Global effect of irrigation and its impact on the onset of the Indian summer monsoon

In a context of increased demand for food and of climate change, the water consumptions associated with the agricultural practice of irrigation focuses attention. In order to analyze the global influence of irrigation on thewater cycle, the land surface model ORCHIDEE is coupled to the GCM LMDZ to simulate the impact of irrigation onclimate. A 30-year simulation which takes into account irrigation is compared with a simulation which does not.Differences are usually not significant on average over all land surfaces but hydrological variables are significantlyaffected by irrigation over some of the main irrigated river basins. Significant impacts over the Mississippi river basinare shown to be contrasted between eastern and western regions. An increase in summer precipitation is simulatedover the arid western region in association with enhanced evapotranspiration whereas a decrease in precipitationoccurs over the wet eastern part of the basin. Over the Indian peninsula where irrigation is high during winter andspring, a delay of 6 days is found for the mean monsoon onset date when irrigation is activated, leading to a significantdecrease in precipitation during May to July. Moreover, the higher decrease occurs in June when the water requirements by crops are maximum, exacerbating water scarcity in this region. A significant cooling of theland surfaces occurs during the period of high irrigation leading to a decrease of the land-sea heat contrast in June,which delays the monsoon onset