Estimation of Regional Evapotranspiration Based on Tri-Angle Method Using Thermal and VNIR Data

Abstract

Evapotranspiration is a critical component in the hydrological cycle, water resources management and climate studies especially in arid and semi-arid regions. This paper aimed at producing a simplified and applicable procedure for estimating spatial distributed daily actual evapotranspiration (ETa) directly at regional scale using thermal and visible-near infra-red (VNIR) data. The findings of this study will be useful for irrigation water management, climate change studies and water resources planning in the study region. Triangle method, which makes a parameterization of priestly-Taylor equation, was used to estimate ETa at daily scale directly by using a simplified approach with realistic hypotheses. This study conducted in Egypt, Salhia, 6th of October Company as an arid region over the winter crops (wheat, potato and sugar beet) cultivated there using multi date Landsat images. The results were compared with ETa values adjusted from crop evapotranspiration ETc “FAO Penmamn-Monteith approach” using the Crop Water Stress Index (CWSI). The results showed high accuracy and good agreement against assessment method. The correlation factor (R2) values for wheat, potato and sugar beet were 0.88, 0.98 and 0.99 and Root Mean Square Error (RMSE) were 0.2, 0.26 and 0.37 respectively over the different dates. In the 16th of April, 2014 there was a significant difference in wheat curves as the RMSE were 0.8 and we explained the reasons of this difference as it is a result of the sprinkler irrigation system effect on the mature wheat. This results show that the proposed procedure is accurate enough at least in most cases of our study for estimating the regional surface ETa but it need to evaluate for wheat under other irrigation systems like surface or drip irrigation systems .