Water Balance COngo

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

The Water Balance and Stable Isotope Hydrology of Lake Edward, Uganda-Congo James M. Russell1,* and Thomas C. Johnson2 1Department

of Geological Sciences Brown University Box 1846 Providence, Rhode Island 02912 2Large

Lakes Observatory University of Minnesota Duluth 10 University Drive, RLB Duluth, 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 published estimates of Lake Edward’s water balance are used to constrain the physical hydrology of the lake, and particularly the relative proportion of surface outflow to evaporative water losses. Stable isotope calculations suggest that Lake Edward loses roughly 50% of its water income by evaporation, while reviews of published hydrologic data together with our calculations suggest that evaporation comprises 54% of water losses. The similarity of these two sets of calculations lends credence to their validity, and provides a new water budget for the lake. Our results have important implications for the chemistry and hydroclimatic sensitivity of Lake Edward. INDEX WORDS:

Lake Edward, East Africa, rift lake, stable isotope, hydrology.

INTRODUCTION The Great Lakes of East Africa are rich sources of information about past variations of the African monsoons. The potential for these lakes to record past variations in monsoon intensity is partly due to their hydrologic sensitivity that is driven by hydrologic budgets for the lakes in which water losses are dominated by evaporation (Spigel and Coulter 1996). Lake Edward, located on the equator at the border between Uganda and the Democratic Republic of the Congo, has received perhaps the least attention of the East African Great Lakes, despite paleoclimatic studies that have revealed a rich and varied paleoclimatic history for the lake (e.g., Russell et al. 2003, Laerdal et al. 2002). A thorough understanding of the modern hydrology of Lake Edward is critical to interpreting paleoclimate data using Lake Edward’s sedimentary record; however, estimates of Lake Edward’s hydrologic budget are few and often contradictory (Lehman 2002). *Corresponding

This paper seeks to refine calculations of the modern hydrologic balance of Lake Edward using past measurements and stable isotope data. First, we summarize previous estimates and measurements of Lake Edward’s hydrology. We then present new stable isotopic data for the lake and watershed that help to constrain the water balance of the lake. Our data and literature review suggest that evaporation and outflow each account for roughly 50% of water loss from Lake Edward, estimates that are similar to some previous studies and help us to better understand the lake’s physical and chemical structure. Background: Regional Geology and Hydrology Lake Edward (0°–0°40′S, 29° 20′–29° 50′E, 912 m a.s.l.) is situated in a Cenozoic half-graben in the Western Arm of the East African Rift Valley (Fig. 1). The lake is presently open, draining northward to Lake Albert via the Semliki River. Lake Edward

author. E-mail: [email protected]

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FIG. 1. Map of the East African Great Lakes region. Dashed lines in the left-hand figure indicate the position of the eastern and western arms of the rift. Shaded gray regions indicate major water bodies. A close-up at right of the equatorial lakes region shows political boundaries (dashed lines) bisecting Lake Edward. is bounded by the Lubero border fault to the west and the Kigezi highlands to the east, the Ruwenzori mountains to the north, and the Virunga volcanoes to the south (Fig. 2). These four regions, together with the Lake George catchment to the northeast, comprise five major catchment areas that provide runoff 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 Edward rise from the rift floor to heights of over 5,000 m and are currently glaciated. Principal inflows from the Ruwenzoris to Lake Edward are from the Nyamugasani and Lubilia rivers (Fig. 2), while considerable additional inflow from the Ruwenzoris is delivered to Lake Edward via Lake George. Mountains to the west of Lake Edward along the Lubero border fault rise steeply from the lake to heights of 2,500 to 3,000 meters within 15 km of the lake shore, and are drained by numerous short, steep rivers. The Kigezi highlands to the east rise more gently to form a low divide between Lakes Edward and Victoria. Principal inflows from the Kigezi highlands to Lake Edward are from the Ishasha, Ntungwe, Nchwera, and Nyamweru rivers. The Virunga volcanoes to the south divide Lakes Edward and Kivu and are very important to the hydrology and chemistry of Lake Edward (Kilham and Hecky 1973, Lehman 2002). Principal inflows

to Lake Edward from the Virunga region are the Lula, Rwindi, and Ruchuru rivers. Lake Edward: Bathymetry, Morphology, Limnology, and Climatology Lake Edward has a surface area of 2,325 km2 and maximum depth of 117 meters, located within 5 km of the western shore (Fig. 3; Lehman 2002). Lake Edward has an oxycline commonly found at about 40 m depth and is thought to be oligomictic (Beadle 1981, Hecky and Degens 1973). Although stratified, the temperature difference between surface and deep waters is only about 1°C, with an average annual surface temperature of about 26.5°C (Verbeke 1957, Beadle 1966). Chemically, Lake Edward is a Na-Mg-HCO3 system with a salinity of approximately 0.8 g/L and a pH averaging 8.9 (Talling and Talling 1965). Rainfall in the Lake Edward region falls in two rainy seasons coinciding with the passing of the Intertropical Convergence Zone, from October to December and March to May (Viner and Smith 1973, Nicholson 1996). Rainfall in the region, and throughout East Africa, varies strongly with altitude (Viner and Smith 1973, Nicholson 1996). Thus, the highlands surrounding Lake Edward receive considerably more rainfall than the lake itself, which experiences a more arid climate than much of the region (Hurst 1927, Viner and Smith 1973).

