Estimating net irrigation requirement of winter wheat using model- and satellite-based single and basal crop coefficients

Abstract Evapotranspiration (ET) is one of the key parameters in water and energy balance equation. According to FAO 56, crop evapotranspiration (ET c ) is calculated from multiplying reference evapotranspiration (ET 0 ) by crop coefficient (K c ). But, due to excessive simplification of K c curve in the FAO approach, potential evapotranspiration (ET p ) would be miscalculated. Therefore, accurate estimates of ET p entail improving K c estimates. In this study, K c curves of early- and late-planted winter wheat were obtained based on two main satellite-based methods: (1) ratio approach (2) vegetation indices (VIs) approach. In the ratio approach, basal crop coefficient (K cb ) and single crop coefficient (K c ) was directly calculated from the ratio of potential transpiration (T p ) to ET 0 (using SWAP) and ET p to ET 0 (using SWAP and the Priestly-Taylor equation), respectively. The VI approach makes use of Landsat 7 (ETM+) and 8 (OLI) and also MODIS imagery in order to extract soil adjusted vegetation index (SAVI). The K cb curves were evaluated against field measured leaf area index (LAI) in 2014-15 growing season. After each K c curve was modeled, net irrigation requirement (NIR) was calculated on daily and season basis. Results showed that the SWAP approach was weak in estimating the K cb and K c curves especially at the late-season stage. The VI approach could properly detect changes in vegetation cover during an entire growing season. But, when it came to K c curve modelling, the VI approach was limited to the values given in FAO 56. However, the Priestly-Taylor approach compensated for this limitation therefore yielded more sensible trends in K c curves. Results indicated that the VI approach reduced estimates of NIR of late-planted winter wheat compared with the FAO-recommended approach by 5.37%. The Priestly-Taylor approach resulted 21.72 and 0.32% lower NIR compared with the FAO-recommended approach respectively for early- and late-planted winter wheat. The decrease in NIR from satellite-based approaches derived from more realistic K c curves during the entire growing season. Overall, making use of the satellite-based approaches could improve water management on regional scales.