Study On Characteristics Of Rainfall Runoff And Flux Of Non-point Nitrogen And Phosphorus In Controlled Watershed Of Shihaoqiao In Gan River,Hubei Province

Being a representative of the agricultural non-point pollution,Xianning city of Hubei province as a typical key area of agricultural production has already been the main source of water environment pollution rather than point pollution.This thesis discusses the experiment of controlling basin water carried out in Shihaoqiao section Gan River in Xianning city of Hubei province,studies the relationship between rainfall in Shihaoqiao watershed and runoff combining with GIS technology,historical records,in-situ monitoring and laboratory tests.The main findings are as follows:(1)Shihaoqiao Watershed has a characteristic mountain area performance.The watershed area is 605 km2.Land-use types are forest land,meadowland and such kind like original ecological land.The territory of forestry in Shihaoqiao is relatively higher in natural landscape.Forest area,paddy field,urban and industrial land,water area,grassland,bare land area account for different percent of total area which are 11.00%,7.38%,2.59%,1.00%,12.32%,0.05% respectively.Forest land area is 397.24 km2,of which dominated by forest and open forest.Grass land area is 75.54 km2,which is still very much area with high-covered grassland and low coverage grassland.Agricultural land area is 111.20 km2,paddy field and dry land of which occupies 66.55 km2 and 44.65km2.Bare soil area of 0.30 km2 is mainly involved by red soil,limestone soil,paddy soil,ratio of which are 67.3%,17.2% and 14.5% respectively.The red earth compromises 2 subgroups of red earth and red soil.The lime soil is dominated by brown limestone soil.The paddy soil is represented by waterlogged type.These two has shallow layer,which is easy to be formed as the loss of surface runoff and soil erosion when it meets heavier rainfall,but paddy soil won't be like this.(2)The main characteristics of rainfall runoff in Shidaoqiao watershed is the uneven distribution of rainfall and runoff during the year.The annual runoff coefficient is positively correlated with rainfall level.During the flood season from April to September,the proportion of total rainfall accounts for 69.2 while 30.8% in non-flood period.The maximum annual rainfall can reach 2116.3mm and the minimum is only 953.3mm.The maximum runoff is 1633.31 mm and the minimum is 601.7mm.The runoff coefficient of this basin varies from 0.51 to.57 and the water consumption of agricultural irrigation increased during the year of dry water.Human activities affect the runoff obviously.(3)The amount of river runoff is positively influenced by the magnitude of rainfall.The results show that the process of river flow are basically the same in 6 in-situ monitorings,and the response time of water level and flow is 1 ~ 3h later rainfall,and also the time of flood peak is 8.5 ~ 11.5 h later than the rainfall peak.The flood peak has a direct relationship with short duration and high intensity rainstorm.Furthermore,the maximum event rainfall intensity is positively correlated with discharge of flood peak.The impact of previous rainfall is an important indicator of the degree of soil moisture,which affect greatly on event rainfall runoff.At the same time,the runoff process and amount are quite different because of different rainstorm center.57% of the rainfall turned into river runoff when the rainstorm center moves to the upstream,however only 37% stayed in the upstream.There is also a different in both flood peak and flood peak time.(4)The load of nitrogen and phosphorus is positively correlated with rainfall and runoff.The larger the rainfall,the more the runoff is produced.Although the nitrogen and phosphorus concentration is reduced,the pollutants migrate more in the river and also the total amount of pollutants is larger.Analyzing historical data and N and P fluxes of event rainfall runoff,it is found that the fluxes of total nitrogen,nitrate nitrogen,nitrite nitrogen and ammonia nitrogen are consistent with the discharge,while the flux of total phosphorus changes more smoothly.The peak time of ammonia nitrogen and total nitrogen is earlier than that of runoff.Large and medium-sized floods produce pollutant far more than ordinary floods or low flood.The average annual discharge of the river basin is 18.06m3/s and the total nitrogen load module calculated from data of multi-years is between 470.2 and 932.5kg/a·km2.(5)Nitrate nitrogen is the main component of the nitrogen pollutants in the river runoff.