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FIG. 2. Map of the Lake Edward region showing catchment areas, major rivers, high elevation areas, swamps, and water sampling sites. Previous Work and Data Sources The first estimates of the hydrologic budget of Lake Edward were made by Hurst (1925, 1927) as part of a survey of the Nile River headwaters. Hurst’s work contains single-sample river gauge data and runoff estimates for several rivers in Lake Edward’s catchment as well as Lake Edward’s outflow. Viner and Smith (1973) provided a hydrologic

budget for Lake George based upon 5 years of detailed hydrologic and climatic monitoring. Their data include daily to monthly river gauge data, the only such data available for the Edward basin. Data from these authors, supplemented by other estimates of Lake Edward’s hydrologic and limnologic characteristics (Worthington 1932, Damas 1937, Verbeke 1957, Hydromet 1982) form the basis for

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Russell and Johnson TABLE 1. Morphometric and Catchment information for Lakes Edward and George, UgandaCongo. Lake Edward Surface Elevation Surface area Volume Max Depth Catchment Area (less lake) Ruwenzori Catchment Area Western Escarpment Area Eastern Rivers Southern Rivers (including Ishasha)

FIG. 3. Bathymetric Map of Lake Edward, with depth contours in meters. The position of crater lakes within the basin are also shown.

our study. Lehman (2002) combined hydroclimatic data with his own hydrologic calculations and an energy balance model into the first physical, hydrologic, and chemical model of Lake Edward. Lehman’s calculations suggest Lake Edward’s outflow exceeds annual water losses by evaporation by a factor of nearly five, an estimate that differs considerably from previous researchers. Bathymetric and morphometric measurements for Lake Edward were calculated by Laerdal (2000) and Lehman (2002) (Table 1). Catchment areas for Lakes Edward and George were measured from Defense Mapping Association maps L-4, L-5, and M-5 (Fig. 2). Our estimate for the catchment area of Lakes Edward and George differs slightly from previous studies (Lehman 2002). Based upon the DMA maps, it appears that Lehman (2002) underestimated the catchments of the Ntungwe and Rusangwe rivers by about 90% and 116%, respectively. We are uncertain as to why these discrepancies exist, but we note that our revised catchment for the Rusangwe River matches that of Viner and Smith (1973), who explored the area extensively. Our revised catchment area increases the proportion of low-elevation areas to the east that drain into Lakes Edward and George, which could affect surface runoff into the lake. The isotopic composition of Lake Edward, inflowing rivers and springs, and occasional rainfall samples were sampled and analyzed between 1996 and 2003. 20-mL samples from rivers were taken at

Lake George Surface area Catchment

912 m a.s.l. 2,325 km2 767 × 108 m3 117 m 15,840 km2 1,231 km2 1,136 km2 5,680 km2 7,793 km2

250 km2 9,976 km2

road crossings within 15 km of Lake Edward’s shore, and lake waters were sampled from open water at least 5 km from shore. Water samples were collected and stored in high-density polyethylene vials prior to analysis. Analyses were conducted on a Finnegan Delta S mass spectrometer at the University of Arizona; results are expressed in delta notation with respect to the SMOW standard. Analytical error was 0.1 ‰ for δ18O and 1.0 ‰ for δD. The Hydrology and Water Balance of Lake Edward The fundamental equation for the hydrology of Lake Edward assumes the lake is in a steady state with respect to its volume: Evaporation + Outflow = Direct precipitation + Catchment inputs

(1)

Previous estimates of the magnitudes of each of these terms will be discussed below. Direct Precipitation Hulme (1998) estimates precipitation in the Lake Edward region at 1.217 m/yr, similar to the estimates of Lehman (2002) of 1.214 m/yr, as well as the 1.1 m/yr estimated by Hurst (1927). However, the highland regions surrounding Lake Edward receive far more rainfall than the lowlands and the lake itself (Viner and Smith 1973). The estimates above are based upon weighted averages of rainfall stations in all of southwestern Uganda, including

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TABLE 2. River inputs to Lakes Edward and George from Hurst (1927), Viner and Smith (1973), and Lehman (2002). River Ruchuru, dry season Ishasha, dry season Ntungwe, dry season Nyamugasani Sebwe (George catchment) Rukoki/Kamulikwezi (George) Mubuku (George) Ruimi (George) Mpanga (George) Kyambura (Kazinga Channel inflow) George basin (Viner and Smith, 1973) Edward basin, Lehman (2002) Edward basin, Hurst (1927)

