Study On Watershed Evapotranspiration Based On The Budyko Hypothesis

Evapotranspiration links with both water and energy cycles in a watershed, plays a key role in the climate-soil-vegetation interactions under a certain climate and landscape condition. Currently, increase in air temperature and decrease in pan evaporation was found to be common worldwide during the past half century, which is termed as pan evaporation paradox. This results in controversy in view of the changes to the hydrological cycle expected by using the Penman and Bouchet hypotheses. In order to reconcile this controversy, this study introduced Fu's analytical solutions to Budyko's coupled water and energy balance hypothesis.Building on the previous investigations, this study is aimed to understanding the relationships between the actual and potential evapotranspiration, to develop a coupled water-energy balances method for watershed evapotranspiration, and to predict the hydrologic response to climate change and its regional feature.First of all, the change of climate condition over China from 1956 to 2005 has been examined. It has been shown that the pan evaporation paradox phenomenon occured from 1956 to 1990, whereas disappears since 1990 gradually. In the longterm trends, precipitation and potential evapotranspiration show large difference with significant regional feathers. For the whole China, they show inverse distribution at the long-term timescale, are negatively correlated at annual timescale, and positively related at the month timescale.Secondly, in terms of water balance analysis in 108 catchments of nonhumid regions in China and the theoritial analysis based on Fu's equation, this study has explained the evapotranspiration at the catchment scale as: in humid regions, change in actual evapotranspiration is controlled mainly by change in potential evapotranspiration rather than precipitation, and this is identical to the Penman hypothesis; in non-humid regions, change of actual evapotranspiration is dominated mainly by change in precipitation rather than potential evapotranspiration, and the Bouchet complementary relationship between actual evapotranspiration and potential evapotranspiration comes about because actual and potential evapotranspiration are correlated via precipitation. Thus the consistency of the two opposite hypothesis has been unified into the Budyko's hypothesis.Further, it has been found that besides the annual climate conditions, the regional pattern of annual water-energy balance is also closely correlated with the relative infiltration capacity, relative soil water storage, and the average slope. This enables one to estimate the parameterω|- from catchment characteristics without calibration from the long time series of water balances. By stepwise regression analysis, a semi-empirical formula for the parameterω|- in terms of the dimensionless landscape parameters is proposed. Applications of Fu's equation together with the parameterω|- estimated by this empirical formula have shown that Fu's equation can predict both long-term mean and annual value of actual evapotranspiration accurately. This implies that the Fu's equation can be used for predicting the annual water balance in ungauged basins.Together with the experimental flux data observed at an irrigated agricultural field, the relationships in evapotranspiration are examined at different timescales, namely long-term, annual, daily and hourly timescale. It has been found that they show complementary at both the long-term annual and annual timescales, whereas at both daily and hourly timescales they show propotional relationships. This apparently paradoxal phenomenon has been explained by using the Budyko coupled water-energy balances hypothesis.Finally, in terms of the theoretical analysis for the hydrological response to the climate change based on Fu's equation and the water balance analysis in nonhumid regions, it is found that the impact of potential evapotranspiration on actual evapotranspiration at the annual timescale has been magnified in previous researches. This study estimate the annual actual evapotranspiration throughout China on the basis of Fu's equation together with the semi-empirical formula ofω|- . It is also found that the relationship in evapotranspiration is positive in humid regions and then gradually turns to negative in non-humid regions at the annual timescale.