干旱区生态环境知识资源中心

Northwest China is located in the inner center of the Eurasia continent, its ecosystem is sensitive to climate variability and anthropogenic alterations. It is the transition zone of the humid East Asia monsoon region and the arid central Asia area, the land-atmosphere interactions are strong there. In this research, the effects of vegetation change on land-atmosphere coupling strength and related land surface hydrological processes in northwest China were examined, and we also tried to confirm the most efficient areas of vegetation increasing for desertification mitigation in northwest China.

Using a state-of-the-art Community Atmosphere Model (CAM3) developed by National Center for Atmospheric Research, the effects of vegetation change in northwest China were studied by three numerical experiments: with realistic vegetation characteristics varying monthly (CTL run), without vegetation (NOVEG run), and with increased vegetation (VEG run). Firstly, the influences of vegetation change in northwest China on local hydrological variables (precipitation, surface water surplus, surface runoff, surface soil moisture and so on) were examined. Then, the effects of vegetation change on local land-atmosphere coupling strength were analyzed, land-atmosphere coupling strength is new standard to evaluate the degree of local land-atmosphere interaction, it is based on the ratio of the covariance between annualy precipitation and evaporation anomalies (from their climatological means) over the variance of precipitation anomalies. The coupling strength is easy to compute from observations and standard model output and has a relatively clear physical meaning, and the stronger the local land-atmosphere interaction is, the higher the land-atmosphere coupling strength will be. At last, an Evapotranspiration-Moisture flux divergence Index was calculated to evaluate the influences of vegetation change on evapotranspiration and moisture transport in the atmosphere. This index could reflect the effects of vegetation change on local land-atmosphere interactions and large scale atmosphere circulations to a certain degree, and it also could be used as an indicator of evaluating the effects of artificial ecological restoration programs.

The land-atmosphere coupling strength increased from southeast to northwest in this area. Desertification led to reduced precipitation and evapotranspiration in most area of northwest China. In the southeastern area, infiltration and soil moisture were decreased for about 40 similar to 80 mm and 5 similar to 20 mm(3) . mm(-3) respectively, yet surface runoff increased for about 10 similar to 40 mm. Land-atmosphere coupling strength also increased. These might cause happening of soil erosion, this might go against for vegetation recovery in this area. An increased precipitation and evapotranspiration were found in the inland area of northwest China by vegetation increasing, yet surface water surplus was reduced slightly, mainly because evapotranspiration was increased more compare with precipitation. In the southern part of Xinjiang and western part of Inner Mongolia which are the edge of desert, increased infiltration and soil moisture increased for about 5 similar to 20 mm and 5 similar to 20 mm(3) . mm(-3) respectively, the coupling strength reduced. The Evapotranspiration-Moisture flux was also high in this region, and it seemed mainly because the local land-atmosphere interaction was altered by vegetation change.

Vegetation has a significant influence on land surface hydrological processes in northwest China, the existence of vegetation could accelerate the hydrological cycle, reduce the variability of evapotranspiration and land-atmosphere coupling. Given limited financial and human resources, concentrated shrub-grass planting and preventing overgrazing in this particular region where the coupling strength could be substantially reduced by vegetation, might be an effective strategy to mitigate serious desertification in Northwest China. In the further research, we should consider the differences between the model land surface vegetation cover and the real situation, the feedback of ocean variability to vegetation change. ensemble simulations are necessary to make this research robust.