River Flow (m3/sec) 40.000 8.000 7.000 8.330 2.040 4.100 12.500 6.000 11.500 9.500 61.800 280.000 141.000

several stations in the highlands surrounding Lake Edward, and therefore probably overestimate direct precipitation to the lake’s surface (Nicholson 1996). Viner and Smith estimate direct precipitation onto the surface of Lake George averages 0.82 m/yr. Rainfall stations nearest to the surface elevation of Lake Edward, Kasese and Kabale, receive 0.87 and 0.99 m/yr, respectively (National Climate Data Center archive). We have averaged these three values and estimate that direct precipitation to Lake Edward is 0.9 m/yr. Catchment Inputs Catchment inputs include river inputs, surface runoff, and groundwater inputs. Groundwater, although it may be important to the chemical balance of Lake Edward, is assumed to be negligible in the hydrologic budget (Lehman 2002). Catchment inputs comprise the largest source of water to Lake Edward (Lehman 2002), yet they are by far the most difficult to estimate due to a nearly complete lack of river gauge data from the Lake Edward basin. The available catchment-normalized surface runoff data from Lake George demonstrate the heterogeneity of the region’s hydrology (Table 2; Viner and Smith 1973). All of the rivers draining into Lake George measured by Viner and Smith (1973), except the Mpanga and Kyambura, drain the Ruwenzori Mountains and have very high surface runoff rates, ranging from 0.514 to 1.54 m/yr. However, when the less steep, low-elevation east-

Catchment (km2)

Runoff (m/yr)

Annual Input (10 9 m 3 /yr)

507 83 183 256 266 4,670 660

0.514 0.777 0.707 1.540 0.711 0.080 0.450

0.260 0.060 0.129 0.394 0.660 0.374 0.297

9,976 15,840 15,840

0.196 0.514 0.280

1.948 8.850 4.435

ern part of Lake George’s drainage basin is taken into account, the average runoff for the entire George basin is only about 0.2 m/yr. This is likely due to the steeper elevation gradients of the Ruwenzoris, which yield higher runoff, as well as higher average annual rainfall at higher elevations within the lake’s catchments. The only river that flows into Lake Edward that has annual gauge data is the Nyamugasani River, which drains the Ruwenzori Mountains (Viner and Smith 1973). Lehman (2002) applied the runoff derived from the Nyamugasani catchment, 0.514 m/yr, to the entire Lake Edward basin, and calculated inputs to the lake totaling 8.85 × 109 m3/yr. Based on the example of Lake George it seems likely that this is an overestimate, given that the slope, climate, and bedrock geology of the Ruwenzori mountains is prone to high runoff as compared to the Lake Edward catchment as a whole. In point of fact, the slope of the Nyamugasani River is about 6% over the river’s catchment, while the average slope of the rivers draining into Lake Edward from the east is only 1.5%. The average slope of rivers draining into Lake Edward from the south is 3%, while rivers draining from the west have slopes equal to, or higher than, the Nyamugasani River. If we assume that rivers draining from the Ruwenzoris and the western mountains into Lake Edward have surface-area normalized runoff yields equal to the Nyamugasani River, that rivers draining the eastern slope provide runoff equal to that of the Mpanga River, and that the southern rivers provide runoff intermediate between these two areas, we

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Russell and Johnson TABLE 3. Evaporation Estimates for Lakes Edward and George, from Hurst (1927), Viner and Smith (1973), Lehman (2002), and Penman and energy balance calculations of this study. Author Lehman (2002) Hurst (1927) Viner and Smith (1973) This Study This Study

Annual Method Rate, m/yr Mass Transfer 1.16 comparison to Lake Victoria 1.20 Penman 1.83 Energy Balance 1.98 Penman 2.10

calculate an average runoff for the Edward catchment of 0.25 m/yr, very similar to the value of 0.28 suggested by Hurst (1927). Hurst’s value is intermediate between that of Lehman (2002) for Lake Edward and Viner and Smith (1973) for Lake George, and seems reasonable given that the Lake Edward catchment contains a slightly higher proportion of steeply sloping terrain than the Lake George catchment. Therefore, we assign a runoff value equivalent of Hurst’s estimate of 0.28 m/yr, or 4.435 × 109 m3/yr, to catchment inputs to Lake Edward excluding inputs from Lake George. In addition to general catchment inputs, the Kazinga Channel delivers 1.70 × 109 m3/yr to Lake Edward (Viner and Smith 1973), a value determined at its exit from Lake George both by hydrologic modeling and gauge data. This represents the combined inputs of rivers and precipitation to Lake George, less evaporation from Lake George’s surface (Viner and Smith 1973).

TABLE 4.

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Water Loss, km3/yr 2.59 2.79 4.24 4.60 4.87

Outputs Surface Evaporation Published estimates for evaporation from Lake Edward vary widely (Table 3). The most common methods of estimating evaporation from a lake surface are energy balance and Penman’s (1948) method. The latter combines a formula for potential evapotranspiration with energy balance and watermass transfer. Both methods require numerous input variables, including air vapor pressure, lake temperature, cloudiness, and surface radiation. Input data for evaporation calculations includes surface pressure, dew point, cloud fraction, and wind-speed data from the Kasese weather station (Table 4), which lies between Lakes Edward and George. Lake water temperature is derived from mean monthly measurements reported in Verbeke (1957), which are slightly cooler than more recent values reported from Lehman (2002). Insolation

Meteorological input data used in evaporation calculations.

