HR: 16:45h
AN: H34D-04    [Abstracts]
TI: Understanding the relationship between actual and potential evapotranspirations from long- term water balance analysis and flux observation
AU: * YANG, D
EM: yangdw@tsinghua.edu.cn
AF: Department of Hydraulic Engineering, Tsinghua University, Qinghuayuan, Haidian District, Beijing, 100084, China
AU: YANG, H
EM: yhb99@mails.tsinghua.edu.cn
AF: Department of Hydraulic Engineering, Tsinghua University, Qinghuayuan, Haidian District, Beijing, 100084, China
AU: SUN, F
EM: sunfubao98@mails.tsinghua.edu.cn
AF: Department of Hydraulic Engineering, Tsinghua University, Qinghuayuan, Haidian District, Beijing, 100084, China
AB: Increase in air temperature and decrease in pan evaporation was found to be common worldwide during the past half century. This results in controversy in view of the changes to the hydrological cycle. Increases in precipitation have been expected due to the Clausius¡§CClapyeron relation in that the specific humidity increases exponentially with the greenhouse-gas induced temperature increasing and confirmed by measurements over northern extratropical land areas. The hydrologic cycle is expected to be intensified (or accelerated). However, the decreased pan evaporation is found to be well related to the global dimming, i.e., the decreased solar radiation induced by the pollution increasing, thus evaporation (i.e., the latent heat flux) should be steadily decreasing from the energy balance perspective. Many researchers explained that the potential evaporation (usually measured by pan) is decreased with increasing of precipitation; however, the increased soil moisture (due to precipitation increasing) can be evaporated because of extra energy available. Therefore, the actual and potential evaporation are in complementary relationship, which is expected to unify the controversy between global warming and dimming. This means that pan evaporation decrease implicates acceleration of the global hydrologic cycle, i.e., increase in the terrestrial evaporation. Based on the complementary theory, many operational formulae have been introduced to estimated actual evaporation from the potential evaporation. Our recent water balance analysis of 108 catchments in non-humid regions of China has shown that there are no general opposite trends between potential and actual evaporation in the same period. A novel phenomenon has been found that the complementary relationships in evaporation are distinctly confirmed when the annual actual and potential evaporation are plotted against annual precipitation; However, complementary relationships disappear in many catchments when actual and potential evaporations are plotted against the time (year) during the same period. This means that complementary idea cannot provide universally correct predictions on the trend of actual evaporation only from the potential one. In this research, we examine the coupled water-energy balance based on Budyko hypothesis and proposed a conceptual model for predicting the inter-annual variability of annual water balance, and the change trends of water balances due to climate changes. The wet environment evaporation was defined as the boundary condition in the Bouchet hypothesis and introduced into complementary relationship (CR), which combined the actual evaporation with potential evaporation in an equation. However, the CR was derived in a closed system where no horizontal energy advection existed. The effect of the horizontal advection on the CR in a real open system was also analyzed in this study. Using the long-term water balance analysis in the 108 study catchments and flux observation at 7 sites in Asia monsoon region, the regional and seasonal variability of the complementary relationship was examined. Key Words: climate change, evapotranspiration, water balance, flux observation, Budyko hypothesis, Bouchet hypothesis
DE: 0495 Water/energy interactions (1878)
DE: 0530 Data presentation and visualization
DE: 1807 Climate impacts
DE: 1818 Evapotranspiration
DE: 1876 Water budgets
SC: Hydrology [H]
MN: 2007 Fall Meeting