Monitoring evapotranspiration of irrigated crops using crop coefficients derived from time series of satellite images. I. Method validation

Crop coefficients (the ratio of actual and reference evapotranspiration, ET/ETo) can be derived from vegetation indices (VIs) obtained by remote sensing. If ground meteorological stations are available to calculate ETo, then the FAO method for estimating the ET of actual crops may be applied in combination with series of satellite images if it is assumed that the crops are not water stressed. This approach was evaluated for two annual crops (cotton and garlic) and three tree crops (mandarin, olive, and peach) using measurements of evapotranspiration, made with the eddy covariance method, as ground truth. Thirty images acquired by the Landsat 5 TM and Landsat 7 ETM+ sensors were used to calculate VIs and derive crop coefficients. The assessment (based on 557 data pairs) led to an overall positive valuation of the method. The root-mean-square deviation over all estimates was 0.75mmday−1. It was concluded that the VI-ETo method is valid and robust for estimating spatially distributed evapotranspiration in large, irrigated areas. Weaknesses of the method were identified and new research to overcome these deficiencies is proposed.