Top of Atmosphere Insolation W/m2 416.2 431.8 438.2 427.1 406.2 392.5 397.0 414.7 429.8 430.3 418.2 409.0

Surface pressure mb 903.6 903.5 903.4 904.7 905.7 904.9 905.6 905.1 905.1 904.3 903.8 904.3

Surface Air Temp °C 23.36 23.58 23.63 23.68 23.57 23.24 22.81 22.83 22.73 22.88 23.01 23.26

Dew Point °C 19.01 17.81 19.02 19.73 19.66 19.00 18.15 17.44 18.72 19.13 19.40 19.15

Cloud Fraction 0.413 0.305 0.481 0.333 0.257 0.292 0.318 0.494 0.353 0.370 0.517 0.420

Temp Lake °C 25.9 26.0 26.1 26.5 27.1 27.2 25.8 25.3 25.8 26.8 27.2 26.5

Wind-speed m/s 2.41 2.14 2.48 2.14 2.00 1.65 1.66 2.22 2.68 2.67 2.33 2.33

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

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

Energy Balance The energy balance method for estimating evaporation assumes that heat inputs from net radiation are balanced by latent heat loss and sensible heat transfer. Equations for our energy balance calculations are discussed extensively in Yin and Nicholson (1998) and will not be repeated here. Briefly, top of atmosphere solar radiation calculated for 0° latitude is modified by cloud cover and lake albedo before entering the lake as incoming radiation. The net longwave flux from the lake is determined as a function of lake temperature, humidity, cloud cover, and water emissivity. The difference between these two terms is the net radiation income to the lake. Calculated radiation income in Lake Edward varies from 140 to 190 W/m2. The ratio of the energy loss from conduction to that from evaporation is referred to as the Bowen ratio, which compares humidity differences in air with a saturated lake surface:

where s is a parameter determined from the slope of the saturated vapor pressure-temperature curve at the mean air temperature, ∆ is the psychrometric constant, Qn is net solar radiation, Qx is change in heat stored in the water body, U is windspeed at 2 m height above the water body, e 0 is saturated vapor pressure, ea is the measured vapor pressure at air temperature and humidity, and L is the latent heat of vaporization. Both Penman and energy-balance derived evaporation estimates exceed previous estimates of evaporation rates from Lake Edward (Table 5) (Hurst 1927, Lehman 2002). Hurst’s estimate is based upon extrapolation of evaporation estimates from Lake Victoria to Lake Edward; however, subsequent estimates have shown that evaporation rates for Lake Victoria exceed Hurst’s estimate by at least 30% (Yin and Nicholson 1998). Lehman (2002) estimated evaporation using mass transfer calculations, and used diel temperature variations for Lake Edward calculated from his physical model of the lake. While this approach should yield better estimates of evaporation than our calculations above, the diurnal temperature fluctuations of Lake Edward are not known. Moreover, evapora-

B = (Ca*(TL – Ta))/(L*(e0 – ea))

(2)

where Ca is the specific heat of dry air, TL is the lake surface temperature, Ta is surface air temperature, L is the latent heat of vaporization, e0 is the saturation vapor pressure, and ea is the measured air vapor pressure. Monthly Bowen ratio values for Lake Edward vary between 0.1 and 0.16. Solution of the Bowen ratio allows for the calculation of evaporation by converting latent heat loss to evaporated water using the latent heat of evaporation at measured lake temperatures. Values calculated for monthly evaporation vary from 0.131 to 0.191 m/month (Table 5). Our estimate of annual evaporation exceeds previous estimates for Lake Edward, but is similar to Penman and water-balance-based estimates for Lake George (Viner and Smith 1973). Penman Evaporation The Penman (1948) approach has been used in numerous studies (Winter et al. 1995, Turner et al. 1996, Yin and Nicholson 1998). Here we rely on a

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

(3)

TABLE 5. Monthly evaporation estimates for Lake Edward calculated from using both Penman and energy balance methods.

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

Penman, m/month 0.1695 0.1836 0.1788 0.1718 0.1886 0.1726 0.1496 0.1642 0.1865 0.2004 0.1921 0.1796 2.1373

Energy Balance, m/month 0.1609 0.1795 0.1564 0.1812 0.1913 0.1701 0.1689 0.1380 0.1727 0.1770 0.1313 0.1555 1.9828

