The Study On Evaporation And Effective Precipitation In Semi-Arid Region

Water cycle links "Geosphere-Biosphere-Atmosphere" system as one of the core issues of global change. For there is a lot of cultivated land but very few water resource and ecological environment is fragile in Northern arid and semi-arid areas, it’s very worthy of attention to study water circulation problems in this region. Firstly, we compare the spatial and temporal variation of potential evapotranspiration of Qinghai Lake and Poyang Lake area for nearly50years, and analyze the influence of different climatic factors on potential evapotranspiration; then we analyze the characteristics and causes of pan evaporation change in Dingxi of typical semi-arid Loess Plateau region with conventional meteorological data from1960-2010and lysimeter evapotranspiration data from2008-2012. We discuss the changes in evapotranspiration and the climatic causes on the bases of the relationships between pan evaporation and actual evapotranspiration under global warming. Thirdly, we calculated soil moisture changes caused by a precipitation and determined the minimum effective precipitation of different depths at different times in semi-arid region, by utilizing soil moisture and precipitation data of nearly five years. Finally, the responses of soil moisture to precipitation and its influence on surface energy budget under different vegetation were analyzed with field observations of grassland and farmland in semi-arid Tongyu. The main conclusions are as follows:(1) There are significant differences in spatial and temporal variations of potential evapotranspiration in recent50years around Qinghai Lake in semi-arid northwest and Poyang Lake areas in the humid southern. The much higher temperature in Poyang Lake area leads to much higher potential evapotranspiration in this area than that in Qinghai Lake. The annual potential evapotranspiration in Poyang Lake region shows decreasing trend, but in Qinghai Lake area it is reducing very slowly. Among the main meteorological factors affecting ET0, sunshine duration, temperature and wind speed were positively correlated with potential evapotranspiration, while the vapor pressure showed a negative correlation, by regression analysis. That the ET0of Poyang Lake area decreased more obviously is mainly caused by more significant declination of Sunshine duration and wind speed in this area.(2) With global warming, the increasing trend of pan evaporation changes in semi-arid Loess Plateau is very different from that in most parts of the world. Besides temperature among the climatic factors affecting pan evaporation, precipitation, low cloud cover, sunshine and wind speed are contrary to that of most other places and relative humidity decreases more significant. A multi-factor model of pan evaporation and temperature, relative humidity, wind speed and low cloud cover is established with multi-factor fitting method and used to simulate the independent impacts of different climatic factors on pan evaporation changes in the past50years.The experiment indicated that the temperature, humidity, wind speed and low cloud amount variations respectively had a contribution rate of17.1%,53.7%,1.9%and27.3%to pan evaporation increase. Unlike the pan evaporation, actual evapotranspiration obtained by estimating does not show clear upward trend, which is much smaller than pan evaporation and more consistent with annual precipitation. This may be related to reduced local precipitation and soil moisture.(3) Soil characteristics, temperature, vegetation are important factors affecting the effective precipitation. The minimum available precipitation of5cm,10cm, and20cm soil layers are4mm,5mm,8mm. In the seasonal distribution, the minimum available precipitations in summer are the highest, respectively5,6,10mm, for the impacts of temperature and vegetation. In warmer year, minimum available precipitation is higher than that of low temperature year, and in growing season, it is higher than the value of non-growing season. With increasing soil depth, the difference of minimum available precipitation between the years of high temperature and low temperature is getting smaller. After precipitation into the soil, shallow soil can implicitly more water. When the shallow soil achieves saturation, water infiltrates into deep soil, but less frequent. When precipitation level is small, only10%-20%of precipitation can eventually convert to soil moisture for the0-20cm layer of soil column. When precipitation level increases, the proportion of water loss is reduced, and the conversion rate can reach70%.(4) In this paper, the response of soil moistures to precipitation and its influence on surface energy budget under different vegetation are analyzed with field observations of grassland and farmland in semi-arid Tongyu. It shows that the variation of soil moisture has important roles in surface radiation and energy budget process. The net radiation and latent heat fluxes are larger, the sensible heat flux is smaller and soil heat flux is about the same when soil moisture is higher. The evaporation ratio also increases, illustrating the proportion of latent heat in effective energy for evaporation increased. Since vegetation can retente precipitation and regulate soil erosion, soil moisture of different underlying surface responses differently to precipitation. Soil moisture in farmland is higher than in the grass, with larger evapotranspiration. With the increase in soil moisture, effective energy of farmland and grassland shows different trends, which may be due to soil moisture and vegetation evapotranspiration at farmland station are higher and that results in more obvious increasing in latent heat flux with soil moisture increases. Net radiation increases exponentially with increasing soil moisture. Soil moisture will also affect surface albedo and have an impact on distribution of total surface energy. With increasing soil moisture, surface albedo decreases, but it is higher in grassland than in farmland. It manifests that vegetation not only directly impacts on surface albedo, but also indirectly affect albedo by changing soil moisture.