Positive or negative correlation between actual and potential evaporation? Evaluating using a nonlinear complementary relationship model

Understanding whether a positive or negative correlation between actual (E) and potential (E-p) evaporation is of great importance in detecting changes of E from E-p. In this study, such correlation was evaluated via the linear regression slope (k) between E and E-p derived from a nonlinear complementary relationship model. k varies with the relative degree of variability in the radiation term (E-rad) and aerodynamic term (E-aero) of E-p, and is further affected by water availability. The sign of k is always positive under conditions with high E-rad variability but at the same time with low E-aero variability (commonly true on an hourly basis). Under conditions with high E-aero variability but at the same time with low E-rad variability, the sign of k tends to change from negative to positive with more readily water availability. On a daily or annual basis, the sign of k would be related with water availability. Under water-limited conditions, the E-aero variability is more significant than the E-rad variability, and negative k is expected. Under energy-limited conditions, the E-rad variability is compared to or even much obvious than the E-aero variability, and positive k is expected. This implies a negative correlation between E and E-p under water-limited conditions but a positive correlation under energy-limited conditions on the daily or annual basis. The above analysis is confirmed in a grassland site in Northeast China at daily and half-hourly time scales. The correlation between annual E and E-p over China was evaluated using data of 690 meteorological stations from 1956 to 2005. The k values change from negative in arid regions to positive in humid regions. According to k and the decreasing trends in annual E-p, it is deduced that E increased for most stations in arid regions but decreased for most stations in humid regions.