Application of time-lagged satellite image-based crop coefficients for estimating actual evapotranspiration through FAO-56 method

Correct estimation of actual evapotranspiration (AET) at the farm and watershed levels is an important step towards making proper water resources management decisions. However, accessing reliable AET values remains a major challenge, especially at the watershed level. The present work examined the possibility of using time-lagged crop coefficients (Kc) extracted from MODIS satellite images (April to October 2009) for estimation of daily AET rates in the corresponding months of 2019 by the FAO-56 method in Neishaboor watershed, Iran. In order to evaluate the results, AET values calculated with the SEBAL (surface energy balance algorithm for land) algorithm in 2019 were used as benchmarks. According to the results, average AET estimation error in the 10 studied subbasins ranged between 0.53 and 1.47 mm.day− 1. Mean error rates in the mentioned months and for [0–2), [2–4), [4–6) and ≥ 6 mm intervals were 0.71, 0.68, 0.73 and 0.64 mm.day− 1, respectively. Comparison of the results and review of the literature showed that when the current AET values are not available, using time-lagged (in this study, 10 years ago) satellite image-based Kc values in the form of FAO-56 method will be useful in making proper management decisions at the watershed level. By the proposed method, compared to the other surface energy balance algorithms, AET estimation is facilitated through reducing the computational steps. Accuracy of the results, however, is highly dependent on land use changes over the pertinent time period, which should be carefully monitored in each region.