Mapping crop evapotranspiration by integrating vegetation indices into a soil water balance model

• Significant relationships are reported in literature between the basal crop coefficient and vegetation indices.
• This paper deals with the integration of the FAO-56 “dual” Kc approach with satellite-derived VIs.
• The outcomes of the study were validated using independent measurements of ET by an eddy covariance system and in situ soil water monitoring.
• ET estimates and soil water depletion in the crop root zone were quite satisfactory simulated by the proposed model.
• The proposed approach may have a great potential as low-cost and prompt tool for irrigation water demand scheduling.

The approach combines the basal crop coefficient (Kcb) derived from vegetation indices (VIs) with the daily soil water balance, as proposed in the FAO-56 paper, to estimate daily crop evapotranspiration (ETc) rates of orange trees. The reliability of the approach to detect water stress was also assessed. VIs were simultaneously retrieved from Worldview 2 imagery and hyper-spectral data collected in the field for comparison. ETc estimated were analysed at the light of independent measurements of the same fluxes by an eddy covariance (EC) system located in the study area. The soil water depletion in the root zone of the crop simulated by the model was also validated by using an in situ soil water monitoring. Average overestimate of daily ETc of 6% was obtained from the proposed approach with respect to EC measurements, evidencing a quite satisfactory agreement between data. The model also detected several periods of light stress for the crop under study, corresponding to an increase of the root zone water deficit matching quite well the in situ soil water monitoring. The overall outcomes of this study showed that the FAO-56 approach with remote sensing-derived basal crop coefficient can have the potential to be applied for estimating crop water requirements and enhancing water management strategies in agricultural contexts.