Simulation Research Of Land Surface Process Over Two Typical Underlying Covers In The Arid And Semi-arid Areas

The arid and semi-arid regions are nearly40%of the global land area, its surface cover are diverse and complex, including various landscapes such as grassland, farmland, desert, Gobi, and permafrost. For these complicated underlying surface, their characteristics of land surface process varies with area and season obviously, and is significantly affected by natural changes and the changes of land use and land cover which is caused by the human activities, and thus their land surface process plays a very important role in the global climate system. But for the most of present land surface process models, there is still existence of a large deviation in land surface process simulation over the typical underlying surface in the arid and semi-arid areas. Furthermore, there is the energy imbalance in the surface layer in observation experiment; this makes the land surface process simulation more difficult. How to improve the land surface parameterization schemes, and develop land surface process model more applicable to the arid and semi-arid regions is an important task in the study of land surface processes.In order to improve the performance of land surface model simulation over the typical underlying surface in the arid and semi-arid areas, the land surface model was studied from three aspects in this paper. Firstly, a land surface process model TBLSHAW was developed and verified with the observation data in the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL). Secondly, focusing on the influence of the energy imbalance, which was commonly existed in the land surface observation experiment, the impact of energy imbalance upon land surface process simulation in semi-arid region were studied by using the SACOL surface layer observation data and land surface model TBLSHAW. Finally, in view of the fact that the large number of arid areas surface was covered by plastic mulch to inhibit evaporation, a new model TBLSHAW_MULCH was developed by improving the parameterization scheme with the observation at a cropland underlying surface covered by plastic mulch in the Gulang Heterogeneous Underlying Surface Layer Experiment (GHUSLE) in the arid region to simulate the land surface process of the cropland underlying covered by plastic mulch in arid region. We draw the following conclusions:(1) Based on the energy and material balance principle between land surface and atmosphere, a land surface model TBLSHAW was developed by means of SHAW and CoLM models. The TBLSHAW model, which was a land surface model including soil-vegetation-atmosphere interactions, absorbed the two-big-leaf model to calculate the material and energy balance between the vegetation and the atmosphere and to consider the vegetation canopy radiation transfer, precipitation interception, evaporation, transpiration, photosynthesis and other physical processes in detail; It used the general soil heat and water coupling transfer model to calculate soil moisture and temperature, and included the process of soil freezing and thawing, evaporation and precipitation infiltration; It combined part of the land surface parameterization schemes which was developed using the long-term observations of soil temperature and moisture and surface layer at SACOL; and it employed SHAW model framework.(2) Using observational data in SACOL, the performance of the TBLSHAW model was verified by simulating the land surface process of the typical dry (bare soil), wet season (grass land), freezing and thawing. Comparing among the observation and the simulations by TBLSHAW and different land surface process models, the results indicated that TBLSHAW model improved the simulation of soil water and heat transfer process in SCAOL. The TBLSHAW model could simulate the net shortwave, net longwave, soil temperature and moisture well, and could reasonably reproduce the energy distribution of the Loess Plateau both in dry and wet seasons. The simulation was close to the observation, and the model was efficient; The TBLSHAW model could also well simulate soil freezing and thawing process, and the bias between simulated soil temperature and the observation was small, so as the soil moisture.(3) In view of the fact that the observed surface layer energy was not closed in the arid and semi-arid regions (even the most of the global land surface), the TBLSHAW model was employed to study the impacts of energy closure ratio upon land surface simulations in semi-arid region through numerical simulation and comparative analysis method with the observation at SACOL in the spring,2007. The results showed that the observed energy imbalance could conducted into land surface process model by turbulent parameterization scheme in semi-arid area, when the sum of turbulent fluxes were lower than the available energy, the energy imbalance would make the land surface process model significantly overestimated the surface longwave radiation and soil temperature; when the actual observed energy imbalance part distributed into observed turbulent fluxes in some way, that meant correcting the observed turbulent fluxes so as to make the surface energy balance, and the turbulent fluxes parameterization scheme was developed based on the modified turbulent heat flux, then the land surface process model could well simulate the surface longwave radiation and soil temperature without changing any physical and biochemical properties of soil.(4) Based on the observation at a cropland underlying surface covered by plastic mulch in the GHUSLE, a new model TBLSHAW_MULCH was developed by coupling a plastic mulch sub-model to simulate the land surface process of the cropland underlying covered by plastic mulch in arid region; A comparative run the TBLSHAW model was performed to investigate the effect of the plastic mulch. The results showed that TBLSHAW_MULCH model could well simulate the land surface process characteristics of the cropland underlying surface covered by plastic mulch. And the model was efficient and the deviations between simulated and observed values were small. Compared the simulation of the TBLSHAW model with that of the TBLSHAW_MULCH model, it was suggested that the plastic mulch obviously affected the land surface process, and consequently the role of the plastic mulch should be considered in the land surface model.