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

Zheng Duan,Hongkai Gao,Changqing Ke

doi:10.3390/rs10071060

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.

Highlights

The water stored in lakes and reservoirs plays a vital role in the economic development and many services that contribute to the well-being of communities up to kilometers downstream and in close proximity to lakes and reservoirs [1]
Based on the lake water balance, the lake outflow (Qo) was estimated as: Qo = Qi + Alake(Plake − Elake) + Groundwater Inflow (Gi)-Go − dVlake/dt where Qi is the inflow into the lake from the surrounding catchments; Precipitation over the Lake (Plake) is the precipitation directly over the lake area; Elake is the lake evaporation; dVlake is the change in water storage over the time period dt; Alake is the surface area of the lake, which varies with water level; and Gi and Go are the groundwater inflow and outflow, respectively
When taking together findings from the aforementioned three most relevant studies [2,28,29] and our current study, we could conclude that estimating the monthly outflow from lakes and reservoirs as a residual of the water balance suffers from large uncertainties, which highlights that more efforts are needed to improve the qualification of each individual water balance component either from in-situ measurements, satellite remote sensing, or ideally a combination of both

Summary

Introduction

The water stored in lakes and reservoirs plays a vital role in the economic development and many services that contribute to the well-being of communities up to kilometers downstream (even across administrative borders) and in close proximity to lakes and reservoirs [1]. To achieve the quantification of the water balance of lakes and reservoirs, relevant data on each component (inflow, precipitation, water level/volume, and outflow, etc.) are required, ideally from ground measurements. Such required data are often not readily available because of, for example, the following situations: (1) variables are not measured at all, which is the case in ungauged basins and inaccessible regions; or (2) measured data are not freely shared. The surface outflow from lakes and reservoirs, a measure of water supply, is a particular variable of high importance for downstream multiple sectors. Poor communications and information cause distrust and tension between multiple water use sectors

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