Agricultural irrigation‐induced climatic effects: a case study in the middle and southern Loess Plateau area, China

Irrigation and rain-fed agriculture, the main agricultural practices in the middle and southern Loess Plateau area, impact the regional climate in different ways. Based on 38 meteorological stations in the middle and southern Loess Plateau area, the effects of agricultural irrigation on climatic variables from 1960 to 2005 were evaluated. Climatic trends from non-irrigated areas were used as a reference. Trends in climatic variables for the growing season and the summer period are negatively correlated with the percentage of cultivated land within a 3 km radius of the meteorological stations. Trends for the average summer temperature (T-a), maximum temperature (T-max), minimum temperature (T-min) and the diurnal temperature range (DTR) decreased by 0.065, 0.073, 0.054 and 0.025 degrees C per decade, respectively. Summer net solar radiation (SR) decreased by 0.0325 MJ m(-2) day(-1) per decade and summer relative humidity (RH) increased by 0.255% per decade with a 10% increase in the area of cultivated land. The climatic effects on meteorological stations in irrigated areas are compared to the trends of non-irrigated areas in the study region. The expansion of irrigation has had a noticeable cooling effect on summer T-a, T-max, T-min and DTR with values of 0.192, 0.193, 0.142 and 0.058 degrees C (10 years)(-1), respectively. Irrigation increased RH by 0.916% per decade and net radiation reduced by 0.230 MJ m(-2) day(-1) per decade. On monthly scales, these effects were more obvious in June and August. The dependence of climatic trends on the proportion of cultivated land can be attributed to agricultural irrigation. The changes in climatic differences for T-a, T-max, T-min, RH and SR corresponded with the expansion of irrigation. The cooling effects induced by irrigation have also partly masked the effects of greenhouse gas-induced warming.