). Comprehensive surveys of the Semliki Riverwere made by the World Meteorological Organiza-tion’s Hydromet survey program at the Semliki’sentrance to Lake Albert (Said 1993, Hydromet1982). Between the two lakes, the Semliki drainsroughly an area of about 7,000 km2, including theextremely wet western side of the RuwenzoriMountains. Thus, although Hydromet measure-ments cannot directly tell us of the Semliki’s dis-

charge from Lake Edward, they do provide upperlimits for the amount of water that exits Lake Ed-ward assuming that water losses by evaporationfrom the Semliki River are at least balanced bywater inputs from the catchment between the twolakes.

Assuming that our hydrologic estimates for directprecipitation, river inflows, and evaporation arecorrect, solution of equation 1 provides an estimateof Semliki River discharge of 3.9 × 109 m3/yr, simi-lar to those of Hurst (1927) and Worthington(1932). If the additional drainage received from theSemliki catchment (assuming inputs of 0.3 m/m2/yr,similar to the Edward catchment, from 7,000 km2),is subtracted from Hydromet (1982) gauge mea-surements, the Semliki discharge from Lake Ed-ward is about 3.7 × 109 m3/yr, very similar to ourestimate of 3.9 × 109 m3/yr based upon Lake Ed-ward’s water balance.

Stable Isotope Hydrology of Lake Edward

Numerous authors have used stable isotopic andhydrologic measurements of lakes to constrain less-easily measured components of lake’s hydrologicbudgets (see Gat 1995). Although a lack of compre-hensive data for the Edward catchment precludes adetailed discussion of the lake’s isotope hydrology,stable isotope data nevertheless provide importantconstraints on Lake Edward’s water budget.

Assuming groundwater is a negligible hydrologic

TABLE 6. Estimates of the annual rate of Semliki outflow. Sources are given atleft.

Flow Rate Annualized flowAuthor Site and Date (m3/sec) (109 m3/yr)

William Garstain, reported in Hurst, 1927 L. Edward, dry season, 1903 97 NA

Hurst, 1925 L. Albert, Mar 1924 175 NA

Hurst, 1925 L. Albert, Apr 1923 90 NA

Hurst, 1927 L. Edward, estimated NA 5.0

Worthington, 1932 L. Edward, dry season, 1930 104 3.3

Damas, 1937 L. Edward 65 NA

Hydromet, 1982, reported in Said, 1993 L. Albert, measured 1956–60 NA 3.8

Hydromet, 1982reported in Said, 1993 L. Albert, measured 1962–70 NA 5.9

Lehman, 2002 L. Edward NA 10.8


input and output, the isotopic mass balance of alake can be described with the following equation:

dVδlake/dt = Qrainδrain + Qinflowsδinflows –Qoutflowδlake – Qevapδevap (4)

where V is the volume of the lake, dt is the time pe-riod of interest, Q represents hydrologic fluxes, δrepresents the isotopic composition of a given vari-able, and the isotopic composition of a lake’s out-flow is assumed to be identical to that of the lakewater. Applying this equation to Lake Edward, andassuming steady state conditions (current dV/dtequals zero), this equation can be expressed as:

Qrainδrain + QKazingaδKazinga + Qother inflowsδother inflows =QSemlikiδlake + Qevapδevap. (5)

Assuming that the inflow fluxes are relatively wellconstrained, this equation can be rearranged to

solve for the ratio of water losses by outflow rela-tive to evaporation:

Qrainδrain + QKazingaδKazinga + Qother inflowsδother inflows =QSemlikiδlake + (1 – QSemliki)δevap. (6)

The isotopic composition of Lake Edward was mea-sured in 1996, 2001, and 2002, and displays littlevariation, with an average of 4.3 ‰ for δ18O and 30‰ for δD (Table 7). Wet and dry season measure-ments of Lake George in 2002 and 2003 also showlittle variation, while the Kazinga Channel variedslightly and averaged about 0.5 ‰ for δ18O and 10‰ for δD. River samples include wet and dry sea-son measurements in 2002 and 2003 from all themajor tributaries from the eastern side of Lake Ed-ward, several rivers draining the Ruwenzoris, andspringwater samples from near the eastern borderfault. Together, these samples cover 65% of LakeEdward’s catchment area, and average –2.2 ‰ forδ18O and –2.8 ‰ for δD. It should be noted that

TABLE 7. Results of stable isotopic analysis (δδ 18O, δδ D) of lakes, rivers, and springsfrom the Lake Edward catchment sampled in 1996–2003.

