From the reference surface defined as “a hypothetical reference grass with an assumed crop height of 0.12 m, a fixed surface resistance of 70s m−1 and an albedo of 0.23”, the daily reference evapotranspiration (ET0) is estimated by the Penman-Montieth (P-M) combination method, which is recommended by the FAO as a standard to calculate ET0 wherever the required input data are available and is proven to have good performance for various climatic conditions worldwide9. The P-M equation can be expressed as (Allen et al.)62: