| Evapotranspiration is a key link combining atmospheric process and land surface process in the hydrological cycle of the climate system. Evapotranspiration research has an important significance for the understanding the climate change and the impact of climate change. Meanwhile, the evapotranspiration research has very important application in many subjects such as water resources, agriculture, and ecological environment. This paper taked three typical river basins, the Pearl, Haihe and Tarim River basins, as research area. As the representatives of wet area, semi-wet and semi-arid area, and arid area, the Pearl River basin, the Haihe River basin, and the Tarim River basin were selected as the research areas of this paper. The spatio-temporal variation of the evapotransipiration in the three river basins were analysed, based on which the paper also tried to find the reasons to explain why the variation existed. The main findings are showed as follows:(1) Water balance equation and Penman formula were used to calculate annual actual evapotranspiration (ETa) and annual potential evapotranspiration (ETp) of the three river basins. Results demonstrate that ETa has an obviously complementary relationship with ETp under the condition of surface water supply changing. After the regression equation between the water supply conditions of underlying surface (P) and the evapotranspiration (ET) was constructed, the T-test method was used to determine that the complementary relationships between ETa and ETp were symmetrical in three subbasins of the Pearl River basin such as Hanjiang, Liujiang and Panjiang. But asymmetrical complementary relationships between ETa and ETp were found Dongjiang, Xijiang and Beijiang, which are subbasins of the Pearl River basin and same situations were found in Haihe and Tarim River basin. The form of complementary relationship model of asymmetric conditions was discussed. More physical significance was given to the parmmeter "a" of the Advection-Aridity model thus the applicable scope extended to "asymmetric complementary relationship". In the Pearl River, the Haihe River and the Tarim River Three River Basin of Advection-Aridity model was calibrated. The results show that the model is applicable in the three basins. (2) The actual evapotranspiration have significantly decreased trend in Pearl River, Haihe over the past50years from1961to2010, but has a significant increasing trend in the Tarim River Basin. In the three basins, the decreased (increased) trend of actual evapotranspiration in summer and autumn is very obvious, but slightly in spring and winter. ETa of the Pearl River was very close to the peak with the ETa of the Haihe River Basin, and the difference between the ETa of the two river basins is mainly reflected in the spring and winter. ETa in Spring and Winter of the Haihe River Basin was close to the ETa in the Tarim River basin, so the difference between the ETa of the two river basins is mainly reflected in the Summer and Autumn. The ETa isoline showed a direction from northeast to southwest in the Pearl River basin and showed a direction from northwest to southeast in the Haihe River basin. In the Tarim River Basin, the spatial distribution characteristics of ETa is that higher ETa values occurred mainly in northern, western and southwest of the basin (root of the Tianshan Mountains and Kunlun Mountains), low ETa values occurred mainly in the hinterland of Taklimakan Desert which is located at the center area of the basin.(3) The temporal trend of the ETa results from the variation of the radiation energy term and the aerodynamic term, but different terms gave different impact on the ETa. According to the Adevection-Arid Model, negative (or positive) trends of ETa related to the negative (or positive) trends of the radiation energy term or related to the positive (or negative) trends of aerodynamic term. In the Pearl River Basin decrease of daily range of temperature, sunshine hours caused decrease in radiation energy, contribute to the actual evapotranspiration decrease; the increase of the average temperature, maximum and minimum temperatures, caused the increase of aerodynamic term, contributed to ETa decrease; decline of average wind speed decrease aerodynamic item, actually slow down decrease degree of ETa. In the Haihe River Basin decrease of daily range of temperature, sunshine hours caused decrease in radiation energy, contributed to the decline trend of ETa. The temperature in the Tarim River Basin (including average air temperature, maximum, minimum temperature rise) and actual water vapor pressure gave more obvious contribution to the increase of the radiation energy; decreased sunshine duration caused a decrease in radiation energy, in turn reduces ETa; decline in average wind speed is contributing to the aerodynamics item, turn on strengthening increased degree of ETa. According to the actual amplitude of variation of all the climate factors, we found that the sunhour contribute the most variation of the ETa in all river basins.(4) Based on review of the Budyko and Fu Baopu water heat coupling actual evapotranspiration model and analysis of the ETa results calculated from the complementary relationship theory, this paper pointed out such two kinds of theories are fundamentally identical. Used the results of ETa in the Pearl River, Haihe River and the Tarim River Basin, the paper fitting parameters of Budyko-Fu Baopu water heat coupling curve. Thus quantitative analysis can be done to explain how the suface water content conditions impact on the actual evapotranspiration:in the Pearl River Basin average each5%increase in precipitation, can caused ETa increased by about2.3%; in Haihe River Basin, the average increase of5%precipitation, actual evapotranspiration increased by about4.5%, far greater than the Pearl River Basin; and in the Tarim River Basin every additional5%of precipitation, actual evapotranspiration will increase nearly5%. According to the actual amplitude of variation of precipitation from1961to2010, we found that the actual contribution of the precipitation on the ETa is about+4.3mm in the Pearl River basin,-92mm in the Haihe River basin and+32.5mm in the Tarim River basin. |
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