Estimating Actual Evapotranspiration over a Large and Complex Irrigation System of the Nile Delta in Egypt

Abstract

In the Nile Delta region of Egypt, measurements of Actual Evapotranspiration (ETa) and Potential Evapotranspiration (ETp) are difficult, expensive and labor-intensive. The current paper aimed at finding the superior method for estimating the ETa and ETa/ETp in the Nile Delta governorates by comparing different methods. Three different methods were used for ETa estimates being; Remote Sensing approach by Moderate Resolution Imaging Spectroradiometer (MODIS) ETa-product (MOD16A2), FAO (33) method, and Irrigation Water Balance Calculation (IWBC) method. The three ETa methods were applied for five governorates in 2017, where the data for IWBC were available. However, only MOD16A2 was compared with FAO (33) for ETa/ETp ratio and the two selected methods were applied for all eight Nile Delta governorates for the period of 2008 -2017. The MOD16A2 product was derived from the MODIS satellite images using an improved evapotranspiration algorithm based on the Penman-Monteith equation. FAO (33) was based on the relationship between the relative yield loss of any crop to relative reduction of water consumption. The IWBC required estimation of both; the field application and conveyance water losses as the only unknown elements of irrigation water balance in Delta governorates. For comparison between applied methods, descriptive statistics, analysis of variance (ANOVA) for checking difference, and cluster analysis were applied. The results showed a significant difference values between MOD16A2 and FAO (33) for estimating ETa/ETp ratios. However, the difference for ETa estimation was insignificant between the three methods indicating a significant relationship, with a strong correlation between MOD16A2 and IWBC. It was observed that ETa values were impacted by the cropping pattern, since they were very close in governorates having the same dominant crops. In conclusion, both MOD16A2 and IWBC can be utilized for ETa estimation. Both MOD16A2 and FAO (33) are not confirmed for estimation of ETa/ETp ratio due to the significant difference between both results. FAO (33) cannot be utilized for both ETa and ETa/ETp ratio estimates. Further data collection and investigation on ETa and ETp estimates methods are recommended.