Effect Of Vegetation Type Change On Runoff In Pailugou Catchment Of Qilian Mountains

In arid and semi-arid areas of Northwest China,mountainous area is an important water source forming area.Effective protection and reasonable restoration of forest vegetation are beneficial to water and soil conservation,and give full play to hydrological benefits.However,improper management of forest vegetation may lead to runoff reduction and inadequate utilization of water resources.Therefore,it is essential to regulate the relationship between forest vegetation and water resources.Qilian Mountain is a typical mountain water conservation forest area in this region.It is of great significance to carry out forest hydrological research in Qilian Mountains to explore the relationship between forest vegetation and hydrological process,reveal the influence of vegetation type change on water balance index of watershed,and put forward the model of scientific and reasonable water conservation vegetation types,which is important to keep the balance of ecosystem.This paper takes the typical catchment of Qilian Mountains which is named Pailugou as the research area.Based on the traditional method of location and semi-location observation,the hydrological process under different vegetation types of forest,grassland and shrub in Qilian Mountains is observed in order to find out the rules and internal relations of hydrological process.Based on the object-oriented classification method,classical statistics and geostatistical spatial variation theory,high-precision digital mapping and other methods to realize the spatial distribution of vegetation in watershed,spatial stratified heterogeneity of soil physical properties and watershed discretization and parameterization.Combined with the distributed hydrological model,the hydrological process and its mechanism of Pailugou catchment are studied comprehensively and synchronously from the point,index and scale.It is expected that it can provide some new understanding for the characteristics of hydrological process change and forest vegetation restoration in mountainous areas,and provide some scientific basis for improving the efficiency of water conservation and establishing a reasonable watershed water resources management model.The main results are as follows:(1)The classification of vegetation types in Pailugou catchment by object-oriented high-resolution remote sensing image classification is carried out with a total accuracy of89.08 and a total Kappa coefficient of 0.834.The classification results are consistent with the current vegetation distribution in the watershed.Picea crassifolia,grassland and shrub are mainly distributed in the elevation range 2600-3400m,2600-3000m and 2600-3600m,respectively.The shady area,semi-shady area,sunny and semiarid sunny accounted for53.5%,20.7%,12.5%and 13.3%of the total watershed area respectively.Picea crassifolia is the dominant species in shady area and semi-shady area,and grassland is mainly distributed in sunny and semiarid sunny,and shrubs are distributed in all four slope aspects.(2)The soil water characteristic curves of soil layers at different depths showed that under the same soil water potential,the soil water content of Picea crassifolia,grassland and shrub in 0-10cm soil layer is the highest,and the soil water content continued to decrease as the soil depth deepened to 60cm.The soil sand content,silt content,porosity,saturated moisture,saturated hydraulic conductivity,bulk density and organic carbon content of the three plantings were significantly different,but the clay content was not significantly different.Under the same water potential,the order of soil water capacity was as follows:Picea crassifolia,shrub,grassland.(3)Based on environmental variables and C5.0 classification decision tree algorithm,the overall accuracy of soil type map after verification is 89%,and the Kappa coefficient is0.83,indicating good classification effect.The prediction models of soil bulk density and soil texture spatial distribution were constructed by classification regression tree algorithm and linear stepwise regression algorithm respectively.The model were evaluated from two indexes of mean absolute error and root mean square error through field verification points,and the ideal mapping results are obtained.(4)According to the analysis of meteorological and runoff measured data in Pailugou catchment from 2015 to 2017,it is found that the precipitation in the basin is dominated by small precipitation events,accounting for 89%of the total precipitation,and the large and medium precipitation accounts for 11%of the total precipitation.In the 2700-3300m and3300-3500m elevation zones,the average rainfall increases by 25.47mm and decreases by10.48mm for each 100m elevation increase,respectively.The rainfall increases first and then decreases with the elevation increase.With the elevation increasing,the temperature in catchment decreases gradually,and the decreasing rate of temperature is 8.375 mm/km.(5)Based on the DEM extraction of watershed digital river network system,a total of17 sub-flow areas are divided into drainage ditch basin,144 hydrological response units.The monthly and daily runoff data from 2015 to 2017 were used to calibrate and verify the model.The parameters with high sensitivity to runoff change were SCS runoff curve number(CN2),snowfall temperature(SFTMP),maximum snow-melt factor(SMFMX),snow-melt base temperature(SMTMP).The final model simulation results show that the ENS value and R~2 are greater than 0.7,and the fitting effect of the simulated value and measured value conforms to the model evaluation standard.The results show that the runoff simulation in rainy season are better;the results in dry season are smaller;the simulation accuracy is poor in heavy rainfall event.SWAT model is suitable for Pailugou catchment.(6)SWAT model was used to simulate the runoff process under the scenario of vegetation type change.The runoff increased when the vegetation types were grassland,shrub and other vegetation types,but decreased when the vegetation types were Picea crassifolia.The surface runoff maintained between 0.09 and 0.18 under different vegetation types.The vegetation prone to surface runoff was grassland,not Picea crassifolia and shrub.Compared with the control,when the vegetation type was Picea crassifolia or combined with other vegetation types,the value of soil flow decreased.The evapotranspiration is the largest when the vegetation type is grassland.The results show that the vegetation combination model can reduce evapotranspiration,balance water budget and increase water yield to a certain extent,while a single vegetation model can reduce water yield and reduce the effective utilization of water resources in the middle and lower reaches of the catchment.At the same time,five optimal vegetation types for water conservation in Qilian Mountains were put forward:Picea crassifolia,shrub+grassland,Picea crassifolia+shrub,shrub,Picea crassifolia+grassland.