Study On Water Cycle Of Typical Grass Land In Source Region Of Yellow River

Hydrologic process in cold region is a linking medium between ecological system and climatic system. Hydrothermal condition directly determines the status of the ecological environment in cold region. This study aims to explore water cycle of typical grassland in source region of Yellow River. Firstly, obtained change of climatic periods by analyzing air temperature and precipitation of weather station located in the town of Dari from 1956 to 2001. Secondly, the infiltration and soil moisture in the two typical watersheds were studied by numerical simulation. Also the relationship between environment factors and soil moisture were studied. The main conclusions are as follow:1. During the past nearly 50 years, the increase rate of precipitation and evapotranspiration is 0.557mm/a and 0.132mm/a, so the climate of Dari changed from drought to moist. The period of temperature is 20a,15a,7a and 5a, and the period of precipitation is 15~20a and 7~8a.Also studied potential evapotraspiration by FAO-penman-monteith combined with former studies on solar radiation over the Tibetan Plateau. The result shows the decreasing rate of flooding period and spring-winter is 0.0467mm/a and 0.1084mm/a.2. The steady infiltration rate is different for different vegetation, the rates are O.lmm/min and 0.017mm/min for two watersheds. At the beginning of precipitation, the infiltration rate of high plant cover is smaller than low plant cover. The beginning infiltration rate of black earth is bigger. Kostiakov infiltration model is better for meadow grassland in cold region, but for black earth beach Horton infiltration model is better. For different precipitation-infiltration processes, the results of two models are different. The runoff yields of meadow and black earth are 1913~2921Land 9313-15244L.3. These environmental factors are divided into four main composition factors successively: Factor of plant, factor of the vegetation, factor of slope above sea level and the wind speed and topographical factor. The order of water content of four different vegetation type : Shrub forest > Black earth beach > Degradate on the meadow > Small high grass, the sectional change of the soil moisture has two kinds of characteristics: the first is the fluctuation type including light high grass and degradate the grass, the second is reducing type including shrub forest and black earth beach;The relation between the different depth soil moisture and the height above sea level obey the unary quadratic regression model , the relation between the moisture of 60cm soil layer and the top layer soil (10-30cm ) and the absolute height above sea level is better than others, and the soil moisture and dependence of height above sea level are relatively strong;Change characteristic of height above sea level of the type soil moisture of different vegetation: Shrub forest and high grass vegetation type soil moisture present that the trend reduces first and then increases as the increase of the height above sea level;the trend of the soil moisture of degradation meadow reduced with its distribute increase of height above sea level;Black earth beach soil moisture is reduced with increase of height above sea level distributed;According to the order of the grassy marshland of shrub forest, the grassy marshland, not slightly degradation degree, slightly degradation degree and black earth beach, the soil moisture sectional change of different degradation degree appears symmetric variation tendency on the symmetric axis that is the not slightly degradation degree, it is the variation tendency of fluctuating and reducing to both sides sequentially.