Surface Energy Balance Algorithm for Land (SEBAL) based Evapotranspiration Estimation in Lower Gilgel Abay Catchment Lake Tana Sub-Basin, Ethiopia

Abstract

On land, Evapotranspiration (ET) plays an important role in the water cycle and is an important parameter in water resources management. Remote sensing is one of the important sources of data and techniques to estimate many climate elements including evapotranspiration. The estimation based on remote sensing is vital for the management of water resources in the catchment. This study estimated the spatio-temporal variation of evapotranspiration in the lower Gilgel Abay catchment, Lake Tana sub-basin from January to March 2016. The evapotranspiration was quantified using the Surface Energy Balance (SEBAL) algorithm and Landsat 8 imagery with climate data. For this analysis, ASTER GDEM, GRASS-python & reference weather parameters from Bahir Dar weather station were used. Parameters including surface radiance, surface reflectance, surface albedo, NDVI, LAI, surface emissivity, surface temperature, net radiation, soil heat flux, sensible heat flux, and latent heat flux were computed. Consequently, the hourly, daily, monthly and seasonal evapotranspiration in the study area were calculated with SEBAL python. The pixel wise calculation shows that the values of the spatial variation of mean ET varied from 0 mm/day to 7.39 mm/day with a mean value of 4.78 mm/day for 23 December 2016. The computed ET values for the months December to March, the maximum estimated ET over the whole catchment ranged from 6.51 mm/day to 7.82 mm/day. The mean ET ranged from 4.37 mm/day to 4.78 mm/day while the seasonal ET was 539.92 mm for 2016. ET values were computed for different conventional methods using REF-ET software. The value of Standard P-M for the weather station was used as a reference to compare the values obtained from other conventional methods as well as SEBAL method. The mean value of the study area from SEBAL calculation approached the point values of CIMIS penman, standard P-M and Priestly Taylor methods. The analyses are vital from the perspective of water resources management on various surfaces of the earth that need to be understood to achieve sustainable development of water resources in the basin and recommended to apply in the remaining sub-basins in the region.
Keywords: Evapotranspiration, Gilgel Abay, GRASS-GIS, Landsat-8, RS, SEBAL.