Estimation of agricultural flood irrigation water consumption in the Heihe River Basin, China, using satellite based daily land surface evapotranspiration and soil moisture

Study region: The Heihe River Basin, China. Study focus: Irrigation data are often from census surveys at coarse administrative or river basin scale, and as such, the amount of water used for agricultural irrigation difficult to quantify. We improve the Soil Moisture to Rain (SM2RAIN) method to estimate irrigation water use in the Heihe River Basin from 2003 to 2020 using thermal infrared and microwave satellite data. The results showed that this approach has satisfactory performance in estimating the annual irrigation water volume (mean volume=0.657 km3/year, R2=0.83, RMSE=0.03 km3/year) when compared with the field measurements at irrigation district administrative scale, due to its reliability in determining the infiltrated water around the root zone used by crops. New hydrological insights for the region: Through an analysis of irrigation water use trends, the results indicate that most farmland areas exhibited a declining trend in water use per hectare (-55 m³/ha/yr). Interestingly, we observed that while water use efficiency improved significantly at the field scale, overall irrigation efficiency showed a decreasing trend. This study reveals a paradox in the Heihe River Basin, where enhanced irrigation efficiency rarely translates into reduced total water consumption at river basin scale. Our study advances agricultural irrigation volume estimation and irrigation mapping across district and river basin scales in arid and semi-arid areas, which should assist in irrigation scheduling and water resource management.