Sample Date δ18O, SMOW δD,SMOW

Lake Edward surface (5 m depth) May-96 4.3 29Lake Edward hypolimnion (45 m depth) May-96 4.5 31Lake Edward surface (1 m depth) Jan-01 4.2 29Lake Edward surface (1 m depth) Jan-02 4.2 30Lake George surface (0.5 m depth) Jan-02 1 14Lake George surface (0.5 m depth) May-03 1.0 10Kazinga Channel Jan-01 0.6 11Kazinga Channel May-03 0.3 8

Nyamugasani River, East tributary Jan-02 –2.7 –4Nyamugasani River, West Tributary Jan-02 –2.2 –1Lubilia River Jan-02 –2 0Bwera River (Lubilia Tributary) Jan-02 –1.4 2Mubuku River (Lake George inflow) May-03 –4.4 –16Nyamweru River Jan-02 –1 5Nyamweru River May-03 –2.8 –7Ishasha River Jan-02 –1.7 1Ishasha River May-03 –2.9 –9Ntungwe River Jan-02 –1.1 3Ntungwe River May-03 –2.7 –8Nchwera River Jan-02 –1.1 3Nchwera River May-03 –2.4 –6Maramagambo forest spring, “Blue Pool” Jun-03 –2.1 –2Maramagambo forest unnamed spring Jul-03 –2.2 –3Rain, Ft. Portal 4 Jan 2002 2.2 33Rain, Ft. Portal 1 Jan 2002 2.1 28Rain, Ft. Portal 29 Dec 2001 –1.6 3Rain, Ft. Portal 17 May 2003 –3.1 5


this average does not include the Kazinga Channel,which is strongly affected by evaporation of watersimpounded within Lake George.

The isotopic composition of rainfall in the regionis poorly constrained. Four rainfall measurementstaken in the Lake Edward basin in 2001–2003 aver-age –0.4 ‰ for δ18O and 17.25 ‰ for δD but ex-hibit considerable scatter. The nearest rainfallstation to Lake Edward, at Entebbe, Uganda, has amean weighted composition of –2.91 ‰ for δ18Oand –11.2 ‰ for δD (Rozanski et al. 1996), but islikely influenced by water evaporated from LakeVictoria. Moreover, evapotranspired moisturewithin the Congo River basin may bring isotopi-cally heavy rainfall from the west into the Edwardregion (Rozanski et al. 1993), thereby further dis-tancing the isotopic composition of rainfall nearLake Edward from Entebbe.

Lake Edward, Lake George, river, and springsamples are plotted in δ18O vs. δD space togetherwith the global meteoric water line (GMWL, δD =8 * δ18O + 10) of Craig (1961) and the African me-teoric water line (AMWL, δD = 7.4 * δ18O + 10.1)(Fig. 4). The latter was adopted by Cohen et al.(1997), who showed that stations in the interior ofEast and Central Africa define a δD vs. δ18O trendthat differs from the GMWL due to the extremecontinentality of rainfall in interior Africa. The va-lidity of the AMWL for Lake Edward is confirmedby the fact that rivers from the Edward basin ploton or closer to the AMWL than the GMWL (Fig.4). Following the reasoning of Craig (1961), the in-tersection of the line linking Lake Edward to in-

flowing rivers yields the mean isotopic compositionof Lake Edward’s source waters. Solution of theseequations gives –0.91 ‰ for δ18O and 3.36 ‰ forδD.

These values are somewhat heavier than the aver-age values of rivers draining the northern and east-ern catchments of Lake Edward. Moreover, if weassume that the mean isotopic composition of riverssampled within the Edward basin equals that ofrainfall, we can estimate the weighted isotopic com-position of inputs to Lake Edward (the KazingaChannel, river inputs, and rainfall) to be –1.56 ‰for δ18O and 0.1 ‰ for δD. The assumption that theisotopic composition of rivers is not strongly al-tered by evaporation, and therefore can be used toestimate the composition of rainfall, is supported bythe position of those rivers on or near the AMWLand GMWL in Figure 4. Were the rivers stronglyaffected by evaporation, we would expect them toplot off the meteoric water lines along the regionalevaporative trend defined by Lake Edward (Craig1961). The differences between these compositionalestimates of the source waters for Lake Edwardsuggest an unmeasured heavy isotopic source-waterto Lake Edward, likely related to moisture from theCongo basin from the unsampled catchments to thesouth and west of Lake Edward (Rozanski et al.1993). At present there is no objective method fordetermining the precise isotopic composition ofsource waters to Lake Edward, so we assume thiscomposition is intermediate between our weightedcomposition and the composition calculated usingthe AMWL: –1.24 ‰ for δ18O and 1.73 ‰ for δD.We note that this estimate is conservative in that itis isotopically heavy relative to our measured val-ues. Isotopically lighter input values will result inhigher estimates of the importance of evaporation,calculated below.

