| Precipitation runoff is an important factor leading to the loss of nitrogen and phosphorus in slope land.While the loss of nitrogen and phosphorus is an important cause of non-point source pollution in the upper reaches of water source areas.Therefore,the study on surface runoff(SR)and subsurface runoff(SSR)in the upper reaches of water source forests of reservoirs and the law of nitrogen and phosphorus loss has attracted much attention.In this paper,the Yaoxiang small watershed in the upper reaches of Tai'an City huangqian reservoir was taken as the research area,which was the hilly area in middle-southern part of Shandong Province.The pinus densiflora forest(PDF),which is a typical tree species in this area,was selected as the research object,and the characteristics of surface runoff,subsurface runoff,and nitrogen and phosphorus loss of pinus densiflora forest were studied by artificial simulation of secondary rainfall test method,with the wasteland as the control.This paper discusses the proportional load and migration law of nitrogen and phosphorus transfer from forestland surface runoff and subsurface runoff in order to provide theoretical basis for water source conservation forest configuration and non-point source pollution control and benefit evaluation.The main research results are as follows:(1)Under the condition of secondary rainfall,the runoff yield characteristics of surface runoff and subsurface runoff in Pinus densiflora forest and wasteland are significantly different.With the development of rainfall,the surface runoff yield of the Pinus densiflora forest and wasteland under the condition of secondary rainfall increases continuously.the time of abortion runoff in the Pinus densiflora forest and the wasteland lagged behind the surface runoff,and the fluctuation of the runoff in the subsurface runoff was less than that of the surface runoff.The surface runoff of Pinus densiflora forest is lower than that of wasteland,which is 92.66% of that of wasteland.The runoff rate in the subsurface runoff of Pinus densiflora forest was 1.32 times higher than that in the wasteland.There was a positive correlation between subsurface runoff infiltration and vegetation coverage in Pinus densiflora forest and wasteland.Compared with wasteland,Pinus densiflora forest has bettereffects on increasing rainfall infiltration,reducing runoff loss and water conservation.(2)Compared with wasteland,Pinus densiflora forest can effectively reduce the loss of nitrogen in runoff under secondary rainfall.Under the condition of secondary rainfall,the total nitrogen(TN)in surface runoff showed a high output concentration in the early rainfall,and the output concentration decreased and gradually tended to be stable as the rainfall progressed.The TN loss in the subsurface runoff remained relatively stable throughout the runoff process.There was a positive correlation between TN loss and loss concentration of surface runoff in Pinus densiflora forest and wasteland.The TN loss of surface runoff and subsurface runoff in Pinus densiflora forest is lower than that in wasteland;the TN loss of surface runoff in Pinus densiflora forest is 81.37% of that in wasteland,and the TN loss in subsurface runoff in Pinus densiflora forest is 86.23% of that in wasteland.It shows that Pinus densiflora forest has better regulation effect on TN in runoff compared with wasteland.(3)Compared with wasteland,Pinus densiflora forest can effectively control the loss of phosphorus in rainfall runoff under secondary rainfall conditions.Under the condition of secondary rainfall,the total phosphorus(TP)output concentration of surface runoff showed a higher trend in the early rainfall and gradually decreased with the rainfall,and the TP output concentration of subsurface runoff showed a dynamic equilibrium trend in the process of rainfall.Under the condition of secondary rainfall,the TP output concentration was positively correlated with the amount of runoff.The TP loss of surface runoff and subsurface runoff in Pinus densiflora forest was lower than that of wasteland,the TP loss of surface runoff in Pinus densiflora forest is 53.32% of that in wasteland,and 72.35% of that in the subsurface runoff of Pinus densiflora forest.This shows that the regulation effect of TP in Pinus densiflora forest is more significant than that in wasteland.(4)Compared with the early rainfall,the rainfall runoff and nitrogen and phosphorus loss of both Pinus densiflora forest and wasteland increased under the secondary rainfall,but the ratio of runoff and nitrogen and phosphorus loss in wasteland was larger,which further indicated that the regulation effect of Pinus densiflora forest on runoff and nitrogen and phosphorus loss was more significant under the secondary rainfall.Specific performance: Under the condition of secondary rainfall,the total runoff yield of the Pinus densiflora forest and the wasteland area was greater than that of the early rainfall,which was 121.0% and 142.0% of the early rainfall,respectively,among them,the surface runoff and subsurface runoff in Pinus densiflora forest were respectively135.0% and 93.7% of the early rainfall.The surface runoff and subsurface runoff in wasteland were 134.0% and 187.0% of the early rainfall,respectively.Under the condition of secondary rainfall,the total TN loss in the runoff of the Pinus densiflora forest and the wasteland was both greater than the early rainfall,which was 102.0% and 119.0% of the early rainfall,respectively,among them,the amount of TN loss from surface runoff and subsurface runoff in the Pinus densiflora forest was 125.0% and 73.7% of the early rainfall respectively,the amount of TN loss from surface runoff and subsurface runoff in the wasteland was respectively 109.0% and 116.0% of the early rainfall.Under the condition of the secondary rainfall,the total TP loss in the runoff of the Pinus densiflora forest and the wasteland was greater than that of the early rainfall,which was 111.0% and 129.0% respectively,among them,the TP loss of surface runoff and subsurface runoff in the Pinus densiflora forest was115.0% and 77.6% respectively of the early rainfall,the TP loss in surface runoff and subsurface runoff in the wasteland was 117.0% and 110.0% of the early rainfall,respectively. |
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