Research On The Actual Evapotranspiration In The National Secondary River Basin Based On The Complementary Relationship

Evapotranspiration (evapotranspiration, ET) is an important process of the hydrological cycle contact processes in the atmosphere and land surface process. It also plays a role key in the study of climate change and hydrological cycle mechanism. The distribution and estimation of actual evapotranspiration in the national watershed model can help improve the utilization of water resources, alleviate tension of ecological environment. This paper aims to obtain the multi-year mean actual evapotranspiration through calculating the equation of water balance and using the 756 meteorological stations and hydrological data. This paper expounds the characteristics of the national watershed distribution of actual evapotranspiration and further validates the complementary relationship of evapotranspiration between the national basins. It also analyzes the changes of the characteristics of evapotranspiration complementary relationship in different climate conditions. According to the secondary basin of the hydrological data, rainfull data and evapotranspiration data, the pan evaporation data are looked as the potential evapotranspiration and at the same time using the water balance equation obtaines basin actual evapotranspiration. The paper analys the correlation between non symmetry of the national two level of complementary watershed evapotranspiration.The following works are completed in this thesis:(1) Considering the water and heat energy balance mechanism and under the condition of a long sequence of time when the soil water stays the same, the paper uses the water balance equation of closed drainage basin to get the multi-year mean actual evapotranspiration by use of the difference of watershed multi-year mean precipitation and runoff depth. Combing with the space distribution of geography and climate characteristics of each river basin and starting from the two points of ten river basins and secondary river basin, this paper respectively discusses nationwide multi-year mean actual evapotranspiration variation characteristics. The analysis shows that the rules of spatial distribution of actual evapotranspiration of the ten big basins. The Southern is generally more than northern, the east is more than the west and the plain in general is more than the mountain. The actual evapotranspiration of secondary river basin have similar change trend, from south to north and from east to west, the actual evapotranspiration is descending.(2) In the discussion of complementary relationship of evapotranspiration and Penman hypothesis, by using the closed basin water balance equation to obtain the actual evapotranspiration and the small evaporation pan data as potential evapotranspiration, the paper discusses respectively the variation characteristics of the actual evapotranspiration and the potential evapotranspiration along with the changes of the underlying surface water conditions in the different wet and dry climate zone. The results show that no matter how wet the basin is the complementary relationship between the actual evapotranspiration and the potential evapotranspiration resists. That is a positive correlation between actual evapotranspiration and precipitation, potential evapotranspiration is negatively correlated with the precipitation. Comparing the different valley, actual evapotranspiration trend line and potential evapotranspiration trend line trend showing closure.(3) Based on the complementary relationship between actual evapotranspiration and potential evapotranspiration, at the watershed scale, the complementary relationship evapotranspiration presented non symmetry. According to a collection between the actual evapotranspiration and the potential evapotranspiration under the surface water conditions which means the precipitation, the paper establishs the asymmetric complementary relationship model. Throuogh the hydrological and meteorological data by the secondary river basin, we can get the following two results: First of all, in the national scope the asymmetry of complementary relationship is universal; Secondly, in different climatic conditions, each parameter of the model decreases with the changes of the underlying surface water conditions, but decreased amplitude is not always the same.(4) According to the complementary relationship evapotranspiration, on the basis of previous research, the paper proposes to establish the general model of the actual evapotranspiration. Using hydrological and meteorological data, the paper can obtain the various coefficients of universal model. Through the model simulation results, the estimation of actual evapotranspiration basin is within 10%, which is a good simulation results. At the same time the paper uses ten big basins of meteorological land hydrological data to verificate the general model. In order to eliminate the effect of soil water storage variables, the essay slides the validation data within the 10 years and the validation of the model results are within 15%. The results show that, the model estimation of actual evapotranspiration have the better results.