| Vegetation dynamic change and water cycle process interact tightly with each other,which is an important ecological process in natural system.The influence of vegetation change on hydrological process is very complex,and usually needs to be studied specifically,for the research conclusions can be inconsistent and depend on the scales and positions of watersheds,as well as study periods.Poyang Lake watershed is one of the important ecological reserves,which has international significance.The ecohydrological process in the watershed affects the material exchange and energy transfer processes,and the biological succession in the watershed is also driven by water cycle process.Thus,the study of eco-hydrological process in Poyang Lake watershed is of great significance for the ecological safety and the construction of ecological economic zone in Poyang Lake watershed.This research based on the relationships between Leaf Area Index(LAI)of vegetation and the evapotranspiration and interception in the water cycle process.And the impact of vegetation inner-annual changes on water cycle process was studied by building hydrological model coupled with LAI remote sensing products.This study comprehensively analyzed the precipitation,pan evaporation,LAI and runoff process inner annual changes in Poyang Lake watershed,and summarized the relationships between each hydrologic meteorological elements and LAI.The evapotranspiration model which is suitable for Poyang Lake watershed was built by combining the relationship between the inner-annual change of evapotranspiration and the precipitation,pan evaporation,LAI.Meanwhile,the rainfall interception model which incorporated with LAI was improved according to the traditional Aston intercept model and the measured data in Poyang Lake watershed.Furthermore,the evapotranspiration model and the improved interception model were coupled into the traditional Storedfull Runoff Model in order to establish the hydrological model considering LAI change.At last,the influence of LAI change on hydrological process in Poyang Lake watershed was analyzed by comparing the difference between the simulation results of traditional Stored-full Runoff Model and improved hydrological model.The conclusions are as follows:(1)The inner-annual changes of monthly rainfall,pan evaporation,runoff,LAI in Poyang Lake watershed during 2000-2014 all follow the trend of increasing firstly and then decreasing.The peak values of precipitation and runoff appear in June,while the peak values of pan evaporation and LAI appear in July.The inner-annual change trend of runoff in each sub-watershed is basically consistent with precipitation,but there still exist differences between the two variables in the specific month.(2)The inner-annual change trend of evapotranspiration have high consistency with LAI,while have low consistency with precipitation.The correlation coefficients between the monthly ET and the monthly LAI,pan evaporation,precipitation,and moving average of the precipitation per 2 months were 0.969,0.894,0.380 and 0.591,respectively.The moving average of the PRE per 2 months was used to replace the monthly PRE as a variable in the model.The accuracy of the evapotranspiration model was high where the R value for the model was 0.98 and the relative error of the model was 10.2%when the data of 2014 were used to validate the model built according to the data during 2000-2013.(3)The established interception model which based on the Aston interception model incorporated with LAI could consider the actual process of rainfall interception more accurately.The improved model only considers actual interception while ignores the evaporation loss from interception.And the ability of the model to calculate rainfall interception in a single day are expanded to calculate interception of rainfall in continuous days by considering the water holding capacity of vegetation before precipitation in each day.(4)The hydrological model which could reflect the influence of LAI was established by coupling the evapotranspiration model and the interception model into Stored-full Runoff Model.Daily flow data in the outlets in each sub-watersheds during 2000-2001 were used to calibrate the hydrological model.According to the evaluation index including NSE and R values,the simulation results in Ganjiang watershed,Xinjiang watershed and Fuhe watershed are relative accurate,and the NSE values and R values all exceed 0.6 and 0.9 respectively.Then daily flow data of three subwatersheds in 2007 were used to validate the model.The verification results show that the hydrological model perform good in the three watersheds and meet the precision requirement,for the NSE and R values indexes also reach 0.6 and around 0.9,respectively.Compared with the original Stored-full Runoff Model,the improved hydrological model can simulate flood peak processes more accurately.(5)The simulation results of improved hydrological model and original Storedfull Runoff Model were compared,and the result shows that the impact of vegetation LAI change on flow is more significant in summer than other seasons.According to the inner-annual change of the difference between monthly runoff simulation results of two models,the impact of vegetation is more significant in summer when LAI is high and the effect would fade in winter when LAI is low.Vegetation is luxuriant in summer,and runoff would be reduced through the processes of interception and evapotranspiration.While in winter,leaves wither and the influence of vegetation on runoff process is weaken but the impacts of other factors like precipitation on runoff increase.The runoff depth difference of two models in Ganjiang watershed which has relative high LAI is larger than other two sub-watersheds,which also shows that the runoff process would be affected by LAI more significant in regions with higher LAI values.The innerannual change of the ratio of the sum of evapotranspiration and interception to precipitation was analyzed,and result reflects that the influence degree of the evapotranspiration and interception on runoff process fluctuates within the year,which results from the desynchrony of the changes of LAI and precipitation.Under the control of LAI and precipitation,the contribution of evapotranspiration and interception on runoff process appears relatively complex inner-annual change characteristic. |
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