Modeling Runoff Responses To Forest Changes In The Upper Zagunao Watershed Under Nonstationary Conditions

The relationship between forest changes and water has been an important issue in the field of ecological hydrology for the past 100 years.As the watershed scale increases,the spatial heterogeneity of watershed characteristics increases,resulting in different hydrological responses to forest changes.In addition,the climate-hydrological and vegetation-hydrological response relationships of the watershed under changing environments are nonstationary,enlarging difficulty to the quantification of hydrological processes in large watersheds.Traditional mathematical statistical methods and hydrological simulation methods cannot take into account the non-stationarity and spatial heterogeneity of the watershed in quantification.The lack of appropriate research methods and a more comprehensive understanding of hydrological processes will result in the failure of the forest and water resources management practices in the watershed to achieve the desired results,and may even cause serious negative impacts on the ecology of the watershed.Therefore,combining the non-stationarity and the spatial heterogeneity of the watershed to quantitate the hydrological response of changing watershed is still a difficult and hot point in hydrological research.This study takes the upper Zagunao watershed in northwestern Sichuan as the research object,and uses the Modified Double Mass Curve method(MDMC),the PageHinkley Test concept drift detection algorithm and the ARIMA Intervention break point detection method to identify the non-stationarity of the vegetation-hydrological response relationship.Based on the results of the non-stationarity test,the study period is divided into three sub-periods,which is used to calibrate and verify the parameters in a distributed model.A model(SWAT)with three different sets of parameters is built which can reflect the hydrological characteristics of the unsteady watershed.In order to reveal the impact of the spatial heterogeneity of watershed characteristics on the hydrological response to watershed forest changes,this paper constructs forest change scenarios with different elevations,slopes,slope aspects,and precipitation gradients to simulate and predict the streamflow response to forest changes in different terrain and climate gradients.The amount of hydrological response will ultimately provide a more direct scientific basis for forest restoration and water resources management in a changing environment.The main conclusions of this study are as follows:(1)Forest harvesting lead to the increase of annual runoff and wet season runoff in the upper Zagunao watershed,but the response trend of dry season runoff is related to the vegetation type after conversion.Among them,the conversion of forest to dry land leads to an increase in dry season runoff,and the conversion of forest to grassland and shrubs leads to a decrease in dry season runoff.Forest restoration will reduce annual runoff and wet season runoff,and increase dry season runoff.As the proportion of forest changes increases,the degree of runoff response increases.A logging rate of 15% can cause significant changes in annual runoff,a logging rate of 10-15% can cause significant changes in dry season runoff,and a logging rate of 25% can cause significant changes in wet season runoff.This ratio is different due to factors such as vegetation type after conversion,topography and climate.(2)The degree of runoff response of forest harvesting varies with terrain and climate gradients:(a)Among various forest conversion scenarios,only the annual and seasonal changes in runoff when the forest is converted to dry land are significantly different in each altitude gradient zone.In general,the response of runoff to forest logging in the high altitude(3400-3700m)area is significantly lower than that of other gradient zones.For example,in the area of 2800-3100 m above sea level,converting 6% of forest to dry land will increase the annual runoff by 16.79 mm,while in the area of 3400-3700 m above sea level,this value is only 5.73 mm.(b)When forest is converted to dry land,the annual and seasonal runoff responses are significantly different in each slope gradient zone.When forest is converted to grassland and shrubs,only the seasonal runoff has a slope effect.The results show that the runoff response degree in the steep slope area(30-45°)is significantly higher than that in the gentle slope area.For example,the conversion of 10%of forest to dry land on a gentle slope(15-30°)leads to an increase in annual runoff by19.86 mm,while conversion in steep slope areas leads to an increase in annual runoff by22.51.mm.(c)The impact of the conversion of forest to dry land on annual and seasonal runoff has significant differences in various slope aspects.(d)The change in annual and seasonal runoff caused by the conversion of forest to dry land has significant differences in the precipitation gradients,while the conversion of forest to grassland and shrubs only has a significant difference in the impact of dry season runoff with different precipitation.The overall performance is that areas with less precipitation(less than 800mm)have a greater degree of runoff response,but as the cutting area increases(greater than 15%),areas with greater annual and wet season runoff responses will shift to greater precipitation.For example,the conversion of 5% of forest to dry land in areas with precipitation less than 800 mm and 800-1000 mm leads to an increase in annual runoff by 11.45 mm and 2.64 mm,respectively.When the conversion ratio rises to 15%,the annual runoff will be respectively increase 45.64 mm and 45.87 mm.In the dry season,the maximum runoff response is always in areas with low rainfall.(3)The degree of runoff response of forest restoration is affected by some topographical factors and climate:(a)When grassland and shrubs are converted to forest,there is no significant difference in the response of annual and seasonal runoff to forest changes with changes in altitude and slope.(b)The response of seasonal runoff varies significantly with the change of slope.The steeper the slope,the higher the hydrological response.For example,the conversion of 10% of the grassland in the steep slope area to forest land has a 1.81 mm higher impact on the wet season runoff than the gentle slope area.(c)The annual and wet season runoff response varies significantly with the slope direction,and the runoff response degree in the shady slope area is significantly higher than that in the sunny slope area.(d)The annual and seasonal runoff responses to forest reforestation vary significantly with precipitation,and the hydrological response is generally higher in areas with large precipitation.(4)The hydrological response of forest harvesting and forest restoration has spatial heterogeneity due to different terrain and climate.Therefore,for the practice of forest harvesting,the conversion of forest in high-altitude areas(3400-3700m)and gentle slope areas can be considered in practical applications to minimize its impact on the hydrological function of the watershed;In each slope aspect and precipitation gradient zone,the actual harvesting ratio and the converted vegetation type should be used to determine practical operation in which areas will cause less hydrological impact.Regarding the practice of forest restoration,in order to reduce its impact on the hydrological process of the watershed,forest restoration should be avoided in steep slope areas,shady slope areas and areas with high precipitation(1000-1200mm).