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tion calculations using mass transfer equations are problematic when shore-based climatic data are used (Winter et al. 1995). Penman and energy balance formulations are less problematic in this regard due to the prominence of the radiation terms in those equations. Monthly Penman estimates for evaporation rates exceed most previously published values (Table 5), but are again in rough agreement with both our energy budget calculation and Penman estimates for Lake George (Viner and Smith 1973). Due to the potential problems with Penman-based estimation of evaporation (Winter et al. 1995, Nicholson and Yin 1998) the energy-budget derived estimate is used in water balance calculations below. Outflow The final term in our hydrologic budget for Lake Edward is outflow via the Semliki River. Annualized discharge estimates for the Semliki vary widely (Table 6), ranging from 3.3 × 10 9 m 3 /yr (Worthington 1932) to 10.8 × 109 m3/yr (Lehman 2002). Comprehensive surveys of the Semliki River were made by the World Meteorological Organization’s Hydromet survey program at the Semliki’s entrance to Lake Albert (Said 1993, Hydromet 1982). Between the two lakes, the Semliki drains roughly an area of about 7,000 km2, including the extremely wet western side of the Ruwenzori Mountains. Thus, although Hydromet measurements cannot directly tell us of the Semliki’s disTABLE 6. left.

charge from Lake Edward, they do provide upper limits for the amount of water that exits Lake Edward assuming that water losses by evaporation from the Semliki River are at least balanced by water inputs from the catchment between the two lakes. Assuming that our hydrologic estimates for direct precipitation, river inflows, and evaporation are correct, solution of equation 1 provides an estimate of Semliki River discharge of 3.9 × 109 m3/yr, similar to those of Hurst (1927) and Worthington (1932). If the additional drainage received from the Semliki catchment (assuming inputs of 0.3 m/m2/yr, similar to the Edward catchment, from 7,000 km2), is subtracted from Hydromet (1982) gauge measurements, the Semliki discharge from Lake Edward is about 3.7 × 109 m3/yr, very similar to our estimate of 3.9 × 109 m3/yr based upon Lake Edward’s water balance. Stable Isotope Hydrology of Lake Edward Numerous authors have used stable isotopic and hydrologic measurements of lakes to constrain lesseasily measured components of lake’s hydrologic budgets (see Gat 1995). Although a lack of comprehensive data for the Edward catchment precludes a detailed discussion of the lake’s isotope hydrology, stable isotope data nevertheless provide important constraints on Lake Edward’s water budget. Assuming groundwater is a negligible hydrologic

Estimates of the annual rate of Semliki outflow. Sources are given at

Author William Garstain, reported in Hurst, 1927 Hurst, 1925 Hurst, 1925 Hurst, 1927 Worthington, 1932 Damas, 1937 Hydromet, 1982, reported in Said, 1993 Hydromet, 1982 reported in Said, 1993 Lehman, 2002

Flow Rate (m3/sec)

Annualized flow (109 m3/yr)

L. Edward, dry season, 1903 L. Albert, Mar 1924 L. Albert, Apr 1923 L. Edward, estimated L. Edward, dry season, 1930 L. Edward

97 175 90 NA 104 65

NA NA NA 5.0 3.3 NA

L. Albert, measured 1956–60

NA

3.8

L. Albert, measured 1962–70 L. Edward

NA NA

5.9 10.8

Site and Date

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TABLE 7. Results of stable isotopic analysis (δ 18O, δ D) of lakes, rivers, and springs from the Lake Edward catchment sampled in 1996–2003. Sample Lake Edward surface (5 m depth) Lake Edward hypolimnion (45 m depth) Lake Edward surface (1 m depth) Lake Edward surface (1 m depth) Lake George surface (0.5 m depth) Lake George surface (0.5 m depth) Kazinga Channel Kazinga Channel

Date May-96 May-96 Jan-01 Jan-02 Jan-02 May-03 Jan-01 May-03

Nyamugasani River, East tributary Nyamugasani River, West Tributary Lubilia River Bwera River (Lubilia Tributary) Mubuku River (Lake George inflow) Nyamweru River Nyamweru River Ishasha River Ishasha River Ntungwe River Ntungwe River Nchwera River Nchwera River Maramagambo forest spring, “Blue Pool” Maramagambo forest unnamed spring Rain, Ft. Portal Rain, Ft. Portal Rain, Ft. Portal Rain, Ft. Portal

Jan-02 Jan-02 Jan-02 Jan-02 May-03 Jan-02 May-03 Jan-02 May-03 Jan-02 May-03 Jan-02 May-03 Jun-03 Jul-03 4 Jan 2002 1 Jan 2002 29 Dec 2001 17 May 2003

δ18O, SMOW 4.3 4.5 4.2 4.2 1 1.0 0.6 0.3

δD,SMOW 29 31 29 30 14 10 11 8

–2.7 –2.2 –2 –1.4 –4.4 –1 –2.8 –1.7 –2.9 –1.1 –2.7 –1.1 –2.4 –2.1 –2.2 2.2 2.1 –1.6 –3.1

–4 –1 0 2 –16 5 –7 1 –9 3 –8 3 –6 –2 –3 33 28 3 5

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

solve for the ratio of water losses by outflow relative to evaporation:

dVδlake/dt = Qrainδrain + Qinflowsδinflows – Qoutflowδlake – Qevapδevap

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

(4)

where V is the volume of the lake, dt is the time period of interest, Q represents hydrologic fluxes, δ represents the isotopic composition of a given variable, and the isotopic composition of a lake’s outflow is assumed to be identical to that of the lake water. Applying this equation to Lake Edward, and assuming steady state conditions (current dV/dt equals zero), this equation can be expressed as: Qrainδrain + QKazingaδKazinga + Qother inflowsδother inflows = (5) QSemlikiδlake + Qevapδevap.

