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Positive or negative correlation between actual and potential evaporation? Evaluating using a nonlinear complementary relationship model
Author(s) -
Han Songjun,
Tian Fuqiang,
Hu Heping
Publication year - 2014
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1002/2013wr014151
Subject(s) - environmental science , sign (mathematics) , correlation , negative correlation , positive correlation , atmospheric sciences , mathematics , physics , medicine , geometry , mathematical analysis
Abstract 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.