Evapotranspiration Under Changing Climate

Abstract

This chapter described methodology to calculate evapotranspiration (ET) values similar to the values calculated with Penman–Monteith equation (P–M), using ET values calculated by Hargreaves–Samani equation (H–S) under current and climate change. The BISm model was used to calculate monthly values of ET using P–M and H–S equations using weather data averaged over 10 years, from 2004 to 2013 for each of the 17 studied governorates and the values were compared. The comparison showed that there were deviations between monthly ET values calculated for each equation in each governorate. Thus, a linear regression equation was established with ET values resulted from P–M plotted as the dependent variable and ET values from H–S equation plotted as the independent variable. The quality of the fit between the two methodologies was presented in terms of the coefficient of determination (R²) and root mean square error per observation (RMSE/obs). ECHAM5 climate change model was used to develop A1B climate change scenario for each governorate for the years 2020, 2030 and 2040, where ET values were calculated. The results indicated that R² was between close to one and RMSE/obs values were close to zero. The results also indicated that the calibration coefficients were capable to account for the effect of relative humidity, wind speed and potential sunshine hours, which were not included in the H–S equation. Furthermore, under A1B climate change scenario, the values of ET were increased. The above methodology could solve a large problem that faces researchers and extension workers in irrigation scheduling in Egypt and in other developing countries under current climate and in calculation of water requirements under climate change.