| Because of the typical geographical and climatic conditions of the Loess Plateau,the imbalance of the ecological environment has become increasingly serious.The problem of soil and water loss in the loess hilly-gully region was the most prominent,and the serious soil and water loss restricted economic and social development.The purpose of soil and water conservation was to improve the ecological environment.As a typical human activity,the soil and water conservation measures changed the characteristics of the underlying surface of the watershed and affected the hydrological cycle of the watershed.Small watersheds are the basic unit for soil and water conservation.Therefore,studying the influence of soil and water conservation measures on the runoff generation in small watersheds can provide scientific guidance for the management of soil and water loss and the reasonable allocation of soil and water conservation measures.In this paper,by taking Wangjiagou watershed,located in the first sub-region of the loess hilly-gully region as the study area.Using qualitative and quantitative analysis,and using models to simulated runoff generation and confluence.Explore the process of runoff generation and confluence under the influence of soil and water conservation.The main conclusions were as follows:(1)Rainfall and runoff characteristics of Wangjiagou and comparative observation watersheds were analyzed.The runoff mechanism of the Wangjiagou watershed changed around 1969,and the distribution interval of rainfall duration and runoff changed.The rainfall and rainfall duration changes in the watersheds were not obvious,but the distribution of runoff intervals changed significantly.In the comparative observation watersheds,the average annual runoff depth has been reduced by 54.7% compared with the uncontrolled watershed.(2)In comparative observation watersheds,the NRCS(Natural Resource Conservation Service)model was used for simulation after divided rainfall types.After calibrated and validated the model parameters,the suitable parameter combinations for different rainfall types were determined.After the soil and water conservation,the parameters changed significantly.The initial abstraction ratio of each rain type of Yangdaogou watershed varied greatly,ranged from 0.05-0.20,curve number was 40.72-92.00;the suitable value of initial abstraction ratio was 0.01 and curve number was 23.40-82.24 in Chacaizhugou watershed.(3)The modified NRCS-RF(Natural Resource Conservation ServiceRandom Forest)model was used to simulate runoff in each watershed.The average curve number of Wangjiagou,Yangdaogou,and Chacaizhugou watersheds was 66.42,81.63,64.42,and the average initial abstraction ratio was 0.09,0.15,0.06.Compared the conservation watersheds with different areas,the distribution form of curve number was similar,and the parameters of Wangjiagou watershed with the larger areas had large fluctuations and the value of the curve number was larger than the others.The distribution form of the initial abstraction ratio in the comparative observation watersheds was similar,and parameters of the uncontrolled watershed were larger,the value of initial abstraction ratio decreases after soil and water conservation and was less than the standard value.(4)The duration of the flood,peak time,peak flow,and lag time of the comparative observation watershed were analyzed,and the mean value of flood characteristics in the Chacaizhugou watershed increased by 14.3%,decreased by 3.0%,decreased by 62.2%,and increased by 7.1%,respectively.Soil and water conservation have the greatest impact on the duration of flood and peak discharge.The flood characteristics under different rainfall levels in the Wangjiagou watershed changed at different stages of governance.(5)Flood confluence was used NRCS unit hydrograph to calculate in the comparative observation watersheds to determine the parameters under different watersheds and rainfall types.From the time of concentration,lag time,and peak time,the parameters of watershed after soil and water conservation had been changed obviously.The average time of confluence,lag time,and peak time of Chacaizhugou watershed were 3.7 times,3.2 times and 2.6 times of Yangdaogou watershed,respectively.The instantaneous unit hydrograph was used to determine the parameters at different stages of the Wangjiagou watershed.The parameters of different stages and rainfall levels change with no clear rules,but n K gradually decreases during the flurry and heavy rain during each treatment stage,and n K gradually increases during moderate rain.Soil and water conservation had changed the micro-topography and increase the water storage capacity of the watershed,the process of runoff generation and confluence have changed significantly.(6)Analysis of the impact of different soil and water conservation measures on runoff.Runoff situations of the different soil and water conservation measures were significantly different.After a comprehensive analysis,there was a significant difference between sweet clover,ridge and cultivate grass,fish-scale.The rainfall characteristics that affect different soil and water conservation measures were different.The number of runoff times was the most under moderate rain,and heavy rainfall and above were the main sources of runoff.Determine the curve number of different soil and water conservation measures,sort by the curve number: level trench>slope farmland>sweet clover>level terrace>ridge> fishscale>cultivate grass. |
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