Environmental Effects And Spatial Allocation Of Pond System In Three Gorges Reservoir Area

Pond landscape plays an important role in the hydrological process and biogeochemical cycle in the basin.It has become a major scientific problem to study the effects of ponds lands on hydrological process and reduction of non-point source pollution.These scientific issures is helpful to reduce the pollution,to ensure the water quality and the water source of the Three Gorges Reservoir area.This study not only enriches the spatial allocation of ponds,but also understands the scientific significance of hydrological effects in different spatial locations.In view of the wide distribution of the pond landscape in the Three Gorges Reservoir area,this paper takes the Jinglinxi small watershed(20km2)in the Three Gorges Reservoir as an example.Based on the combination of high definition remote sensing image and field investigation,the quantity and distribution pattern of the pond are discussed,and the characteristics of the catchment area of a single pond are extracted with the aid of the flow direction algorithm.The sediment deposition in the basin is obtained by the method of combining the field observation with the model in the typical pond.The SWAT model was calibrated by the surface runoff,the output of nutrient elements and some other data obtained from the long-term field site monitoring.The hydrological process and the output process of nutrient elements are simulated,and the effect of the reservoir landscape in the pond and the basin in the material transport of the basin is revealed.The following conclusions are obtained:(1)The change of pond landscape in Jinglin watershed is obvious.The main characteristic is the increase in quantity and area.The density of the pond in the watershed has increased from 16 pond/km2 in 1983 to 31 pond/km2.The total area of the pond increased from 31.5ha in 1983 to 56.2ha in 2016.Most ponds have a smaller area,and 70%of the pond area is less than 1000 m2.The total drainage area of ponds accounted for 13.22%in 1983,while the total drainage area of ponds covering 35.4%.(2)The siltation of the existing ponds is serious,and the sediment sediment depth is between 0.8-5.0m in 19 ponds in the last 35-63 years.The sediment yield modulus(SSY)from the sediment deposition information in the pond is from 2.07 t ha-1 a-1 to 67.8 t ha-1 a-1.There is a significant correlation between the sediment yield modulus and the area of the catchment area,the slope farmland and the proportion of the paddy fields.Sloping farmland is the most important source of sediment.The amount of sediment interception by pond is 1.35x 104ta-1,which accounts for 9%of the total soil erosion in the whole basin.The sediment output of the whole basin accounts for 4%of the total basin erosion and the sediment output modulus is 2.9t ha-1 a-1.Both the pond and the reservoir changed the sediment output modulus of the river basin.The existence of ponds and reservoirs made the sediment output account for more than 1%and 2%of the total erosion volume.(3)The ponds and reservoirs in the Jinglin watershed have obvious interception of the flood peak flow.The ponds can reduce the peak flow rate by 26%,and the reservoir can reduce the peak flow rate of 41%.When the reservoir and ponds exist at the same time,the flood peak flow can be reduced by 57%.The existence of watershed changed the hydrological balance of watershed,including surface runoff,lateral flow,groundwater flow,infiltration,soil moisture,evapotranspiration and water yield.First,the existence of ponds reduces the surface runoff generated by the whole basin from 179.5mm to 160.4mm.The amplitude of the lateral flow is very large,up to 112mm.The pond increased the amount of groundwater recharge by 16.4mm.In general,the distribution of ponds significantly reduced the water yield of the basin,and the average water yield decreased from 535.8mm to 416mm.(4)The average output of TN in Jinglinxi watershed from 2012 to 2017 is 1.28×104kg,and the average output concentration is 1.52mg/L.The average output of NO3--N is 9123kg,and NO3--N is the main form of TN output.The total output of TP is much smaller than that of TN.The average level in the past 6 years is only TN 1/8,the total output is 0.15×104kg,and the average output concentration of TP is 0.17mg/L.Pond affects the output of nutrients in Jinglinxi watershed.But the reduction rate of each nutrient element is different,the reduction rate of the pond to TN reaches 31.7%.The existence of the pond reduces the output of NO3--N in the basin by 0.34x104kg,and the reduction of the pond to the NO3--N is 27%.The maximum reduction rate of the pond to the TP is 46%.The reduction rate of the reservoir to TN is 7.6%.The output ofNO3--N is only 0.07x 104 kg higher than that of the current situation,and the reduction rate of NO3--N to reservoir is only 6.9%.Compared with the first two nutrient elements,the reduction effect of reservoir on TP is more obvious,the reduction rate is 15.5%.(5)In view of the spatial optimization of ponds in the basin,we designed a watershed simulation-spatial optimization algorithm,which successfully realized the selection of the location of the newly built ponds and the existing ponds restoration.We have chosen the suitable location of the new pond,there are 106 new ponds in the potential position.The new pond can increase the storage capacity of 23.85×104m3,which can increase the area of the catchment area by 4.2 km2.In the watershed simulation-spatial optimization algorithm,the connection of SWAT simulation and non dominated sorting genetic algorithm is successfully realized by using the area of the catchment area of the pond as a decision variable.