Estimation of Lake Outflow from the Poorly Gauged Lake Tana (Ethiopia) Using Satellite Remote Sensing Data

Abstract

Lake Tana is the largest lake in Ethiopia, and its lake outflow is the source of the Blue Nile River that provides vital water resources for many livelihoods and downstream/international stakeholders. Therefore, it is essential to quantify and monitor the water balance of Lake Tana. However, Lake Tana is poorly gauged, with more than 50% of Lake Tana Basin being ungauged from in-situ measurements, making it difficult to quantify the lake inflow from surrounding basins. The lack of in-situ measurements highlights the need for the innovative application of satellite remote sensing. This study explores how freely accessible satellite remote sensing can be used to complement routine weather data to quantify the water balance of Lake Tana and its surrounding catchments. This study particularly investigates whether the outflow from Lake Tana can be estimated with sufficient accuracy as the residual of the lake water balance. Monthly inflow into lake was computed as the total runoff from the surrounding catchments; the runoff was estimated as the residual of the land-based catchment water balance using satellite precipitation improved with an integrated downscaling-calibration procedure, satellite evapotranspiration, and a correction term for changes in land total storage (soil moisture storage and deep percolation). The outflow from Lake Tana was estimated as the residual of lake water balance by combining satellite-based lake precipitation, changes in water storage, and lake inflow with estimated lake evaporation. Evaluation using limited available measurements showed that estimated annual runoff for two gauged subbasins agreed well with measurements, with differences within 4%. The estimated annual outflow from Lake Tana was also close to measured outflow, with a difference of 12%. However, the estimated monthly runoff from catchments and monthly lake outflow were unsatisfactory, with large errors.

Water Balance for a Tropical Lake in the Volcanic Highlands: Lake Tana, Ethiopia

“…Annual evaporation of 1650 mm a −1 was selected [8] that was calculated with the modified Penman-Monteith method [53] using data from the Ethiopian National Meteorological Agency. The evaporation rate of [8] was in the same order as reported by other studies [23,24,28,30] ( Table 1). The rate computed by the surface energy balance system (SEBS) algorithm of 1637 mm a −1 using Moderate Resolution Imaging Spectroradiometer (MODIS) land surface products and some ancillary meteorological data was similar as well [54].…”

“…Since including rain gauges in and near the lake improves the accuracy of the amount of rain falling on the lake, the rainfall based on a combination of gauges near the lake and the land (i.e., lake + land-based rainfall) is a better representation of the lake rainfall compared to the only land-based stations. Most current studies on lake evaporation based on a full energy balance of the lake with satellite data [27,30,54] agree that Penman understates the lake evaporation and reported evaporation rates ranging from 1563-1688 mm a −1 (Table 1). Thus, water balances with evaporation rates of around 1650 mm a −1 and lake rainfall validated by stations that are either on the Island of the lake or close to the shore are likely the most accurate Figure 3 for the location for the locations of the stations.…”

“…As can be seen from Figure 3, Bahir Dar has the greatest rainfall from all land-based stations. The highest lake rainfall of 1688 mm a −1 (Table 1 [31]) was determined with the Tropical Rainfall Measuring Mission (TRMM) 3B43 dataset [30]. The TRMM 3B43 dataset has been shown to perform poorly in the Lake Tana basin [57] Monthly precipitation for Lake Tana for six land-based weather stations and eight "lake + land"-based weather stations consisting of the Jigrfa station on the east shore and a station on Dek island plus six land-based stations.…”

“…In the same year, Sutcliffe, et al [20] confirmed that the only streamflow data that were available for the Blue Nile basin before 1964 were for the outflow of Lake Tana. Subsequent water balances of Lake Tana were made by Kebede, et al [7] for the period 1960-1992; Aragie [21] for the period 1964-2000; Pelgrum, et al [22] for the year 2001; SMEC [8] for 1960 to 1995; Gieske, et al [23] for 1992-2003; Wale, et al [24] for 1992-2003; Chebud, et al [25] for 1993-2003; Setegn, et al [26] for the years 1978 to 2004; Rientjes, et al [27] for 1994-2003; Tegegne, et al [28] for 1996-2002; Nigatu [29] for 1994-2004; Duan, et al [30] for the year 2006; Dessie, et al [31] for 2012 and 2013; Kebede, et al [32] for 2002 and 2006; and finally Mamo, et al [33] for 1995-2009 (Table 1). Dessie et al [31] used two scenarios: scenario 1 used observed river discharges for most of the rivers and in scenario 2 some of the water balance terms were adjusted to decrease the unaccounted water losses.…”

The Application of Satellite Imagery in Surface Water/Lake Modelling: A Review of Previous Studies on Lake Tana and Its Basin

“…For analyzing lake Tana and its basin many types of data were employed to assess the physical, climatologically, hydrological and environmental phenomena of the lake and its basin, such as monthly rainfall and precipitation data e.g. [28], weather and soil data e.g. [74], and surveying and GPS data e.g.…”