Estimating Spatially Explicit Irrigation Water Use Based on Remotely Sensed Evapotranspiration and Modeled Root Zone Soil Moisture

Abstract Estimation of irrigation water use (IWU) is critical to assessing water use efficiency and optimizing water resource allocation. However, diversity in irrigation scheduling, irrigation system, and crop type has imposed restrictions on deriving the accurate spatial distribution of IWU. Existing approaches of deriving IWU may have large uncertainty, due mostly to methodological limitations, inadequate inputs, and the mismatch in spatiotemporal scale between IWU estimates and in situ counterparts. Remote sensing has the potential to provide valuable information on moisture transport, which could unravel water consumption by crops. First, we developed a robust relationship between root zone soil moisture (RZSM) and the ratio of actual evapotranspiration (ET a ) to reference ET from the land components of European ReAnalysis‐5 (ERA5L) without an irrigation module across the Contiguous United States (CONUS). Second, RZSM under irrigation was derived from the developed relationship and remotely sensed ET a from the operational Simplified Surface Energy Balance (SSEB op ) model during 2000–2020. Third, IWU over the two decades was derived based on increases in RZSM and ET a due to irrigation using remotely sensed and model‐based variables, along with our derived formulas considering groundwater recharge originating from irrigation. The IWU estimates were fairly reliable compared with reported IWU from the Farm and Ranch Irrigation Survey at the state‐level in four years, with the coefficient of determination ranging from 0.74 to 0.84, root mean square error ranging from 1.90 to 2.33 km 3 /yr, and bias ranging from −0.11 to 0.15 km 3 /yr. Integration of remotely sensed ET a with ERA5L output resulted in IWU estimates of much higher accuracy than published studies. The IWU estimates across the CONUS reflected an increasing trend (∼2.71 km 3 /yr 2 ) during 2000–2007 but a decreasing trend (∼0.90 km 3 /yr 2 ) during 2008–2020, due to efficient irrigation practices and increased precipitation.