| The soil stone mountain area is an important headwaters area and headwaters of rivers in China.It is also a typical type area of soil erosion in China.Runoff evolution regulation and material migration-transformation regulation in headwater areas are of great significance to water resources security and ecological civilization construction in watersheds.Based on land use data in 2018 and monitoring data from July 2019 to December 2020 in the Minjia River watershed of the upper reaches of Danhan River,we used integrated landscape pattern analysis,isotope tracer analysis,spatial clustering analysis,redundancy analysis(RDA),and partial redundancy analysis(pRDA).The characteristics of land use composition,landform,and landscape configuration at different scales in the watershed were analyzed.The effects of comprehensive landscape patterns on seasonal water quality were elucidated,and the sirgle and comprehensive effects of land use,topography,and landscape configuration on seasonal water quality were further quantified.The law of water conversion and replenishment and its effect on the amount of nutrient migration were studied.The main achievements of this study are as follows:(1)The land use,landscape configuration,and topography vary significantly at different spatial scales.Forested land is the dominant land-use type at different spatial scales,with the proportion of forested land at the sub-watershed and buffer scales above 85.46%and 74.58%,respectively.Compared with the sub-watershed,the landscape fragmentation degree of the buffer zone is larger,the landscape heterogeneity is higher and the landscape shape is more irregular.The forested land is the dominant landscape at both scales.Topographic factors vary significantly with spatial scale,and the values of each topographic factor in the buffer zone are larger than those in the sub-watershed scale.(2)The sources of runoff replenishment varied significantly with the seasons,and the surface and subsurface h ydrological pathways had important effects on the nutrient migration concentration.Different nutrients had inherent specific mechanisms.More than 68.49%of the surface water recharge during the rainy season comes from atmospheric precipitation,while more than 77.60%of the recharge during the dry season comes from shallow groundwater.The stable isotopes of hydrogen and oxygen in different water bodies in the basin varied significantly with seasons(P<0.05).Temperature,relative humidity,and solar radiation are the main factors affecting the stable isotope content of precipitation.(3)The spatial and temporal variation of surface water quality indicators is obvious.The water quality in the upstream and midstream areas is better than that in the downstream areas,and the water quality in the rainy season is worse than that in the dry season.Compared with the dry season,the total nitrogen(TN)content increased by 34%and the total phosphorus(TP)decreased by 23%during the rainy season.TN,electrical conductivity(EC),water temperature(WT),dissolved oxygen(DO),and acidity(PH)were greater in the middle and lower reaches than in the upper reaches,and the seasonal differences were significant(P<0.05).nitrate nitrogen(NO3--N)was greater in the upper reaches,and the seasonal differences of each index were significant;TP and ammonia nitrogen(NH4--N)spatial differences varied with season.(4)Buffer zones better explain water quality changes than sub-basin-scale landscape indicators.The integrated landscape pattern explained more than 91.2%of the water quality changes during the wet season,and the total explanation decreased by 2.6 to 3.9%during the dry season.The total explanation of water quality changes by landscape indicators at the sub-watershed scale exceeded 87.3%.Compared with the sub-watershed scale,the explanation at the buffer scale increased by 7.6-8.9%.The most important explanatory variables for water quality changes at the sub-watershed scale are the landscape shape index of woodland and the proportion of woodland area,and the most important explanatory variables at the buffer scale are the proportion of woodland area and the hypsometric integral(HI).(5)Significant factors of seasonal water quality variation were attributed to the combined effect of landscape configuration,land use composition,and topographic factors,which contributed more than 59%to seasonal water quality variation.Among the three types of factors,the landscape configuration alone explains more of the water quality changes,while the influence of topographic factors and land use composition on water quality varies by seasonal and spatial scales.At both scales,the separate explanatory degree of the rainy season was ranked as follows:landscape configuration>land use composition>topographic factor,and the land-use composition explained the least water quality changes in the dry season. |
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