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

The isotopic composition of Lake Edward was measured in 1996, 2001, and 2002, and displays little variation, with an average of 4.3 ‰ for δ18O and 30 ‰ for δD (Table 7). Wet and dry season measurements of Lake George in 2002 and 2003 also show little variation, while the Kazinga Channel varied slightly and averaged about 0.5 ‰ for δ18O and 10 ‰ for δD. River samples include wet and dry season measurements in 2002 and 2003 from all the major tributaries from the eastern side of Lake Edward, several rivers draining the Ruwenzoris, and springwater samples from near the eastern border fault. Together, these samples cover 65% of Lake Edward’s catchment area, and average –2.2 ‰ for δ18O and –2.8 ‰ for δD. It should be noted that

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

this average does not include the Kazinga Channel, which is strongly affected by evaporation of waters impounded within Lake George. The isotopic composition of rainfall in the region is poorly constrained. Four rainfall measurements taken in the Lake Edward basin in 2001–2003 average –0.4 ‰ for δ18O and 17.25 ‰ for δD but exhibit considerable scatter. The nearest rainfall station to Lake Edward, at Entebbe, Uganda, has a mean weighted composition of –2.91 ‰ for δ18O and –11.2 ‰ for δD (Rozanski et al. 1996), but is likely influenced by water evaporated from Lake Victoria. Moreover, evapotranspired moisture within the Congo River basin may bring isotopically heavy rainfall from the west into the Edward region (Rozanski et al. 1993), thereby further distancing the isotopic composition of rainfall near Lake Edward from Entebbe. Lake Edward, Lake George, river, and spring samples are plotted in δ18O vs. δD space together with the global meteoric water line (GMWL, δD = 8 * δ18O + 10) of Craig (1961) and the African meteoric 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 of East and Central Africa define a δD vs. δ18O trend that differs from the GMWL due to the extreme continentality of rainfall in interior Africa. The validity of the AMWL for Lake Edward is confirmed by the fact that rivers from the Edward basin plot on or closer to the AMWL than the GMWL (Fig. 4). Following the reasoning of Craig (1961), the intersection of the line linking Lake Edward to in-

flowing rivers yields the mean isotopic composition of Lake Edward’s source waters. Solution of these equations gives –0.91 ‰ for δ18O and 3.36 ‰ for δD. These values are somewhat heavier than the average values of rivers draining the northern and eastern catchments of Lake Edward. Moreover, if we assume that the mean isotopic composition of rivers sampled within the Edward basin equals that of rainfall, we can estimate the weighted isotopic composition of inputs to Lake Edward (the Kazinga Channel, river inputs, and rainfall) to be –1.56 ‰ for δ18O and 0.1 ‰ for δD. The assumption that the isotopic composition of rivers is not strongly altered by evaporation, and therefore can be used to estimate the composition of rainfall, is supported by the position of those rivers on or near the AMWL and GMWL in Figure 4. Were the rivers strongly affected by evaporation, we would expect them to plot off the meteoric water lines along the regional evaporative trend defined by Lake Edward (Craig 1961). The differences between these compositional estimates of the source waters for Lake Edward suggest an unmeasured heavy isotopic source-water to Lake Edward, likely related to moisture from the Congo basin from the unsampled catchments to the south and west of Lake Edward (Rozanski et al. 1993). At present there is no objective method for determining the precise isotopic composition of source waters to Lake Edward, so we assume this composition is intermediate between our weighted composition and the composition calculated using the AMWL: –1.24 ‰ for δ18O and 1.73 ‰ for δD. We note that this estimate is conservative in that it is isotopically heavy relative to our measured values. Isotopically lighter input values will result in higher estimates of the importance of evaporation, calculated below. The isotopic composition of evaporated water vapor from Lake Edward, δevap, has not been measured. However, it can be calculated using the following equation from Benson and White (1994) that describes the isotopic equilibration of lake-derived evaporated water with regional humidity across a turbulent mixed layer: δevap/1000 = {[(Rlake/ev) – hfRair] / [((1 – h)/k) + h(1 – f)]} – 1

(7)

where R lake = 1 + δ lake/1000 and R air = 1 + δ air/ 1000. In this equation, ev is the equilibrium enrichment factor that depends on lake temperature (ev =

