SURFACE ENERGY BALANCE ALGORITHM FOR LAND (SEBAL) BASED EVAPOTRANSPIRATION ESTIMATION IN LOWER GILGEL ABAY CATCHMENT TANA SUB-BASIN, ETHIOPIA

Abstract

On land, Evapotranspiration (ET) is a combination of evaporation from the soil surface and transpiration from vegetation and it is one of the climate elements that plays an important role in water cycle and an important parameter in water resources management. As a result, many mathematical equations and algorithms have been developed and designed to compute evapotranspiration by different authors. Estimating from conventional methods of ET, which uses meteorological data, cannot estimate at a larger scale rather at certain location. To overcome this problem, remote sensing are one of the important sources of data and techniques to estimate many climate elements including evapotranspiration. The study area was located in Gilgel Abay catchment Tana sub basin in North West part of Ethiopia. The main objective of this study was estimation of evapotranspiration using SEBAL algorism and Landsat 8 imagery with climate data. In this study, SEBAL based remote sensing technique was applied to estimate spatially as well as temporal distributed evapotranspiration. For this analysis, ASTER GDEM, GRASS-python file & reference weather parameters from Bahir Dar weather station was used. The SEBAL python file was used to calculate the values of all parameters including surface radiance, surface reflectance, surface albedo, NDVI, LAI, surface emissivity, surface temperature, net radiation, soil heat flux, sensible heat flux, latent heat flux, which are consequently used to calculate the hourly and daily evapotranspiration in study area. Based on the analysis the results obtained from pixel wise calculation shows the values of the spatial variation of mean ET for different land cover ranged from 4.42 mm/day for closed grassland to 6.25 mm/day for dense forest areas. While the daily ET varied from 0 mm/day to 7.39 mm/day for the study area and its mean, value was 4.78 mm/day for 23 December 2016. For monthly ET calculation selected months were used and results show the maximum estimated actual evapotranspiration over the whole catchment ranged from 6.51 mm/day (Jan) to 7.82 mm/day (Mar). The mean actual evapotranspiration ranged from 4.37 mm/day (Feb) to 4.78 mm/day (Dec). The seasonal ET also was analyzed using the same approach as daily from Landsat 8 data for selected months and results showed with the mean value of 539.92 mm in 2016.