| The typical grassland inland river basin is an important ecological barrier in northern of China,which has a profound feedback effect on the natural and socio-economic system.However,due to the continuous degradation of grassland,the problem of soil and water loss is becoming more and more serious,which leads to a series of problems such as the decline of soil fertility,land productivity,soil nutrient loss and so on,resulting in the gradual weakening of the two-way regulation function of ecological hydrology,which seriously hinders the sustainable development and ecological security of the region.As the basic hydrological unit of the slope,the research on the process of soil and water loss on slope is conducive to find out the interaction among rainfall runoff,soil erosion and nutrient loss,so as to provide guidance for the maintenance of eco-hydrological relationship in grassland watershed.Therefore,this thesis took the natural slope and small watershed of semi-arid typical grassland as the research objects,using the method of combining natural rainfall with in-situ artificial simulated rainfall.The changes of soil moisture,runoff and sediment yield and nutrient loss of different slope positions and vegetation treatments under different experimental conditions were observed.The response process of soil moisture on slope was analyzed,the effects of different infiltration response types on water movement process were clarified,and the process of water-sediment-nutrient transport on slope and its coupling mechanism were explored.Slope rainfall runoff model,soil erosion model,sediment nutrient transport model and their coupling model were constructed,on the basis of which a small watershed scale water-sediment-nutrient coupling model was developed.The main achievements are as follows:(1)The soil water content and soil water storage increased in the downslope direction,and all showed as BG > SG> CK.The dry and wet changes in fast-changing layer(5 cm)and active layer(10 cm)were rapid,while soil moisture below 20 cm was relatively stable and fluctuated only in heavy or continuous rainfalls.The rainfall replenishment threshold and the required initial soil profile water content of soil moisture response in 5 cm,10 cm and 20 cm soil layers were 5.8 mm,8.0 mm,11.4 mm and 8.7 %,9.4 %,10.8 %,respectively.The soil moisture response process varied greatly under different rainfall,rainfall intensity and antecedent soil moisture conditions,which explained 41.1% of the total difference.Soil properties,vegetation characteristics and topography could explain 38.8%,14.5% and 5.6%of the soil moisture variation on the hillslope.In addition,under the comprehensive influence of environmental factors,changes in soil moisture of the upslope were significantly affected by soil sand content,the differences in the midslope were mainly due to soil clay content and belowground biomass,whereas the vegetation characteristics were the main factors in the downslope.(2)According to the temporal patterns and rate distribution of soil moisture response,two types of preferential flow(PF)were identified,namely,non-sequential response(NSR)and fast preferential flow in sequence response(PF-rate).The frequency of PF was 15.9%–44.5%,the frequency of PF-rate was 1.4-2.7 times that of NSR,and its contribution to the total infiltration was more than 50%.The conditions for the occurrence of the two PFs were inconsistent and different from the studies which showed the higher probability of PFs under dryer conditions and with higher rainfall intensities.Small rainfall events with low rainfall intensity and relatively low soil moisture at most locations showed higher NSR frequency,which further highlight the strong dependence of NSR on hydrophobicity and macropore structure of the topsoil.Whereas a higher rainfall intensity increased PF-rate under wet conditions.Meanwhile,the occurrence of PF-rate was also significantly controlled by vegetation coverage,soil texture and saturated hydraulic conductivity of 5-10 cm soil layer.Furthermore,the water recession rate of NSR and PF was slower and faster than that of matrix flow,respectively.(3)Rainfall intensity and initial soil water content were the main factors affecting soil infiltration rate,which could explain 47.0-54.8% of the variation of soil infiltration rate,while vegetation characteristics and soil topography explained 11.6-16.7% and 3.3-15.6%,respectively.The initial runoff time and initial rainfall loss decreased with the increase of rainfall intensity and initial soil water content,and increased with the increase of vegetation.The relative contribution rates of rainfall intensity,soil water content and vegetation coverage to runoff rate were 33.07-53.13%,7.73-14.70% and 6.49-12.59%,respectively.Rainfall intensity,rainfall erosivity and initial runoff rate could jointly explain 65.81% of the variation of initial sediment yield rate,while steady sediment yield rate and average sediment yield rate were more affected by slope gradient and runoff process.The relationship between slope runoff and sediment yield process can be described by linear function.The runoff sediment yield index and soil erodibility obtained from the linear function both decrease with the increase of vegetation and increase with the increase of slope gradient.(4)The runoff rate,runoff water depth,Reynolds number,Froude number,runoff shear force,runoff power and unit runoff power increased with the increase of rainfall intensity,while the Darcy-Weisbach coefficient and Manning roughness coefficient decreased with the increase of rainfall intensity.The response of each hydrodynamic parameter to the initial soil moisture content varied with different locations.Higher vegetation coverage and lower slope gradient can reduce runoff Reynolds number,Froude number,flow shear force and runoff power.In addition,vegetation also had obvious effect of reducing flow velocity and increasing resistance.The Reynolds number and runoff power of hydrodynamic parameters could characterize the flow generation process.However,the hydrodynamic parameters used to characterize the sediment yield process were different in different slope positions.In addition,the loss of organic carbon and total nitrogen can be well characterized by the hydrodynamic parameters.(5)Compared with the standard SCS-CN model,the improved rainfall runoff model based on equation structure and parameter determination method had higher predictive performance.The improved soil erosion model was obtained by replacing the rainfall erosion factor in the original RUSLE model with the rainfall runoff erosion factor,which could accurately predict the soil loss caused by the rainfall event.The improved model of nutrient transport with sediment based on the relationship between enrichment rate and environmental factors also performed well.The improved SCS-CN model and the improved RUSLE model were coupled with the sediment nutrient migration model.The coupled model could reasonably predict the slope runoff,soil loss and sediment nutrient loss of the rainfall event.Based on the above results,the three improved models were integrated into the SWAT model,and an improved SWAT model suitable for the study area was established,which could accurately predict the water,sediment and nutrient loss at the small watershed scale. |
PDF Full Text Request