Derivation And Application Of The Coupled Water-energy Balance Equation

In the catchment scale, study on evapotranspiration is the basis for understanding hydrological cycle and for optimal allocation of water resources. But present theories on regional evapotranspiration have limitations to fully understand the spatial and temporal variability of evapotranspiration and for accurate estimating the actual evapotranspiration. Main objectives of present study are to derive the coupled water-energy balance equation based on existing evapotranspiration theories, and exploring its applicability for estimating the actual evapotranspiration and predicting the change trend of actual evapotranspiration due to climate change.Budyko hypothesis indicates that actual evapotranspiration in a catchment was dominated by precipitation and potential evapotranspiration over a long time scale, and the relationship of these three variables can be expressed as a function. Following this hypothesis, a set of partial differential equations was presented to describe the coupled water-energy balance in a catchment. Solution to these equations has only a parameter if the solution exists. Based on the hydrologic principle, the boundary conditions were given for the partial differential equations. Using the Buckingham Pi Theorem, the analytical solution was obtained, i.e. mean annual coupled water-energy balance equation.Considering the effect of soil moisture on actual evapotranspiration, this thesis also derived the coupled water-energy balance equation at any arbitrary time scale, and the equation was the same as the Budyko hypothesis at long-term time scale and the same as the Penman hypothesis at short time scale.Based on the flux observation at experiment stations in the Asia Monsoon region, the coupled water-energy balance equation was validated over different time scales, i.e., from a monthly time scale to daily time scale. The equation was used to calculate the monthly actual evapotranspiration of the Yerqiang Oasis, and the result was compared with that estimated by the oasis runoff-evapotranspiration model. It showed that the equation was usable for estimating actual evapotranspiration at regional scale. It offers a useful tool for the water consumption analysis for the water resources allocation program. The parameter of the equation has been found to have a positive correlation with the regional integral condition of vegetation.When evaluating the response of actual evapotranspiration (E) to climate change, potential evapotranspiration, a function of atmosphere state, it is both the result of and the cause for E. Therefore, the coupled water-energy balance equation can't estimate E independently. Based on Bouchet and Budyko hypotheses, a model for evaluating hydrologic response to the changes of climatic factors was established.Using the meteorological data during the past 50 years, it was been found that changes in climatic factors have significant regional variability. The increase in greenhouse gases and aerosols may be responsible for climate change. Based on the results estimated by the model, I analyzed the regional variability in the sensitivity of actual evapotranspiration to climatic factors, as well as the trend in actual evapotranspiration and the dominative climatic factors in China.