The isotopic composition of evaporated watervapor from Lake Edward, δevap, has not been mea-sured. However, it can be calculated using the fol-lowing equation from Benson and White (1994)that describes the isotopic equilibration of lake-de-rived evaporated water with regional humidityacross a turbulent mixed layer:

δevap/1000 = {[(Rlake/ev) – hfRair] / [((1 – h)/k) + h(1 – f)]} – 1 (7)

where Rlake = 1 + δlake/1000 and Rair = 1 + δair/1000. In this equation, ev is the equilibrium enrich-ment factor that depends on lake temperature (ev =

FIG. 4. δD vs. δ18O for rivers, springs, and lakessampled within the Lake Edward catchment.


exp(1137TL–2 – 0.4156TL

–1 – 2.0667 × 10–3, Ma-joube 1971), h is relative humidity of the region, fis the fraction of humidity that has been advectedinto the basin, δair is the isotopic composition ofmoisture advected into the basin, and k is the ki-netic fractionation factor that depends on windspeed and equals 0.994 for wind speeds less than6.8 m/s (Merlivat and Jouzel 1979). We used an av-erage relative humidity of 74%, and the annual av-erage lake temperature data of Verbeke (1957) forLake Edward to calculate ev (Table 4). δair is as-sumed to be in isotopic equilibrium with regionalrainfall at surface air temperatures, and regionalrainfall is assumed to have the same average iso-topic composition as rivers in the region (Friedmanet al. 1962, Benson and White 1994).

The calculation of δevap is very sensitive to com-binations of f and h (Benson and White 1994). De-creasing humidity causes isotopically lighter valuesof δevap due to faster exchange across the mixedlayer near the lake surface. The value of f for LakeEdward is unknown, and will depend on factorssuch as regional climate, humidity, and winds aswell as basin morphology. f can vary between 0 and1, but is likely low in large lakes such as Lake Ed-ward (e.g., Ricketts and Johnson 1996, Benson andWhite 1994). By substituting equation 6 into equa-tion 5 and varying f, we can calculate a range of

possible values for the percentage of the water in-come to Lake Edward lost by evaporation (Fig. 5).Using the same suite of regional input variables,humidity and wind speed data from Kasese, and hy-drologic and isotope variables from Viner andSmith (1973) and measured in the present study, weperformed the same calculation for the oxygen iso-tope balance of Lake George. The latter calculationallows us to estimate the validity of our results forLake Edward, as the hydrological fluxes for LakeGeorge are reasonably well-known (Viner andSmith 1973).

Viner and Smith (1973) show that Lake Georgeloses 21% of its water income by evaporation,while solution of equations 5 and 6 for LakeGeorge estimate evaporative losses of 22 to 25% ofwater income as f varies from 0 to 0.7. Our esti-mates are thus remarkably similar to measured val-ues given the uncertainty in our estimates of theisotopic composition of rainfall in the region. Ap-plying these equations to Lake Edward, calculationsof the percentage of the net water income that islost from Lake Edward by evaporation differ forδ18O and δD by an average of 12%. It seems likelythat this is due to errors in calculating the composi-tion of source water to the lake. Regardless, it is ap-parent that, at a minimum, evaporation represents40% of the net water output from Lake Edward.Unfortunately, the value of f cannot be known withcertainty for Lake Edward. However, at values of f< 0.4, which seem likely for a lake the size of Ed-ward, and with δ18O calculations using values set atmean variables listed in Table 4, the most likelyevaporative loss is between 50 and 60% of thewater income.

DISCUSSION AND RECOMMENDATIONS

The East African Great Lakes comprise an im-portant economic resource for riparian countries.Despite their importance, considerable uncertaintyremains with regards to the Great Lakes’ physicalhydrologies, including that of Lake Edward. Withinthe present study, surface runoff, outflow, evapora-tion, and the isotopic composition of water incometo Lake Edward remain poorly constrained. More-over, it should be noted that we have averaged hy-droclimatic data from the Lake Edward regionacross several decades, introducing potential errorsinto our estimates that we cannot quantify. Never-theless, some preliminary conclusions may bedrawn, and we hope that this work will spur future

FIG. 5. Isotopic simulations of water loss byevaporation as a function of f (fraction ofadvected moisture over the lake) calculated for δδ Dand δ18O. Error bars represent the range of varia-tion when the net source composition is allowed tovary between the values calculated by meanweighting of hydrologic inputs and by the inter-section of the AMWL with the evaporative trenddefined by Lake Edward.


research into the physical hydrology of this impor-tant lake.

Our hydrologic estimates for the water budget ofLake Edward based upon literature review suggeststhat evaporation comprises about 54% of the waterlosses from Edward (Table 8). This seems reason-able in light of the results of our isotopic analysesthat constrain the ratio of evaporation/total waterlosses to between 0.5 and 0.6. Our revised hydro-logic estimates for Lake Edward suggest that evap-oration is much more important to water losses thanprevious researchers have indicated (e.g., Hurst1927, Lehman 2002). Based upon our analysis, itappears that previous analysts may have overesti-mated the magnitude of river inputs to Lake Ed-ward and thereby annual discharge from theSemliki River. Indeed, comparing the hydrologicestimate of this study to Lehman (2002) highlightsthe importance of obtaining accurate runoff esti-mate from the Lake Edward basin: The higher sur-face runoff values used by Lehman (2002) yield85% higher water inputs to the lake than the presentstudy.

