Changes Of Hydrological Connectivity Of Microtopography Surface In Sugarcane Area And Its Relationship With Nitrogen And Phosphorus Loss

As the main sugarcane planting area in China,Guangxi has an important economic position.The efficient and clean production of sugarcane is very important.The problem of agricultural non-point source pollution can not be ignored.Traditional runoff detection methods ignore the runoff path and the impact on the process,while hydrological connectivity can represent the smoothness of the hydrological process between different landscape units,describe the runoff path and pay attention to the hillside-river-exit system,so it has received extensive attention.Conducting hydrological connectivity studies is essential to better understand the mechanisms of nitrogen and phosphorus loss caused by fertilization and soil erosion.At present,the research on hydrological connectivity is concentrated on the large watershed scale,and there is less research on the microtopographic scale in the watershed.In order to reveal the influence of surface microtopography and farming measures on changes in hydrological connectivity and the relationship between hydrological connectivity and nitrogen and phosphorus loss,this study took the Nala small watershed in Chongzuo city,Guangxi Zhuang Autonomous Region as the research object.Based on high-resolution UAV images,the optimal DEM resolution of hydrological connectivity was explored,the temporal and spatial variation of hydrological connectivity of sugarcane in different growing periods in 2020 and2021 was analyzed,and the effect of surface micro-topographic feature parameters(slope S,slope length factor L,surface roughness S_R and standard deviation of elevation SDE),planting pattern(new planting and perennial),planting method(cross slope and parallel slope)and vegetation coverage on hydrology.According to the results,the improvement measures of the hydrological connectivity index were proposed,and the relationship between the hydrological connectivity and the loss of nitrogen and phosphorus in the basin was discussed in combination with the nitrogen and phosphorus loss data of the monitoring station.The main results and conclusions are as follows:(1)The IC(index of hydrological connectivity)and surface microtopographic feature parameters(L,S,S_R and SDE)are significantly affected by DEM resolution.With the decrease of the pixel size(5m-0.05m),The mean IC value first increased(5m-0.2m)and then decreased(0.2m-0.05m).The difference in the mean IC value of different land use types reached the maximum(34.72%)at 0.5 m.S and SR increased by 29.13%and 1.62%,respectively,and SDE and L decreased by 98.72%and 72.09%,respectively.The best DEM resolution of watershed is 0.1 m.(2)The hydrological connectivity of the basin has significant temporal and spatial differences with the growth period of sugarcane:Different regions of the basin have different connection potentials,and the hydrological connectivity of the basin fluctuates and decreases with the growth period of sugarcane.The difference of IC among different land use types in the same sugarcane growth period ranged from 17.94%to 40.59%;The maximum change rate of IC average value in the same sugarcane growth period in two years can reach 20.36%.The output of total dissolved nitrogen and total dissolved phosphorus at the outlet of each basin changes significantly with the growth period of sugarcane,and the hydrological connectivity has a strong correlation with the output of total nitrogen and total phosphorus(R~2 is 0.67 and 0.70 respectively),which can explain the change of 67%to 70%of the output of nitrogen and phosphorus in the basin.(3)Hydrological connectivity is affected by S,L,S_R,SDE,planting patterns,planting methods and vegetation coverage.The principal component regression results show that the seven influencing factors can explain 90%of the change of hydrological connectivity(R~2=0.90).After normalizing the influence weights of the seven influencing factors on IC,the order is SDE>vegetation coverage>S_R>area ratio of newly planted and perennial sugarcane>L>area ratio of sugarcane along and across slope>S.Therefore,SDE can be added to IC index as a weight for optimization.The results show that the a area ratio of newly planted and perennial sugarcane and the area ratio of sugarcane along and across slope affect the hydrological connectivity of the basin by affecting the vegetation coverage and surface roughness.