A review of remote sensing estimation of crop water productivity: definition, methodology, scale, and evaluation

The scarcity of water resources is one of the biggest problems restricting food production. Crop water productivity (CWP) is a practical and quantifiable indicator for characterizing agricultural water use efficiency. Remote sensing technology provides an accurate, cost-effective, and regional method for estimating CWP. However, remote sensing methods for CWP estimation and their application scenarios need to be summarized. In this paper, the CWP and related parameters are clearly defined. Different types of CWP estimation methods and their application at different scales are reviewed. CWP, as the crop yield ratio to actual crop evapotranspiration (ETa), is typically not directly estimated but calculated by estimating crop yield and ETa. Therefore, crop yield and ETa estimation methods are summarized, respectively. ETa can be remotely sensed using surface energy balance residual methods, semi-empirical formula methods, statistical regression methods, and ground instruments. Crop yield can be remotely estimated using data assimilation, statistical regression, and ground instruments. Moreover, the application of these methods at the point, field, and regional scales is further reviewed from previous literature. Finally, the in-situ measurements of CWP are introduced. This review can provide a detailed reference for follow-up studies related to CWP.