Irrigation enhances precipitation at the mountains downwind

Atmospheric circulation models predict an irrigation-rainfall feedback. However, actual field evidences for local evaporation recycling (moisture feedback) are weak. We present strong field evidence for an increase in rainfall at the mountains located downwind of irrigated zones. We chose two regions, located in semiarid southern Spain, where irrigation started at a well defined date, and we analyzed rainfall statistics before and after the beginning of irrigation. Analyzed statistics include the variation of (1) mean rainfall Δ P, (2) ratio of monthly precipitation to annual precipitation Δ r, and (3) number of months with noticeable rainfall episodes Δ Pmin after a shifting from unirrigated to irrigated conditions. All of them show statistically significant increases. Δ P and Δ r show larger and more statistically significant variations in June and July than in August. They also tend to increase with the annual volume of water applied in the neighbouring upwind irrigation lands. Increases in Δ Pmin are statistically significant during the whole summer. That is, the number of noticeable rainfall events displays a relevant increase after irrigation. In fact, it is this number, rather than sporadic large rainfall episodes what makes the summers wetter. The increase in rainfall, while statistically significant, is distributed over a broad region, so that it is of little relevance from a water resources perspective, although it may enhance vegetation yield.