Where does the irrigation water go? An estimate of the contribution of irrigation to precipitation using MERRA

Irrigation is an important human activity that may impact local and regional climate, but current climate model simulations and data assimilation systems generally do not include it. The European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim) shows more irrigation signal in surface evapotranspiration (ET) than the Modern Era Retrospective-analysis for Research and Applications (MERRA) because ERA-Interim adjusts soil moisture according to the observed surface temperature and humidity. Here, the ET increase due to irrigation is estimated with a simple method, and the result is compared with a dataset from a hydrological model that accounts for irrigation. The two methods produce ET increases with very different seasonal cycles and amplitudes, and the latter is considered to be more realistic and is used for the remaining analyses. A back-trajectory method is used to estimate the contribution of irrigation to precipitation over local and surrounding regions, using MERRA with added irrigation-caused ET increase and observation based corrections. Results show substantial contributions of irrigation to precipitation over heavily irrigated regions in Asia, but the increase in precipitation is much less than the increase in ET over most areas, indicating that irrigation could lead to water deficits over these regions. For the same increase in ET, precipitation increases are larger over wet areas, where convection is more easily triggered, but the percentage increase is similar for different areas. There are substantial regional differences in the patterns of irrigation impact, but for all the studied regions the highest percentage contribution to precipitation is over local land.