The Hydrology of Lake Edward, Uganda

FIG. 5. Isotopic simulations of water loss by evaporation as a function of f (fraction of advected moisture over the lake) calculated for δ D and δ18O. Error bars represent the range of variation when the net source composition is allowed to vary between the values calculated by mean weighting of hydrologic inputs and by the intersection of the AMWL with the evaporative trend defined by Lake Edward. exp(1137TL–2 – 0.4156TL–1 – 2.0667 × 10–3, Majoube 1971), h is relative humidity of the region, f is the fraction of humidity that has been advected into the basin, δair is the isotopic composition of moisture advected into the basin, and k is the kinetic fractionation factor that depends on wind speed and equals 0.994 for wind speeds less than 6.8 m/s (Merlivat and Jouzel 1979). We used an average relative humidity of 74%, and the annual average lake temperature data of Verbeke (1957) for Lake Edward to calculate ev (Table 4). δair is assumed to be in isotopic equilibrium with regional rainfall at surface air temperatures, and regional rainfall is assumed to have the same average isotopic composition as rivers in the region (Friedman et al. 1962, Benson and White 1994). The calculation of δevap is very sensitive to combinations of f and h (Benson and White 1994). Decreasing humidity causes isotopically lighter values of δ evap due to faster exchange across the mixed layer near the lake surface. The value of f for Lake Edward is unknown, and will depend on factors such as regional climate, humidity, and winds as well as basin morphology. f can vary between 0 and 1, but is likely low in large lakes such as Lake Edward (e.g., Ricketts and Johnson 1996, Benson and White 1994). By substituting equation 6 into equation 5 and varying f, we can calculate a range of

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possible values for the percentage of the water income to Lake Edward lost by evaporation (Fig. 5). Using the same suite of regional input variables, humidity and wind speed data from Kasese, and hydrologic and isotope variables from Viner and Smith (1973) and measured in the present study, we performed the same calculation for the oxygen isotope balance of Lake George. The latter calculation allows us to estimate the validity of our results for Lake Edward, as the hydrological fluxes for Lake George are reasonably well-known (Viner and Smith 1973). Viner and Smith (1973) show that Lake George loses 21% of its water income by evaporation, while solution of equations 5 and 6 for Lake George estimate evaporative losses of 22 to 25% of water income as f varies from 0 to 0.7. Our estimates are thus remarkably similar to measured values given the uncertainty in our estimates of the isotopic composition of rainfall in the region. Applying these equations to Lake Edward, calculations of the percentage of the net water income that is lost from Lake Edward by evaporation differ for δ18O and δD by an average of 12%. It seems likely that this is due to errors in calculating the composition of source water to the lake. Regardless, it is apparent that, at a minimum, evaporation represents 40% of the net water output from Lake Edward. Unfortunately, the value of f cannot be known with certainty for Lake Edward. However, at values of f < 0.4, which seem likely for a lake the size of Edward, and with δ18O calculations using values set at mean variables listed in Table 4, the most likely evaporative loss is between 50 and 60% of the water income. DISCUSSION AND RECOMMENDATIONS The East African Great Lakes comprise an important economic resource for riparian countries. Despite their importance, considerable uncertainty remains with regards to the Great Lakes’ physical hydrologies, including that of Lake Edward. Within the present study, surface runoff, outflow, evaporation, and the isotopic composition of water income to Lake Edward remain poorly constrained. Moreover, it should be noted that we have averaged hydroclimatic data from the Lake Edward region across several decades, introducing potential errors into our estimates that we cannot quantify. Nevertheless, some preliminary conclusions may be drawn, and we hope that this work will spur future

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TABLE 8. Our calculated summary water budget for Lake Edward based upon previous surveys and stable isotope mass balance calculations. Direct Precipitation Kazinga Channel Discharge Other catchment inputs Evaporation Semliki River Outflow Water Residence Time

2.04 × 109 m3/yr 1.7 × 109 m3/yr 4.75 × 109 m3/yr 4.61 × 109 m3/yr 3.88 × 109 m3/yr 20 years

research into the physical hydrology of this important lake. Our hydrologic estimates for the water budget of Lake Edward based upon literature review suggests that evaporation comprises about 54% of the water losses from Edward (Table 8). This seems reasonable in light of the results of our isotopic analyses that constrain the ratio of evaporation/total water losses to between 0.5 and 0.6. Our revised hydrologic estimates for Lake Edward suggest that evaporation is much more important to water losses than previous researchers have indicated (e.g., Hurst 1927, Lehman 2002). Based upon our analysis, it appears that previous analysts may have overestimated the magnitude of river inputs to Lake Edward and thereby annual discharge from the Semliki River. Indeed, comparing the hydrologic estimate of this study to Lehman (2002) highlights the importance of obtaining accurate runoff estimate from the Lake Edward basin: The higher surface runoff values used by Lehman (2002) yield 85% higher water inputs to the lake than the present study. Our results have important implications for the modern-day chemistry of Lake Edward, and the potential for developing paleohydrologic records from Lake Edward. Lake Edward waters are slightly brackish (0.7 ppt TDS) with a chemistry dominated by Na+, Mg2+, K+, and HCO3-. Kilham and Hecky (1973) attributed this chemistry to the influence of the alkaline, ultramafic rocks of the Virunga volcanoes. Unfortunately, there are almost no chemical data from the rivers draining the Virunga region into Lake Edward, severely limiting our ability to develop hydrochemical mass balance models of Lake Edward. Lehman (2002) produced the first chemical model for Lake Edward, and balanced the lake’s bicarbonate budget using Hurst’s (1927) measurement of the alkalinity of the Ruchuru River of 17.2 meq. This alkalinity is more than twice that of Lake Edward’s; however, Hurst (1927) also

