Multi-time Temporal Scales Analysis For Forest Water Conservation In Discontinuous Watershed Based On SWAT Model

Forest water conservation function is an important part of forest ecosystem service function.Quantitative calculation of forest water conservation is the main content of forest water conservation function evaluation.Under the background of global climate change,human activities have intensified,and the spatial distribution of underlying surface of watershed has been changed frequently.The spatial distribution of forest vegetation is discontinuous and different.How to reveal the role of forests in the water cycle process from the simulation of water cycle process in watershed,and quantitatively calculate the temporal and spatial changes of forest water conservation in watershed with discontinuous forest distribution,is of great significance to further improve the theory of forest water conservation function.A semi-distributed hydrological model Soil and Water Assessment Tool?SWAT?is selected.Based on the analysis of the relationship between the accuracy of daily runoff simulation and the objective function,Nash efficiency coefficient(E_ns),a method for constructing the SWAT model of daily runoff is proposed,which reflects the spatial distribution of forest patches in the basin.The calculation method of forest water conservation and its response to spatial change of forest area in watershed with discontinuous forest spatial distribution at different time scales?year,month and day?was established,and the corresponding concept of forest water conservation coefficient was put forward,that is,the ratio of forest water conservation to corresponding precipitation was used to reflect the forest water conservation capacity of watershed.Taking Jinjiang River basin in southeastern coast as an example,which is characterized by rapid socio-economic development and obvious discontinuity of forest distribution,and using land use in 1985 and 2006,a daily runoff model reflecting two kinds of forest conditions was constructed.The annual,monthly and daily forest water conservation and its spatial variations under meteorological conditions from 2002 to 2010 were calculated respectively.The results show that:1)E_nss of daily SWAT model The discriminant criteria are different from annual and monthly scales.When the efficiency coefficient reaches about 0.7,the simulation accuracy of the model can meet the actual requirements,and the flood runoff is more sensitive to Ens and less sensitive to dry runoff.2)By setting the area thresholds of land use,soil and slope to 0,the matching of HRU with the actual spatial distribution of forest patches can be achieved,and the corresponding SWAT model can reasonably simulate the role of forests in watershed water cycle.3)There are obvious differences in forest water conservation function at different time scales.The annual conservation of forest water resources is positive.The annual average conservation coefficient of forest water resources is 0.243,and the difference between years is small.The monthly forest water conservation is mostly positive,while in some months it is negative.The positive and negative changes of monthly forest water conservation reflect the function of forest water storage between high water month and low water month.The forest water conservation is positive in flood season,negative in dry season,and the effect of forest water conservation is obvious.Forest water conservation is closely related to rainfall.The correlation coefficient of annual scale is the largest,monthly scale is the second,and daily scale is the smallest.Spatial distribution of forest is an important factor affecting spatial distribution of forest water conservation.4)The decrease of forest area leads to the decrease of forest water conservation.Under the condition of spatial change of forest area,the monthly change of forest water conservation has a strong correlation with monthly rainfall.In the month with large rainfall,the monthly change of forest water conservation is large.The change area is the main factor affecting the spatial distribution of forest water conservation change.5)The method of calculating the response of water conservation quantity and forest area to spatial change in watershed with discontinuous forest distribution based on distributed hydrological model is proposed in this study,which provides a new attempt for the evaluation of forest water conservation function in watershed.