| The interaction between land and atmosphere at different spatial and temporal scales is complex,and it modulates the exchanges of energy,mass,and momentum across the interface;therefore,this interaction has serious impacts on climate.Under the dual coercion of human activities and climate change,land use in China has undergone great changes at different temporal and spatial scales over the past 30 years.China is the largest country in the world in terms of irrigated area,and the North China Plain(NCP)is the region with the largest irrigated area in China.The precipitation is scarce and the water resources are short in Northwest China,which have severely restricted the development of local agriculture,and the plastic film mulching can effectively suppress soil evaporation and maintain soil moisture.The application prospect of the plastic film mulching in the Northwest China cropland is good.The changes in these typical underlying surfaces of China can produce non-negligible impacts on regional climate through the land–atmosphere interaction.The current numerical simulations of the impacts of land use changes on regional climate were mostly concerned with the changes of single land use type,and did not consider the subgrid-scale heterogeneity.The irrigation schemes in the numerical model needed to be improved.There were no studies to investigate the impacts of plastic film mulching cropland on regional climate.Therefore,firstly,we used the regional climate model WRF/Noah(Noah land surface model coupled with the Weather Research and Forecasting model)and designed two types of experiments to investigate the regional climate effects of land use and green vegetation fraction changes over China,and the simulation period was from 0000UTC 1 May 2010 to0000UTC 1 March 2011.One type consisted of three land use experiments,which used the three periods of land use data(1990,2000,2010)to run model separately.The other type was comprised of three green vegetation fraction experiments,which used the three periods of green vegetation fraction data(1990,2000,2010,included from January to February next year)to run model separately.Secondly,a dynamic irrigation scheme,which considers the groundwater processes and dynamic vegetation,was constructed and incorporated into the Noah land surface model with multi-parameterization options(Noah-MP)coupled with the Weather Research and Forecasting(WRF)model.Multiple experiments(with and without irrigation)were designed and run continuously for 11 years(2004-2014)to simulate the impacts of irrigation over the NCP on regional climate.Finally,a dynamic irrigation scheme was incorporated into WRF/Noah.On this basis,two experiments(with and without plastic film mulching)were designed and run separately for 9 years(2007-2015)to simulate the impacts of plastic film mulching cropland over the arid and semi-arid areas in Northwest China on regional climate.The main results and conclusions are as follows:(1)In the land use experiments,the surface albedo,leaf area index,the impermeability of urban ground,and low cloud fraction can affect the surface energy and its distribution,which led to that the influences of land use changes on latent heat flux,sensible heat flux and surface air temperature(2 m)in the part areas of China were significant in summer.The surface albedo and low cloud fraction mainly affected surface net radiation,and leaf area index and the impermeability of urban ground mainly affected latent heat flux,which triggered the changes in surface air temperature.The influences of land use changes on the winter surface energy were mainly achieved through the changes of surface albedo affected greatly by ice or snow,such as southern Tibet.In the green vegetation fraction experiments,leaf area index was consistent with green vegetation fraction,and the green vegetation fraction changes affected latent heat flux by affecting soil evaporation,canopy water evaporation,transpiration(the three components of the latent heat flux in the model),causing changes in the distribution of surface energy.Although the changes direction of soil evaporation affected directly by green vegetation fraction were generally opposed to the changes of total evapotranspiration,the transpiration,which was directly affected by leaf area index,had the greatest influences on latent heat flux.Therefore,the latent heat flux changes and green vegetation fraction changes had good spatial consistency,and the sensible heat flux and surface air temperature changes were essentially opposite to the latent heat flux.The changes of summer precipitation were related to the influences of land use and green vegetation fraction changes on wind fields.The mechanism analysis took Northeast China as an example,land use and green vegetation fraction changes affected the geopotential height by changing the vertical layer temperature,and then wind fields would be changed.(2)The simulated irrigation rate of the NCP was close to observations and its interannual variations were clearly associated with dry or wet years.The groundwater table affected by irrigation varied in the NCP irrigated areas during simulation period but declined overall,especially in some areas of Hebei province,where it declined by greater than 6 m.The increases in soil moisture induced by irrigation not only had a direct influence on soil evaporation,but also increased the green vegetation fraction,which led to increases in transpiration.Therefore,the influences of irrigation on latent heat flux contained both the changes of soil evaporation and transpiration.The irrigation–induced increases in latent heat flux led to the decrease of the surface energy that allocated to sensible heat flux,which caused to decreases in surface air temperature.The spring and summer surface air temperature in the areas with larger irrigation rate decreased significantly by 0.8–1.6°C and 1.2–2.6°C,respectively.The experiment that considered irrigation showed obvious improvement in simulating several key variables(gross primary productivity,green vegetation fraction,sensible heat flux,and summer surface air temperature).In the NCP,irrigation increased water vapor and caused cooling in boundary layer,which had opposing influences on precipitation,resulting in heterogeneous changes in summer precipitation.The changes of precipitation in non-irrigated areas were related to the influences of irrigation on wind fields.The contribution of direct effects of irrigation to the blended influences of irrigation and dynamic vegetation on surface air temperature was 70.9%(63.8%)in spring(summer).(3)In the Northwest China irrigated cropland,the irrigation amount decreased obviously during the early stage of crop growth(from 22 April to 1 July)after mulching plastic film,especially in May,indicating that plastic film mulching can effectively save agricultural water.In the irrigated cropland,irrigation was both considered in cropland with and without plastic film,so there were little differences in soil moisture between cropland with and without plastic film besides the lower the first layer soil moisture in cropland without plastic film during the early stage of crop growth.In the non-irrigated cropland(terraced fields),soil moisture in plastic film mulching cropland was obvious greater than that in cropland without plastic film,which indicated that plastic film mulching can maintain soil moisture.The influences of plastic film mulching on land surface process were mainly reflected in the early stage of crop growth,which led to increases in upward shortwave radiation and decreases in latent heat flux.From the mean results of the early stage of crop growth,after mulching plastic film,the latent heat flux decreased by 12-30 W/m~2,and the sensible heat flux increased by 8-16 W/m~2,and the surface air temperature increased by 0.2-0.8°C in cropland.The plastic film mulching in the cropland reduced the water vapor content in the atmosphere,which may led to that the changes of precipitation in Northwest China was mainly decreased. |
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