Estimation of a catchment scale evapotranspiration using water balance equation over Lake Kyoga catchment

Abstract

This study uses the water balance equation to calculate catchment scale evapotranspiration (ET) with observations of recipitation, runoff, and terrestrial water storage changes (TWSC) for the period 2001-2020. Two ET estimates are generated 1) ETWB estimated considering terrestrial water storage change and ETPQ obtained by subtracting discharge from precipitation. Data used in this study include; TWSC sourced from the Global Land Data Assimilation System (GLDAS), runoff from the Modern-Era Retrospective Analysis for Research and Application Version 2 (MERRA2) and precipitation from the Climate Hazards Infrared Precipitation with Station data version 2 (CHIRPS2). Results show that TWSC fluctuates between negative and positive values ranging from 122.6mm to -116.2mm and discharge has a good consistence with the precipitation fluctuations. The negative values of annual ETWB and ETPQ and monthly ET in the months of November and December are associated to the uncertainties in discharge and TWSC data. The mean annual variations of ETWB and ETPQ values ranging from -524.7mm to 157.5mm and - 456.8mm to 126.1mm respectively. The impact of ignoring TWSC in calculating ET is notable, as the large deviation of the monthly mean of ETWB and ETPQ reaches 44.0 mm/month in January and small deviation of 0.2 mm/month in June and July. The spatial distribution of ET indicates that there is an increase more as you move to the north portion of the Basin