Precipitation will generate runoff after a series of loss such as plant interception filling,evaporation,infiltration,etc.Runoff is the main carrier of non-point source pollution such as nitrogen and phosphorus,which leads to the loss of nutrients in water and soil,and further causes the gradual increase of water environment pollution.Therefore,clarifying the characteristics of natural rainfall,discussing the runoff mechanism and nitrogen and phosphorus migration characteristics under different scenarios has certain theoretical and practical significance for the analysis of regional rainfall runoff law and the scientific control of non-point source pollution.Taking Yongnian District of Hebei Province as an example,based on the hourly rainfall data in the past 39 years,the frequency ranking method is used to analyze the typical rainfall process in different periods.At the same time,using the Linmingguan Artificial Rainfall-Runoff Test Site of Hebei Engineering University as the test platform,a test plan for runoff and nitrogen and phosphorus migration was designed.Through the observation of runoff,detecting the relevant indicators of water samples and soil samples.From the perspective of the change trend of runoff and flow,the distribution characteristics of runoff and soil nutrient concentration,etc.,to discuss the characteristics and laws of typical plots under different vegetation,rain intensity and slope conditions,and to analyze the characteristics of wheat plots in different growth periods.The main research results are as follows:(1)The typical rainfall process in flood season and non-flood season in North China Plain is designed.Based on the frequency analysis method about rainfall and rainfall duration was applied to design the typical artificial rainfall processes in the annual flood and non-flood seasons.The designed rainfall process in flood season illustrates such characteristics: the duration is short,process change is remarkable and concentrating in the rear.The designed rainfall process in flood season is 16.5mm,and it speak value is 6.2mm,the rainfall duration is 3h,and the process has a back single peak.The corresponding characteristics in non-flood season are those: duration is long,process change is slow and mainly concentrating in the middle and rear.the design rainfall of typical process in non-flood season is 15.9 mm,and its peak value is 2.6mm,the rainfall duration is 10 h,and the process has a single center peak.The design rainfall process with different characteristics in different periods can better reflect the time-history characteristics of regional rainfall.(2)The runoff mechanism of the North China Plain under different vegetation,rainfall intensity and slope were discussed.The surface runoff process was highly consistent with the rainfall process,and the greater the rain intensity,the greater the surface runoff flow;the sub-surface runoff had a certain hysteresis,and the smaller the rain intensity,the more obvious the hysteresis.With the increase of slope,the surface runoff time is advanced,and the runoff flow increases;the sub-surface runoff time is behind,and the runoff flow decreases.The vegetation coverage and growth process have a significant impact on the runoff and runoff process.The greater the vegetation coverage,the slower the surface runoff time and the runoff flow.With the growth of the plants,the vegetation root system under the crop rotation system increased the porosity in the middle of the experimental soil tank,and the abortive flow in the soil at the maturity stage of wheat was faster and increased in magnitude than that at the seedling stage,and the effect was more significant.(3)The migration law of nutrients with runoff and soil at different growth stages of wheat was analyzed.The runoff nutrient loss has a positive effect on the runoff,the larger the runoff,the greater the nutrient concentration in the runoff;the soil nutrient loss has a negative effect on the runoff,and the effect is more significant in the deep soil.Due to the absorption and interception of nitrogen and phosphorus by vegetation roots,the loss of nutrients in the soil can be reduced.And with the growth of vegetation and the continuous development of root system,the interception effect becomes more and more significant.Among them,the concentration of total nitrogen is reduced by50%~60%,the concentration of ammonium nitrogen is reduced by 50%~68%,and the concentration of total phosphorus is reduced by 55%~78%.In comparison,the total phosphorus concentration decreases greatly,indicating that phosphorus is more easily absorbed and fixed by the soil.Therefore,promoting water infiltration and reducing surface runoff flow can greatly reduce the output of non-point source pollution such as nitrogen and phosphorus.The research results provide certain reference and reference for the analysis of the runoff law in the North China Plain and the origin and scientific control of nonpoint source pollutants such as nitrogen and phosphorus. |