Simulation Study On The Effects Of Irrigation On Regional Climate And Crop Heat Stress Over China

China is the country with the largest irrigated area in the world,and irrigation makes an important contribution to ensuring China’s food security.At the same time,the proportion of irrigation water consumption is large,accounting for more than 60%of China’s total water consumption throughout the year.Irrigation not only satisfies the water needed for crop growth,reduces the extreme high temperature experienced by crops in summer,which plays a huge role in stabilizing and increasing grain production;it also increases the water vapor in the atmosphere and changes the surface energy budget and water cycle.Thus,irrigation has a significant impact on regional climate and crop growth.However,the impact of irrigation on the climate for the entire China is rarely studied.Although some research from the United States and India illustrates the effects of irrigation on climate,these estimates may not translate to China.In addition,China is located in the East Asian monsoon region,and the summer monsoon onset is a rapid transition from the dry season to the wet season,the mechanism of effects of irrigation on the summer monsoon onset(SMO)is still not clear.Furthermore,the effect of irrigation on crop heat stress has rarely been studied.Climate model is an important tool for studying land-atmosphere interactions,which can be used to explore the impact of irrigation on regional climate and crop heat stress.Therefore,this dissertation mainly employs the Weather Research and Forecasting(WRF)model to carry out three aspects of research work,in which the land surface model uses the Community Land Model(CLM).Firstly,a reasonable irrigation scheme was developed by considering multiple cropping,and we designed the control test without irrigation(CTRL)and the sensitivity test with irrigation(IRRIG).The simulations were carried out for 10 years(2001-2010)and we evaluated the WRF model’s performance on regional climate over China,and we also evaluated the model’s performance for simulating irrigation water use,and finally explored the impact of irrigation on regional temperature and precipitation in China.Secondly,we chose the appropriate definition to determine the regional SMO and to explore the effects of irrigation on SMO for China.Finally,we selected two commonly used crop heat stress indicators,such as Killing Degree Days(KDD)and Heat Stress Days(HSD),to study the impact of irrigation on crop heat stress for China.The main conclusions are as follows:The WRF model could well reproduce the land surface temperature(LST),surface air temperature(SAT)and precipitation over China.When irrigation was added,the WRF model improved the simulations of LST,SAT and precipitation in the irrigated area to a certain extent.Additionally,the annual irrigation water use in the IRRIG simulations were consistent with the observations,and the simulations of irrigation water use in different regions also performed well,indicating that the irrigation scheme developed could better reproduce China’s agricultural irrigation water use.The effects of cropland irrigation were mainly to reduce the LST and SAT in the irrigated areas,and the impact on precipitation was mainly showed that spring irrigation could suppress summer precipitation.After irrigation was added,the latent heat flux(LHF)increased and the sensible heat flux(SHF)decreased over the irrigation areas.The cooling effect of evaporation reduced both the LST and SAT.There were differences in the spatial distribution and magnitude of cooling.The greater the amount of irrigation,the more obvious the cooling,such as the North China Plain,the maximum decrease of annual mean LST and SAT were 2.8 and 2.5℃,respectively.These cooling effects coincided with the increased annual mean LHF of up to 68.3 W m-2 and the decreased annual mean SHF of up to 45.9 W m-2.On regional average,the annual mean LST and SAT both decrease by 0.6℃,the LHF increases by 9.1 W m-2,and the SHF decreases by 6.7 W m-2;the seasonal mean LST decreases by 0.8,0.3,0.7 and 0.7 ℃,respectively,the SAT decreased by 0.9,0.3,0.8 and 0.7℃,the LHF increased by 12.1,2.5,10.4 and 11.3 W m-2,and the SHF decreased by 10.4,3.0,6.4 And 7.0 W m-2.For precipitation,the decreased temperature induced by irrigation reduced the land-sea heat contrast,the consequent summer monsoon circulation is weakened,thereby suppressing summer precipitation(～30%).The WRF model could simulate well the summer monsoon precipitation over China.When irrigation was added,the simulation accuracy is further improved.The development of the SMO was started in southern China in early and mid-May,then moved northward to the Yangtze River Basin at the end of May and early June,and migrated northward to northern and northeastern China from June to August.Changes in the temporal and spatial distribution of the SMO affected agricultural planning and decision-making.Irrigation affected the SMO with a net delay of 0.12 pentad averaged over all of the influenced areas.The SMO saw an averaged delay(advance)of about 3.1(3.6)pentad(5-day period)over late-SMO(early-SMO)areas in the north monsoon region of Yangtze River Basin,and the averaged delay(advance)was about only 2.2(2.3)pentad days in the south,indicating irrigation affected SMO mainly in the relative dry monsoon region.Interestingly,more than 65%of the changed areas of SMO occurred over cropland due to irrigation and certainly,in turn,to influence agricultural production.Irrigation reduces the heat stress on crops,but the reduction is not large.For the impact of irrigation on KDD,in terms of the spatial distribution,the observed mean annual KDDs were below 400℃ d,gradually decreasing from south to north.The CTRL simulation overestimated the mean annual KDDs,while the IRRIG simulation significantly reduced them over part of the irrigated areas.In terms of the time series,the IRRIG simulation improves the simulation of the mean annual KDD averaged over the irrigated areas to a certain extent(722%).For the impact of irrigation on HSD,the observed spatial distribution of the mean annual HSDs were below 140 d,gradually decreasing from south to north.The CTRL simulation overestimated the mean annual HSD,while the IRRIG simulation significantly reduces them over some irrigated areas.In terms of the time series,the IRRIG simulation improves the simulation of the mean annual HSD averaged over the irrigated areas to a certain extent(18-30%).Irrigation has different effects on crop heat stress under different irrigation percentages.The overall performance was that as the irrigation percentage increased,the reduction of crop heat stress by irrigation was more obvious.In addition,crops in nonmonsoon region rely more on irrigation in summer,and the simulation improvements of irrigation on crop heat stress(KDD and HSD)in non-monsoon region(Xinjiang)were increased(15-35%and 20-40%)compared to monsoon region.