Impacts Of Climate And Forest Cover Changes On Streamflow In The Xiaoxingan' Mountains Forested Watersheds

In the context of global climate changes, the intensification of human's activities and the increasing modification of underlying surface, the impacts of climate and vegetation changes on streamflow have been the hot issue among many researches. In the Xiaoxingan'Mountains, the important wood base for water conservation in China for example, the impacts of climate and forest cover changes on streamflow are significant both to the reasonable forest management and the development and utilization of water resources, also important to the preservation of the ecological environment and balance for Northeast region.With the unique feature of the Xiaoxingan' Mountains, this research choose Tangwang, Xinancha, Yongcui watersheds and Yulin verse Xinlin as quasi-pair watersheds for the typical study watersheds. On the basis of hydro logical and meteorological data for 43-year (1964～2006) for Tangwang, Xinancha and Yongcui watersheds,34-year (1973-2006) for quasi-pair watersheds, the variation tendencies of yearly mean temperature, yearly total precipitation and streamflow, the percentage of forest area and their relationships were analyzed with the methods of nonparametric tests (Spearman and Mann-Kendall), time series, regression analytic methods and graph and curve method (FDCs, Flow Duration Curves). Based on the analyses results, we constructed the regression models of yearly total streamflow and precipitation, and predicted and analyzed the reflection of yearly total streamflow combing with the future precipitation. With the graph and curve method (DMCs, Double Mass Curves), the relative contribution impacts of climate and forest cover changes on streamflow were quantitative analyzed in Xinancha watershed based on the regression models of accumulated yearly total streamflow and precipitation before the remarkable changing point on DMC.The average yearly streamflow of Tangwang, Xinancha and Yongcui watersheds is 243.42mm,292.73mm, and 355.55mm, respectively. The variation tendencies of yearly total streamflows in Tangwang and Yongcui showing an unremarkable decreased change, and that of Xinancha watershed appears to be increasing, yet insignificant. However, in the quasi-pair watersheds, namely Yulin and Xinlin, unremarkable rising trend and a falling trend can been seen in Xinlin and Yulin, respectively. The Mann-Kendall test result of the Yulin is also insignificant, but in the Spearman rank test, it is equal to the statistical quantitance |T|, which manifests that, the Yulin watershed with small area is sensitive to the impacts of extraneous factors. A notable rising trend of temperature can be seen in different watersheds in Xiaoxingan' Mountains, in which, the average temperature in Tangwang watershed from 1964 to 2006 is 1.1℃, the temperature rising gradient of slope is 0.468℃/10a (p< 0.0001), while that in Xinancha and Yongcui watersheds is 1.0℃,0.429℃/10a (p< 0.0001),1.1℃,0.468℃/10a (p< 0.0001), respectively. Which indicated that the climate condition of Xiaoxingan'Mountains will approch a warming period in the coming future. However, the precipitation is different. The precipitation in Tangwang and Yongcui watersheds from 1964 to 2006 is unremarkably growing (p=0.8929,p=0.5317), while those in Xinancha and quasi-pair watersheds are not notably falling. The information mentioned above illustrates that the spatial variation exist of precipitation in different watersheds.The impact of precipitation changes on streamflow is significant, and it is the key factor that influence the water resources system in Xiaoxingan'Mountains forested watersheds in the future. With the assumption scenarios, when the precipitation increases 20%, the streamflow in Tangwang and Xinancha watersheds will be increased by 20.81% and 39.30%, respectively, Yongcui and Yulin will be increased by 32.23% and 31.56%, respectively, only 23.89% in Xinlin watershed. With the different characteristics of underlying surface and different impacts of extraneous factors in different watershed, the responses of streamflow to precipitation are different.The impacts of forest cover changes on streamflow in different watersheds are not alike. Between 1976 and 2006, the forest area showed a decreased change in Tangwang watershed, whose change rate is-6.85% and forest area decreases 1326 square kilometers, and which makes the average accumulated yearly total streamflow get less by 232.643mm/3.65%, and that in the year of 2006 is 456.58mm/4.19%. Between 1973 and 2006, the forest areas in Xinancha watershed showed a growing trend from 66.01% to 88.48%, and which raises the accumulated yearly total streamflow, with an average growth of 579.04mm/6.48% and the total rise in 2006 is 801.17mm/7.00%. Between 1973 and 2006, the forest area in Yulin watershed increased from about 20% to 85%. But, when the forest area of Yulin is beyond to the control watershed, namely Xinlin, its streamflow show a relative decreased trend. The relative contribution impacts of climate and forest cover changes are different. Among the years from 1983 to 2006 in Xinancha watershed, the relative contribution impact of climate change on yearly total streamflow is 11.55%/36.OOmm, and that of forest cover changes is 11.04%/34.73mm. However, climate changes have a negative impact on yearly total streamflow, while forest cover changes a positive one. Nevertheless, with the raise of percentage of forest area, the relative contribution impact on streamflow is going down. When the percentage of forest area is beyond 87%, the influence of vegetation is negative; whereas that of climate changes on yearly total streamflow is positive. Therefore, for forest reasonable manage and water resources sustainable development and utilization in the future, the percentage of forest area should be less than 87% in Xiaoxingan' Mountains forested watersheds which alike Xinancha watershed.