Estimating evapotranspiration using SEBS and SEBAL modelling techniques: a case study of South Phuthiatsana River Basin in Lesotho

Evapotranspiration (ET) is one of the critical components of the hydrological cycle; it therefore has to be estimated accurately in order to achieve sustainability of water resources. It is also essential to quantify ET at basin level since it constitutes a major loss to the water balance. In this study, remote sensing data was used to estimate actual evapotranspiration (AET) since it overcomes point measurements due to its ability to provide representative measurements of several relevant physical parameters at extensive areas as opposed to point. The objective of this research is to estimate spatial and temporal distribution of actual ET and quantify actual ET in South Phuthiatsana River Basin using meteorological data and remote sensing. This was carried out using Moderate-resolution Imaging Spectroradiometer (MODIS) and Landsat 7 ETM images. Both Surface Energy Balance System (SEBS) and Surface Energy Balance Algorithm for Land (SEBAL) models were applied in estimating spatial and temporal distribution of actual ET within the basin. For SEBS model ILWIS for SEBS was used and ERDAS Model Maker Tool was used for SEBAL model. The results were validated by comparing with potential evapotranspiration (ETo) since there were no ET ground observations.The results indicated that both SEBS and SEBAL can be used to estimate ET in the basin. The RSME was 1.28 mm/day, 0.54 mm/day, 1.72 mm/day 1.56 mm/day for SEBS using MODIS, SEBS using Landsat, SEBAL using Landsat and SEBAL using MODIS data sets respectively. Furthermore, ET results followed a seasonal trend with higher ET during months of November to January due to more vegetation cover, high rainfall and high temperatures.