A numerical study for climatic effects of irrigation in the Fergana basin，Central Asia

Central Asia is located in the arid climate zone and agricultural production is highly dependent on irrigation. However, the effects of irrigation on local climate remain unclear. The effects of irrigation on warm season (May—September) climate in the Fergana basin are studied using the Weather Research and Forecast model (WRF), in which an irrigation parameterization scheme is implemented and soil data are also updated. A pair of simulations, i.e., the IRRG that includes irrigation and the NATU that excludes irrigation, are carried out for the years of above normal rainfall (i.e. 2009), below normal rainfall (i.e. 2008) and normal rainfall year (i.e. 2007), respectively. The climatic effects of irrigation are then studied by comparing the IRRG and NATU simulations. The results are as follows: (1) Irrigation leads to an average increase in surface latent heat flux (79.2 W/m²) and an average decrease in surface sensible heat flux (−61.3 W/m²). As a result, daily mean temperature cools by 1.7℃ and specific humidity increases by 2 g/kg (accounting for about 36% of NATU) in the warm season. Since May to June is rainy season and July to August is dry season, the irrigation demand from July to August is larger than that from May to June. Thereby, the cooling and wetting effects of irrigation are also larger from July to August. (2) The cooling and wetting effects mainly exist in the irrigation area with cooling of 2℃ and specific humidity increment of 2.4 g/kg, while they are too weak to be detected beyond the irrigation area. Meanwhile, from the ground to the upper atmosphere, the cooling and wetting effects gradually weaken and cannot be detected above the pressure level of 500 hPa (about 4000 m above the ground). (3) In the central plain of the basin, the potential rainy effect caused by air humidity increment and the potential rainless effect caused by convective inhibition offset each other. As a result, local precipitation shows no detectable differences between the IRRG and NATU simulations. However, irrigation lead to precipitation increment (~0.6 mm/d) in remote areas over the northern and southern mountains surrounding the basin. (4) Differences in irrigation demand and their effects on local climate under different climate backgrounds mainly exist in May to June. The irrigation demand in 2008 is more than that in 2009 by 20 mm per month. Consequently, the cooling effect is stronger by 0.3℃ and the wetting effect is stronger by 0.5 g/kg in 2008 than those in 2009. Meanwhile, the rainy effect is weaker in 2008 than that in 2009 by 0.6 mm/d.