Status of crop water use efficiency evaluation methods: A review

Due to the impacts of climate change and human activities on scarcity and uneven spatial distribution of water resources, water use efficiency (WUE) has gained significant attention. Currently, there is a lack of research on estimating WUE with higher accuracy and stronger mechanism on the large regional scale. This study systematically reviewed the status and hotspots from an amount of studies and analyzed various concepts and estimation methods of WUE at different scales. Additionally, we discussed the limitations and challenges of these studies to point out the future opportunities. WUE at different scales could not be directly scale transferred as WUE varied at various scales. Recently, the remote sensing gross primary productivity (GPP) and evapotranspiration (ET) products, the remote sensing crop distribution recognition models, remote sensing ET and yield estimation models have improved the accuracy of WUE estimation at the regional scale, but lack of mechanical explanation. However, data assimilation provided a stronger mechanistic interpretation of WUE estimation at the regional scale by coupling remote sensing data and crop models. Optimized data assimilation method for coupling multi-source remote sensing fusion platform, unmanned aerial vehicle (UAV) platform and crop model would be a better approach to quantify WUE of the large area in the future. It is also necessary to incorporate crop types and irrigation allocations into high spatial resolution crop-specific WUE modeling. Therefore, integrating multi-source remote sensing data, ensemble using multi-crop models, innovating near-real-time seamless efficient data assimilation algorithms at the regional scale, and refining multi-variables assimilation at different crop growth stages are the research hotspots in the future. This study will provide useful guidance for optimizing water resource management in precision agriculture and improving the efficient utilization of water resources.