Our results have important implications for themodern-day chemistry of Lake Edward, and the po-tential for developing paleohydrologic records fromLake Edward. Lake Edward waters are slightlybrackish (0.7 ppt TDS) with a chemistry dominatedby Na+, Mg2+, K+, and HCO3

-. Kilham and Hecky(1973) attributed this chemistry to the influence ofthe alkaline, ultramafic rocks of the Virunga volca-noes. Unfortunately, there are almost no chemicaldata from the rivers draining the Virunga regioninto Lake Edward, severely limiting our ability todevelop hydrochemical mass balance models ofLake Edward. Lehman (2002) produced the firstchemical model for Lake Edward, and balanced thelake’s bicarbonate budget using Hurst’s (1927)measurement of the alkalinity of the Ruchuru Riverof 17.2 meq. This alkalinity is more than twice thatof Lake Edward’s; however, Hurst (1927) also

states that reagents for measuring chemical analyseswere made from local natural waters, potentiallycorrupting the alkalinity data.

Marlier (1951) measured the conductivity of theRuchuru River at 408.7 µS/cm, a value much toosmall to allow an alkalinity of 17.2 meq/l. Otherrivers and lakes in the region with conductivitiesranging from 300–600 µS/cm have alkalinities be-tween 2.1 and 6.8 meq/l, while Lake Edward has aconductivity of ~880 µS/cm and an alkalinity of ~9meq/l (e.g., Damas 1954, Talling and Talling 1965).In sum, our hydrologic estimates imply that LakeEdward’s salinity is significantly concentrated rela-tive to its inputs; we estimate a concentration factorof ~2 for conservative solutes. Furthermore, our es-timates imply that Lake Edward’s salinity andchemistry should be particularly sensitive tochanges in the hydrologic balance and concomitantchanges in salinity concentration factors, particu-larly changes in rainfall as suggested by Lehman(2002).

Considerable ambiguities about the hydrology ofLake Edward remain and will not be resolved with-out additional measurements of the lake’s physicalproperties. While some of these, such as rainfall,lake temperature, and cloudiness, may be most ef-fectively monitored using remote sensing tech-niques (e.g., Lehman 2002), others, such as surfacerunoff and evaporation, will require additional fieldmeasurements using river gauges and lake-basedmeteorological buoys. Both Lehman (2002) andthis study highlight cloudiness, humidity, diurnaltemperature, and unmeasured runoff from thesouthern rivers as key variables needed to clarifyour understanding of Lake Edward. These variablesremain unmeasured, and must be quantified to fur-ther our knowledge of both the modern and paleo-limnology of this Great Lake.

CONCLUSIONS

Calculations based on stable isotopic and hy-drometeorological data provide similar estimatesfor Lake Edward’s water budget. These data indi-cate that Lake Edward loses between 50 and 60% ofits water income by evaporation from the lake sur-face. Hydrologic inputs to the lake are dominatedby river inputs from the catchment. Thus, althoughLake Edward loses significantly more of its waterincome to outflow than other East African Riftlakes, the large evaporative flux from Lake Edwardshould make the lake’s water level and chemistryhighly sensitive to hydroclimatic variations.

TABLE 8. Our calculated summary water budgetfor Lake Edward based upon previous surveys andstable isotope mass balance calculations.

Direct Precipitation 2.04 × 109 m3/yrKazinga Channel Discharge 1.7 × 109 m3/yrOther catchment inputs 4.75 × 109 m3/yrEvaporation 4.61 × 109 m3/yrSemliki River Outflow 3.88 × 109 m3/yrWater Residence Time 20 years


ACKNOWLEDGMENTS

We wish to thank the Government of Uganda,and in particular the Ugandan National Council ofScience and Technology and Ugandan Wildlife Au-thority for permission to conduct field work. DirkVerschuren, Hilde Eggermont, Kristina R. M. Beun-ing, and the International Decade for East AfricanLakes program are also acknowledged for assis-tance with field work. Sharon Nicholson and JohnT. Lehman provided very helpful reviews of an ear-lier version of this manuscript. This research wassupported by NSF Earth System History programgrant # 0314832. Any opinions, findings and con-clusions expressed in this material are those of theauthors and do not necessarily reflect the views ofthe National Science Foundation.

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Submitted: 7 March 2005Accepted: 24 November 2005Editorial handling: William M. Schertzer

the water balance and stable isotope hydrology of lake edward,...

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