Observation And Simulation Of Transient Land Surface Process Over Desert-steppe Transitional Zone In The Arid Areas

The land surface is nearly30%of the earth area. AS an interface about exchange of energy, momentum and matter between land and air, land surface is a very important part of earth science systems. Different underlying surface characteristics largely determine the exchange between soil, vegetation and air, affecting the main features of atmospheric circulation and climate of different spatial and temporal scales. The arid regions accounted for about30%of the global land area, which includes farmland, grassland, desert, Gobi, permafrost and many other complicated landscapes of land surface. For these complex underlying surfaces, their features of land surface process largely varies with spatial and temporal scales. In arid regions, the exchange of energy and matter between land and air is significantly affected by the changes of land use and land cover caused by human activities and natural changes. Desert-steppe transitional zone generally refers to the transitional zone between the desert and grassland/cropland. This underlying surface advances and retreats because of precipitation, which the transient change is very significant. For dry desert surface, a heavy rainfall can cause the surface become moist, resulting in the rapid growth of vegetation in a relatively short period of time (3-5days), so that the moist desert surface become desert meadow surface. If there is no heavy precipitation occurred in the late, then the soil water content reduces, the vegetation degenerates over desert meadow surface, the underlying surface becomes dry desert surface again. As a typical underlying surface in arid regions, the distinguished transient change over desert-steppe transitional zone is bound to affect regional weather and climate characteristics. Therefore, the study of transient land surface process variation features of desert-steppe transitional zone has far-reaching significance.This paper study follows three aspects in order to research the transient change of land surface process over desert-steppe transitional zone in arid regions. First, using the observed data of "Microclimate and Evaporation Experiment" from July to September2012, analyzes the characteristics of transient change of land surface over desert-steppe transitional zone in arid regions. Secondly, using the observed data and the Community Land Model version4.0(CLM4.0) which considering transient change of land use and canopy cover to simulate the features of transient change of land surface process over desert-steppe transitional zone. Finally, because of the importance of soil porosity a, changing the porosity parameterization scheme in CLM4.0by man-made, then examine the effects of soil porosity of different sizes to model simulation capability by sensitivity testing and comparative analysis, preliminarily exploring the causes of deviations between CLM4.0model simulated and observed soil temperature, soil water content over desert-steppe transitional zone. Main conclusions are as follows:(1) The characteristics of transient change of land surface over arid desert-steppe transitional zone were very significant due to precipitation. Soil temperature of shallow soil layers above40cm decreased after the precipitation, then increased with the disappearance of the radiation effects of precipitation, but soil temperature of80,120cm changed little, the mean values were about21and19℃. Soil water content can respond to rainfall obviously, soil water content of10,20cm increased rapidly, then slowly decreased; soil water content of30,40cm decreased rapidly after the increase. Surface albedo firstly decreased and then increased after rainfall. Albedo was higher over desert meadow surface, and closely associated with surface water content, sun elevation angle and vegetation growth parameters. The sensible and latent heat flux had significant change in precipitation, latent heat flux increased and then decreased, sensible heat flux decreased then increased, clearly associated with surface water conditions. Over the dry desert surface, the sensible heat flux was dominant while the latent heat flux was dominant after rainfall. Then the dominance of latent heat flux was gradually being replaced by sensible heat flux because of evapotranspirtion and reduced soil water content. The increase of soil water and growth of canopy can affect the near surface energy closure.(2) CLM4.0model can successfully simulate the variation of the surface radiation, turbulent fluxes, soil temperature and water content with the transient changes over the underlying surface, while there were deviations between the two. In the dry and moist desert surface conditions, deviations between simulated and observed values of the reflected radiation was small, while the model simulations had higher value comparing to the observed values over the desert meadow surface; CLM4.0model simulated the trend of surface longwave radiation, but deviations existed on noon and night. The correlation coefficient between the simulated turbulent fluxes and observed values reached above0.85, but the simulated values were higher than the observed values. The simulated soil temperature and water content were smaller than the observed values. The CLM4.0model had a poor performance of the simulated soil moisture changes caused by heavy precipitation process.(3) CLM4.0model simulated soil temperature and water content were highly sensitive to the soil porosity, the correlation coefficient, MBE and NESS between simulated and observed of soil temperature decreased with soil porosity decreased; the deviations between simulated and observed10cm soil water content became small with increasing soil porosity. There was a certain sensitivity between CLM4.0model simulated radiation, turbulent fluxes and soil porosity. The inapplicability of soil porosity parameterization scheme of CLM4.0model over the desert-steppe transitional zone in arid regions was the reason of deviations between simulated and observed soil temperature, soil water content. Developing applicable soil porosity parameterization schemes over the desert-steppe transitional zone in arid areas, improving the simulation capability of soil thermal and hydraulic conductivity can help to improve simulation performance of soil temperature and water content over such underlying surface.