Observation And Simulation Research Of Land Surface Process On Cropland Covered By The Plastic Film In The Arid Region

The impacts of land use and land cover change caused by anthropogenic activities on environment and climate are one of the hot and hard issues in global climate change research.With the rapid development of modern agriculture,plastic mulch technology is widely used.The plastic mulch not only significantly alters the physical characteristics of the underlying surface such as the surface albedo,but also blocks the energy and mass exchange between the land surface and the atmosphere.These changes must have an obvious influence on the climate and environment by changing the land-atmosphere interaction.In order to accurately simulate and study the energy and mass exchange characteristics between the plastic film-covered cropland underlying surface and the atmosphere,this paper firstly analyzed the land surface process characteristics of the cropland underlying surface covered by the plastic film based on the data from 1 June to 24 July 2015 collected by the land-atmosphere interaction experiment at a plastic film-covered cropland site in the arid region of northwest China.Afterwards,a new detailed model CoLM-mulch was developed by incorporating a plastic mulch layer submodel and a surface albedo dynamic parameterization scheme that was built on the basis of the observational data into the land surface process model CoLM,meanwhile establishing the surface layer,plastic mulch layer and soil layer energy balance equations,presenting the radiation and sensible heat transfer parameterization schemes.Finally,the CoLM-mulch model was used to simulate and study the land-atmosphere interaction characteristics of the cropland underlying surface under with plastic film covering in the arid area.Additionally,the original CoLM model was run with the same atmosphere forcing data,soil and vegetation parameters as a comparison to investigate the effect of the plastic mulch on the cropland land surface process.The main conclusions summarized as follows:?1?The soil temperature at the depth of 20 cm above the shallow surface of the plastic film-covered cropland in the arid region was greatly affected by the irrigation,the daily average soil temperature at the depth of 5cm decreased from 24? to 19? after irrigation,while the soil temperatures at the 40 cm and 80 cm depth in the deeper soil layer were less affected by theirrigation,their daily soil temperatures changed less.The soil moisture increased dramatically at each layer,with significant mutations when the cropland was irrigated,but the changes of the soil moisture in the deeper layer apparently lagged behind the shallow layer,and then with the disappearance of irrigation effect,the soil moisture had an obvious decline process after the peak,especially the soil moisture at the depth of 5cm decreased rapidly,but the soil moisture of the depths of 10 cm,20cm,40 cm and 80 cm reduced slowly.The results showed that the soil temperature and humidity were higher than those of the bare soil,especially the shallow soil,which indicated that the plastic film mulching had some effect on the soil and soil moisture.The longwave and shortwave radiation were all larger than those of the soil The diurnal variation of surface albedo is related to the solar elevation angle and vegetation coverage.By comparing the characteristics among the three distinct types underlying surface of corn cropland covered by plastic film,cropland covered by plastic film but without crops,and bare soil,the soil temperature and moisture of the plastic film-covered underlying surface were higher than those of the bare soil underlying surface,especially at the shallow soil layer,which indicated that the plastic film mulching had definite effect on improving the soil temperature and soil moisture;At the same time,the longwave and shortwave radiations of the underlying surface with plastic film covering were also larger than those of the bare soil underlying surface,and the changes of surface albedo on the plastic film-covered cropland was closely associated with the solar altitude angle and vegetation coverage.?2?Assuming that the plastic film mulch completely cuts off the water-vapor transfer between the soil and the atmosphere,and totally inhibits the CO₂ flux generated by soil respiration,on this basis,the surface layer,plastic mulch layer and soil layer energy balance equations were established,the radiation and sensible heat transfer parameterization schemes were proposed.Accordingly,a submodel describing the plastic mulch layer physical processes was developed.Simultaneously,it was found that the surface albedo of the cropland underlying surface covered by plastic film wasn't affected by the soil moisture,but related to the vegetation coverage and the solar altitude angle,thus a surface albedo dynamic parameterization scheme was put forward.Further,the plastic mulch layer submodel and the improved surface albedo dynamic parameterization scheme were coupled and employed into the CoLM model,and consequently a land surface process model Co LM-mulch was formed for simulating the atmosphere-vegetation-plastic film mulch-soil interaction.?3?The CoLM-mulch model could reasonably simulate the diurnal variations of soil temperature,soil moisture,as well as radiation,water,heat and CO₂ fluxes on cropland underlying surface with plastic film covering.And the Co LM-mulch model simulation efficiency was high,the deviations between the simulations and observations were minor,and the surface albedo dynamic parameterization scheme was appropriate.Compared the simulated values of the CoLM-mulch model with that of the CoLM model,it showed that the statistical evaluations between the simulated values by CoLM-mulch and the observed were much more reasonable than that of the CoLM,especially in the lower vegetation coverage period.However,from the mean bias to see,the soil temperature and moisture,upward longwave radiation,sensible heat flux,as well as CO₂ flux simulated by the CoLM-mulch model were slightly higher than the observations,and the upward shortwave radiation and latent heat flux were lower than the observations,while the CoLM model is the opposite.Simulation results demonstrated that the plastic film mulch could significantly improve the soil temperature and moisture,influence the surface albedo,radiation balance,hinder the evaporation of soil water,suppress the CO₂ produced by soil respiration,and change the water-heat transfer and CO₂ budget balance processes between the land surface and the atmosphere.