Climate Effect Of Farmland Irrigation In Fergana Basin, Central Asia

Irrigation is the important means of land use management,which can’t be ignored in the simulation study of climate effect of land use change.Fergana basin is a typical agricultural area in Central Asia,because of the lack of precipitation,high irrigation demands of crop growth,so this paper plans to take Fergana basin as the research area,firstly,embedding the irrigation parameterization scheme in Weather Research and Forecasting the Model(WRF),updating the regional soil type data and relevant parameters,and then simulating the influence of irrigation on the temperature,humidity,wind speed and precipitation of the area,and discussing the climate effect of agricultural irrigation.(1)Firstly,based on NDVI time series and Landsat OLI image,using the unique phenological characteristics of winter wheat,extracts the spatial distribution information of Winter Wheat in Fergana basin(Uzbekistan)in Central Asia from 2003 to 2018,and estimates its planting area.It is found that the winter wheat in this area is mainly concentrated in the northeast and southwest,and the winter wheat planting in the central area is less;the average winter wheat planting area in this area from 2003 to 2018 is267.42 km~2,accounting for 2%of the total cultivated land area,and with great interannual changes,the largest area in 2009 is 927.57 km~2.This result provides the spatial distribution data for simulating the climate effect of irrigation in winter wheat growth period(spring).(2)Embedding the dynamic irrigation parameterization scheme in WRF/Noah,updating regional soil type data and relevant parameters.According to the planting system of the study area,irrigation is divided into spring and summer irrigation.Simulation results show that the simulation results of temperature,specific humidity,soil humidity,latent heat flux and sensible heat flux of irrigation experiment(updating soil type)are closer to ERA5 reanalysis data,so adding dynamic irrigation parameterization scheme to WRF/Noah model,updating regional soil type data and related parameters can obviously reduce simulation error.(3)Through opening and closing irrigation parameterization scheme in the WRF/Noah simulation experiment,the open irrigation experiment is divided into wheat irrigation and cotton irrigation,analyzes and comparing the results.It is founded that irrigation will lead to the decrease of temperature,the increase of specific humidity,the decrease of wind speed and the increase of precipitation in the irrigation area,and the effect varies according to the irrigation amount.The irrigation amount of wheat irrigation is lower than cotton irrigation,so the effect of temperature reduction and humidity increase is weaker than that of cotton irrigation.After irrigation,the average rate of wheat irrigation is less than that of cotton irrigation.The monthly temperature decreased by0.28℃,cotton irrigation by 0.93℃,wheat irrigation by 0.40g/kg and cotton irrigation by1.38g/kg.Both wheat irrigation and cotton irrigation will reduce the wind speed in the irrigation area,but the effect is weaker than that of temperature and specific humidity.The monthly average wind speed of wheat irrigation decreases by 0.036m/s,and the over average wind speed of cotton irrigation decreases by 0.31m/s.Finally,irrigation will increase the precipitation in the irrigation area by a small margin.The average monthly precipitation increment of wheat irrigation is 1.55mm,and that of cotton irrigation is3.31mm.For areas outside the irrigation area,wheat irrigation will increase the monthly precipitation in the north,East and south of the irrigation area by 10 mm,while cotton irrigation will increase the average monthly precipitation in the north of the irrigation area by 25 mm.