As the average global temperature increases,it is generally expected that the air near the surface should be drier,which should result in an increase in the rate of evaporation from terrestrial open water bodies.However,despite the observed increases in average temperature,observations from many regions show that the rate of evaporation from open pans of water has been steadily decreasing over the past 60 years.It is important to understand why pan evaporation has decreased despite the increases in average temperature in order to make more robust predictions about future changes in the hydrological cycle.One of the explanations is that the decrease in evaporation is caused by large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration.Previous work demonstrated that,in non-humid environments,measured pan evaporation is not a good measure of potential evaporation(I_ETp);moreover,in many situations,decreasing pan evaporation actually provides a strong indication of increasing terrestrial actual evaporation(I_ETa).The key issue in research on the hydrological cycle is how to estimate I_ETa.As we know,it is difficult to obtain a sufficient volume of reliable instrumental I_ETa measurements,so scientists have made use of a range of theoretical,especially climatological,methods for this purpose.There are two main theories to estimate I_ETa from I_ETp.At the regional scale,the Penman hypothesis is a common approach to reducing I_ETp to I_ETa in response to the water stress,which is given by a function of soil water availability.This is often questioned by the Bouchet complementary relationship theory,in that the Penman hypothesis does not consider the complex surface-atmosphere interactions at the catchment scale.The discrepancy between the Penman and Bouchet hypotheses is especially highlighted in non-humid regions.
In this paper,we select the Pearl River basin of southern China as the study area,and the relationship between I_ETa and I_ETp is analyzed in depth.Firstly,the actual evapotranspiration(I_ETa) and potential evapotranspiration(I_ETp) are calculated,respectively based on the water balance model and the Penman formula,in seven sub-basins of the Pearl River Basin from 1961 to 2000.The relationship between I_ETa and I_ETp under the change in water supply conditions is quantified and compared among the seven sub-basins.The results show that:
(1)The annual I_ETa is much lower than the I_ETp,and the mean annual I_ETa values are less than 1/2 of the I_ETp in most of the sub-basins.The area-weighted average I_ETa of the seven sub-basins is 681.4 mm yr^-1,and the I_ETp is 1 560.8 mm yr^-1.The variability of the I_ETa is more significant than the variability of the I_ETp.
(2)I_ETa in five sub-basins(Dongjiang,Xijiang,Beijiang,Liujiang and Panjiang) is positively correlated with the precipitation(P),but in the other two sub-basins (Hanjiang River and Yujiang),the correlations between I_ETa and P are not so obvious.The I_ETp of all sub-basins shows a significant negative correlation with P.Given the condition of increasing P,the annual mean I_ETa of the seven sub-basins shows an obvious increasing trend,while the I_ETp presents a clear downward trend.
(3)A joint regression equation(P-I_ET) between I_ET(containing I_ETa and I_ETp) and precipitation is constructed for every sub-basin,and t-tests of all the regression coefficients are used to determine if the relationship between P and I_ETa or I_ETp belongs to the Bouchet complementary relationship theory or the Penman proportional hypothesis theory.The results confirm the applicability of the former theory in all of the seven sub-basins except Yujiang.In Hanjiang,Liujiang and Panjiang,the relationship between I_ETa and I_ETp fits the complementary relationship theory completely.Furthermore,in three sub-basins(Dongjiang,Xijiang and Beijiang),the relationship between I_ETa and I_ETp belongs to the "asymmetry complementary relationship".
(4)A number of previous studies generally lend support to the I_ETa formula based on the completely symmetric complementary relationship theory,but not the "asymmetry complementary relationship" theory.A schematic diagram of the "asymmetric complementary relationship" between I_ETa and I_ETp is given in the present paper.Based on strict logical inference,and at the same time considering the boundary conditions of extreme drought and extreme wet conditions,this paper gives the I_ETa model under the "asymmetry complementary relationship".