Characteristics Of Regional Evapotranspiration And Its Response To Climate And Land Use Change

Land evapotranspiration plays an important role in the process of water cycle, energy cycle and carbon cycle. Climate change and human activities affect the water cycle and change the land evapotranspiration, while the changes of surface evapotranspiration will have a feedback effect to the climate and the environment. In order to understand how climate change and human activities affect evapotranspiration, the relationship between surface potential evapotranspiration and actual evapotranspiration in different areas of China was analyzed at first. Then the response of evapotranspiration to climate change and land use/cover change in the North China Plain was studied. The main research contents and conclusions are as follows.(1)The relationship between potential evapotranspiration (ETP) and actual evapotranspiration (ETa) was carried out on both annual and seasonal scales in different areas of China. ETP was obtained from the MODIS data product MOD16A2, and ETa was simulated by land surface model CLM. The results showed that the relationship between ETP and ETa varied with space and time. In arid, semi arid, transitional and semi humid areas, the annual ETP and ETa had a symmetrical complementary relationship, while they had a positive correlation in humid area. In spring, ETP and ETa exhibited a symmetrical complementary relationship in arid, semi arid, transitional and semi humid regions, and showed a proportional relationship in humid area.In summer and autumn, ETP and ETa met an asymmetric complementary relationship to a certain degree in all regions. The relationship between ETP and ETa was not obvious in winter.(2) The seasonal response of evapotranspiration to key climatic factors was analyzed. At first, FAO56 Penman-Monteith equation was used to calculate the ETo at 48 weather stations over the North China Plain from 1960 to 2012. Secondly, the interannual changing trend of ETo and its key climate influencing factors:air temperature (T), sunshine hours (n), wind speed (WS) and relative humidity (RH) was analyzed in different seasons. Then sensitivity analysis had been done on ETo to the four climatic variables respectively. Finally, the contributions of climate factors to the change of ETo was analyzed by combining with the sensitivity of ETo to climate factors and the relative changes of climate factors. It was revealed that ETo from 1960 to 2012 showed a downward trend in volatility. T increased while n, WS and RH decreased. ETo was found to have positive sensitivities to T, n and WS and have a negative sensitivity to RH. The maximum sensitivity of ETo to T and the maximum sensitivity of ETo to n appeared in summer, while the maximum sensitivity of ETo to WS and the maximum sensitivity of ETo to RH appeared in winter. It was found that the decline of ETo in spring, autumn and winter was mainly affected by the fall of WS, and the decrease of ETo in summer was mainly attributed to the decrease of n.(3) The influence of land use/cover change on land surface evapotranspiration is studied. Based on the land use data of North China Plain in 1990,2000 and 2010, combined with the evapotranspiration in corresponding period, the variation characteristics of different land types were obtained. The effect of land use change on land surface evapotranspiration was summarized by calculating the index of ecological disturbance degree. The results showed that the raise of the actual evapotranspiration in the North China Plain was mainly caused by the increase of the forest and grassland evapotranspiration during the period from 1980s to 1990s. While the decrease of the actual evapotranspiration during the period from 1990s to 2000s was mainly due to the increase of the construction land and the decrease of the forest, grassland and wetland area. The land use change in the North China Plain gradually made the land surface evapotranspiration reduce, and the effect intensified with the increase of the land use change rate.