Satellite-based global-scale irrigation water use and its contemporary trends

Irrigated agriculture is important for satisfying the increasing world food demand, but it interrupts the natural hydrological cycle by applying additional water to crops. Accurate information regarding irrigation water use (IWU) is a prerequisite to exploit land surface modeling and to quantify the uncertainty of climate projections. In this study, the global IWU was estimated for 2000-2015 by combining satellite-based soil moisture (SM) observations from the European Space Agency's Climate Change Initiative (ESA CCI) and the model estimated SM from European ReAnalysis-5 (ERA5). Considering irrigation as an unmodeled hydrological process in ERA5 and the ability of ESA CCI SM to capture the irrigation patterns, the global IWU was estimated by solving the water balance equations for ESA CCI and ERA5 SM. Owing to the global absence of ground-truth data for IWU, the IWU estimates were compared with the reported irrigation water withdrawals (IWWs) provided by Food and Agriculture Organization. The results indicated that satellite-based SM observations have the potential to identify global irrigation. All three ESA CCI products (active, passive, and merged) discerned the global irrigated areas satisfactorily, and the estimated IWU captured the pattern of the country-level IWWs (R = 0.77, 0.72, and 0.81 for active, passive, and merged products, respectively). However, the estimated IWU substantially underestimated the reported IWWs (bias of -76.55, -76.01, and -73.93 km³ for active, passive, and merged products, respectively) due to the coarse spatial resolution (0.25° × 0.25°) of the microwave remote sensing products and the inclusion of supplemental water in the IWWs, which was lost during distribution to crops. Trend analysis of the IWU indicated an increasing trend of the IWU in the first decade of the 21st century. However, in recent years the trend has reversed due to advances in agriculture technology and the adoption of water-efficient irrigation methods.