Legesse, Dagnachew (PhD)Hailu, Fekadu2018-07-042023-11-092018-07-042023-11-092008-02http://10.90.10.223:4000/handle/123456789/6193The focus of this study is to estimate the daily actual evapotranspiration and assess its spatio-temporal variability in Abaya-Chamo basin, southern section of the Main Ethiopian rift valley using remote sensing techniques. The basin covers an area of 18,600 sq.km and comprises Abaya and Chamo lakes, and number of rivers and streams entering and leaving the lakes. The daily actual evapotranspiration of the basin was estimated and thematically mapped on a pixel-by-pixel basis by applying Surface Energy Balance Algorithm for Land (SEBAL) model on MODIS/Terra images acquired in six days between January and December 2006. SEBAL is an image processing model that uses spatially distributed spectral remote sensing data (visible, near infrared and thermal infrared) to derive radiation, heat and evaporation fluxes. Wind speed and sunshine hour from standard metrological stations were used in modeling. The Evaporative fraction in the land surface exhibited similar regional distribution patterns to the evaporation rate in the basin. In dry seasons, the average evaporative fraction did not exceed 0.4, highlighting the dominance of sensible heat flux across the basin, and the corresponding rise in observed surface temperatures, whereas in wet seasons the EF reached a maximum value of 0.78. The result of this study generally demonstrates that SEBAL could be used to provide information on evaporative loss and moisture condition of the water shade/catchment/basin during modeling processes because image record parameters integrated over a large area. The spatial distribution pattern of actual evapotranspiration is significantly affected by soil moisture content and land use/cover pattern in the basin. In the rift, escarpment and highland, maximum daily evapotranspiration from land surface was recorded in the wet autumn days followed by spring and summer days. In winter days, the rift floor is usually dry and higher evapotranspiration values are limited to swamps/marshy areas. Unlike the rift, relatively high mean daily ETa values in the warm winter days are noted on the highlands. Comparative assessments of the result of this study with the results obtained from river discharge measurements are in good agreement, the difference being less than 10%. But Thornthwaite type soil water balances over estimate ETa by 17% to 37% from mean SEBAL results. This may be due to fracture systems that facilitate water loss in the area in which they do not have well representation in the model. The spatially distributed SEBAL ETa can be used to set water budgets for hydrological modeling, to support water planning, to estimate aquifer depletion and to support ground water modeling. These can be most promising study area in the basin in the future. The integration of remote sensing techniques and distributed hydrological modeling can produce better results. Key words: actual evapotranspiration, lake Abaya-Chamo, MODIS/Terra, SEBALenActual evapotranspirationlake Abaya-ChamoMODIS/TerraSEBALThesis