Review of Water Conservation Value Evaluation Methods of Forest and Case Study

In terms of various understanding of water conservation value of forests and dramatic differences with outcomes from different methods,the conceptions of methods have been analyzed based on the summary of methods.Comprehensive Water Storage Ability Method,Precipitation Storage Method,Water Balance Method and Equivalence Method have been chosen to be applied to Zhifanggou watershed and Nihegou watershed of the Loess Plateau in northern China and Nuerzhai watershed in southern China.It was shown that the results of different methods were inconsistent because of the difference of connotations of these methods.The difference between methods was significant especially in the two small watersheds in the Loess Plateau.For Zhifanggou watershed,the results from Comprehensive Water Storage Ability Method and Equivalence Method were higher,while the results from Water Balance Method I and Precipitation Storage Method were lower,with the range of the four methods being around 1.5 million.In contrast,the results in Nuerzhai watershed had no significant differences from each other.The range of the results from the methods above can be around 0.2 million.But the result from Water Balance Method Ⅱ has been shown abnormal in all watersheds studied.Further analysis showed that Comprehensive Water Storage Ability Method and Equivalence Method aim at the potential ability of water storage,while Precipitation Storage Method I and Water Balance Method I focus on the actual ability of water storage.In arid and semi-arid areas,the result by Comprehensive Water Storage Ability Method is higher than the actual value,but smaller in humid areas,just because that the actual ability of water storage is often lacking in arid and semi-arid areas.Besides,the equivalent factors used in Equivalence Method should be modified according to the climate in the study area in order to cater to the actual situation of different studied areas.Moreover,the calculation formulation of Water Balance Method Ⅱ is inconsistent with the connotation supposed to reflect.That's the reason why the result of Water Balance Method Ⅱ is rather lower,sometimes even negative.Thus,Water Balance Method Ⅱ was not suggested to be used in the evaluation of water conservation value.The method of water storage ability by soil,which has similar connotation to Water Balance Method Ⅱ,could be considered as an alternative way.