states that reagents for measuring chemical analyses were made from local natural waters, potentially corrupting the alkalinity data. Marlier (1951) measured the conductivity of the Ruchuru River at 408.7 µS/cm, a value much too small to allow an alkalinity of 17.2 meq/l. Other rivers and lakes in the region with conductivities ranging from 300–600 µS/cm have alkalinities between 2.1 and 6.8 meq/l, while Lake Edward has a conductivity of ~880 µS/cm and an alkalinity of ~9 meq/l (e.g., Damas 1954, Talling and Talling 1965). In sum, our hydrologic estimates imply that Lake Edward’s salinity is significantly concentrated relative to its inputs; we estimate a concentration factor of ~2 for conservative solutes. Furthermore, our estimates imply that Lake Edward’s salinity and chemistry should be particularly sensitive to changes in the hydrologic balance and concomitant changes in salinity concentration factors, particularly changes in rainfall as suggested by Lehman (2002). Considerable ambiguities about the hydrology of Lake Edward remain and will not be resolved without additional measurements of the lake’s physical properties. While some of these, such as rainfall, lake temperature, and cloudiness, may be most effectively monitored using remote sensing techniques (e.g., Lehman 2002), others, such as surface runoff and evaporation, will require additional field measurements using river gauges and lake-based meteorological buoys. Both Lehman (2002) and this study highlight cloudiness, humidity, diurnal temperature, and unmeasured runoff from the southern rivers as key variables needed to clarify our understanding of Lake Edward. These variables remain unmeasured, and must be quantified to further our knowledge of both the modern and paleolimnology of this Great Lake. CONCLUSIONS Calculations based on stable isotopic and hydrometeorological data provide similar estimates for Lake Edward’s water budget. These data indicate that Lake Edward loses between 50 and 60% of its water income by evaporation from the lake surface. Hydrologic inputs to the lake are dominated by river inputs from the catchment. Thus, although Lake Edward loses significantly more of its water income to outflow than other East African Rift lakes, the large evaporative flux from Lake Edward should make the lake’s water level and chemistry highly sensitive to hydroclimatic variations.

The Hydrology of Lake Edward, Uganda ACKNOWLEDGMENTS We wish to thank the Government of Uganda, and in particular the Ugandan National Council of Science and Technology and Ugandan Wildlife Authority for permission to conduct field work. Dirk Verschuren, Hilde Eggermont, Kristina R. M. Beuning, and the International Decade for East African Lakes program are also acknowledged for assistance with field work. Sharon Nicholson and John T. Lehman provided very helpful reviews of an earlier version of this manuscript. This research was supported by NSF Earth System History program grant # 0314832. Any opinions, findings and conclusions expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. REFERENCES Beadle L.C. 1966. Prolonged stratification and deoxygenation in tropical lakes I. Crater Lake Nkugute, Uganda, compared with Lakes Bunyoni and Edward. Limnol. Oceanogr. 2:152–163. ——— . 1981. The Inland Waters of Tropical Africa: An Introduction to Tropical Limnology. London: Longman. Benson L.V., and White J.W.C. 1994. Stable isotopes of oxygen and hydrogen in the Truckee River-Pyramid Lake surface-water system. Limnol. Oceanogr. 39: 1945–1958. Cohen, A.S., Talbot, M.R., Awramik, S.M., Dettman D.L., and Abell, P. 1997. Lake level and paleoenvironmental history of Lake Tanganyika, Africa, as inferred from late Holocene and modern stromatolites. GSA Bulletin 109:444–460. Craig H. 1961. Isotopic variations in meteoric waters. Science 133:1702–1703. Damas, Z.H. 1937. Recherches dans les Lacs Kivu, Édouard, et Ndalaga. Exploration du Parc National Albert. Mission H. Damas (1935–1936). Institut Royal Colonial Belge, Brussels, Belgium II (1). ——— , 1954. Étude limnologique de quelques lacs rundais, I le cadregéographique. Institut Royal Colonial Belge, Mémoires Collection Tome XXIV, fasc. 2. Brussels, Belgium. Friedman, I., Machta L., and Soller, R. 1962. Watervapor exchange between a water droplet and its environment. J. Geophys. Res. 67:2761–2766. Gat, J.R. 1995. Stable isotopes of fresh and saline lakes. In Physics and Chemistry of Lakes, 2 nd Ed., pp. 139–162. A. Lerman, D. Imboden, and J.R. Gat (eds.), Springer-Verlag, Berlin. Hecky, R.E., and Degens, E.T. 1973. Late PleistonceneHolocene chemical stratigraphy and paleolimnology of the Rift Valley Lakes of central Africa. Woods Hole

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