Study On The Mechanism And Simulation Of Water/heat Transfer Of Spring Wheat With Film Mulching In Northwest China

Shiyang River Basin is located in the arid area of Northwest China,where the extreme water shortage in this region has become the main factor that restricts the development of local agricultural economy.Covering transparent film could increase soil temperature,maintain soil moisture and thus would change the water-heat environment in the farmland.Simulating water and heat transfer in Soil-Plant-Atmosphere-Continuum(SPAC)system when covering transparent film is of vital importance to clarify the interactive relationship between water-heat transfer and crop growth in SPAC system,and to reveal the mechanism of water saving irrigation with plastic mulch,to optimize irrigation schedule and ensure the stable crop yield with high-efficient utilization of agricultural water resources.In this study,the Shiyang River Basin was selected as the study area,and the spring wheat was treated as the experimental crop,with different water supply modes and plastic mulching conditions.The soil water and heat dynamics,surface net radiation and crop physiological indicators were monitored to systematically analyze the impacts of film covering on surface energy distribution,water-heat transfer resistance and crop growth mechanism.The coupled model of CropSMPAC was established to describe these factors quantitatively,through which to realize the dynamic simulation under different water supply modes in SPAC system,especially the water and heat variations and the spring wheat yield differences.The research has achieved the following results and conclusions:(1)Through a two-year field experiment,the patterns of soil water-heat variation with and without transparent film mulching under different water supply modes were studied and their impacts on spring wheat were explored.The results showed that the surface albedo covered by transparent film is 0.2-0.28,which was larger than those without it(0.16-0.25).Covering transparent film decreased the shortwave radiation of soil surface and increased the soil surface temperature.In 2014,with film mulching the accumulated temperature at 10 cm soil depth was 1890.1 C and 1886.2 C for W1 and W5 treatments respectively,and 1835.4? and 1813.1? Cat 20 cm soil,respectively.They were higher than those without transparent film by 98.8? and 118.3?(10 cm),102.7? and 154.3?(20 cm).Similar conclusion was obtained in 2015.The surface soil moisture with film mulching was higher than that without it,indicating that film mulching could effectively inhibit evaporation.The maximum LAI of sptring wheat increased by 0.42-1.57 with film mulching,and WUE also increased by 3.2-27.3%.(2)Based on the mechanism of film mulching on surface energy allocation,water and heat transfer resistance and crop growth,a coupled model CropSMPAC was established.The model was based on the theory of water and heat transfer in SPAC and was able to describe the effect of plastic film mulching on water and heat transport and crop growth in spring wheat field.The calculating equation of soil evapotranspiration function was improved by introducing the mulching resistance parameter rm and the film mulching ratio fm.Also the accumulated air temperature was replaced by the accumulated soil surface temperature for calculating the crop development stage in the new model.The results indicated that CropSMPAC model could better reflect the effects of film mulching on the water consumption,accelerated growth period and increased yield of spring wheat.(3)Using two years of field measured data,the CropSMPAC model was calibrated and verified.The results showed that the model was not only able to simulate the soil water-heat dynamics with or without film mulching under different water supply modes,but also capable to simulate the coupling effects between soil water-heat dynamics and crop growth.The coefficient of determination R2 was 0.68-0.95 when simulating the soil water storage within 1 m,and that for soil temperature was 0.79-1,which indicated that the model had high precision for the simulation.(4)According to the current local irrigation schedule,the adjustment of irrigation schedule and the scenarios of future climate were simulated,to analyze the effects of different scenarios on farmland water dynamics,crop production and water production efficiency.The variation characteristics of crop yield reduction under future climate scenarios were quantitatively described,to confirm the mechanism of irrigation schedule on water consumption of the spring wheat,and finally came up with the reasonable irrigation schedule.At present stage,the irrigation schedule under current climatic conditions should be adjusted to 60 mm,90 mm,90 mm and 60 mm of irrigation quota in May 1st,May 21st,June 13th and July 1st,respectively.It could save 60 mm of irrigation amount compared with the current irrigation schedule during whole growth period,while at the same time high water production efficiency was achieved.