Study On The Health Assessment Of Small Watershed Ecosystem Based On The Occurrence Characteristics Of Phosphorus Pollution

As the first branch of the dam and the largest tributary of the Hubei section in the Three Gorges Reservoir Area, Xiangxi River becomes the backwater area with the water storage of the Three Gorges Project, and its geographical position is very important. Since 2004, the main stream and tributaries of Xiangxi River will bloom phenomenon in different degrees, in every year from February to October. The duration of water bloom is longer and longer, and the continuous range of water bloom is more and more wide, which shows that the dominant species of the duration of water bloom in Xiangxi River has changed from non toxigenic dinoflagellates, diatoms, green algae to toxigenic cyanobacteria, which has seriously affected the stability of the ecological system of Xiangxi River, and also threaten the safety of water quality in the Three Gorges Reservoir area. It is urgent to develop ecosystem health assessment work in Xiangxi River watershed to curb the trend of the deterioration of water quality in Xiangxi River and to maintain the safety of water environment and ecosystem health in Xiangxi River watershed.This paper discusses the related theory of watershed ecosystem health evaluation and methods, taking the small watershed of Xiangxi River as the research object, by selecting the "pressure-state-response" model and using the method of comprehensive index evaluation to make comprehensive evaluation of ecosystem health in Xiangxi River small watershed. The results are as follows:1. To make comprehensive evaluation of water environmental quality in Xiangxi River small watershed. We selected 12 sampling points of the 5 indicators about PH, dissolved oxygen, COD, total nitrogen and total phosphorus in four seasons of the Spring(March), summer(June) and autumn (September) and winter (December) in2015. ① The results of the single factor evaluation method to evaluate the water quality of the river basin is:the water quality indexes of PH, dissolved oxygen, COD can meet the requirements of water quality standard class I; for total nitrogen and total phosphorus, all sampling points are beyond the requirements of water quality standard class III except for some single sampling points to meet the requirements of water quality standard class III. Therefore, total nitrogen and total phosphorus is the main reason for the excessive water basin, the control of total phosphorus and nitrogen emissions is the focus of future water pollution control work of the Xiangxi River basin. ② The results of the comprehensive pollution index method to evaluate he water quality of the river basin is:under the premise of standard for surface water environmental quality standard2.class Ⅲ water quality standards as evaluation criteria, from the average value of 12 sampling points in four seasons we can see that the comprehensive pollution index is autumn(1.2494) less than summer(1.2515) less than spring(1.3822) less than summer winter(1.5157), the water quality is heavy pollution.③ The results of the evaluation of water eutrophication is:all the sampling points of the water trophic status are all in the mesotrophic level and above the mesotrophic level, and it shows an increasing trend from downstream to upstream. The average value of the whole basin TSIM is 66.6, which belongs to the rich nutrition level.3.Follow the principle of establishing the evaluation system and the selection of the evaluation index, constructing the framework of "pressure-state-response" model as an index system, selecting 14 indicators of pressure index (fertilizer use intensity, pesticide application rate, population density, human disturbance index) and state index (dissolved oxygen, chemical oxygen demand, total nitrogen, total phosphorus, with an average annual rainfall, annual evaporation) and response index (the intensity of soil erosion, vegetation coverage, land reclamation index, per capita GDP) to compose the ecosystem health assessment index system in Xiangxi River watershed, the evaluation criteria of each individual index are determined at the same time.4.By using analytic hierarchy process to analyze the weight of index, and the analytic hierarchy process and the comprehensive index method to establish the evaluation model of comprehensive index of multi-index. The comprehensive evaluation standards of watershed ecosystem health standards are determined, and the watershed ecosystem health status is divided into 5 levels of very healthy, healthy, sub-healthy, unhealthy and sick.5. The results of comprehensive evaluation of ecosystem health in Xiangxi River Watershed is:from the average of all sampling points we can see that the Xiangxi River Watershed is in a sub-healthy state; from 12 sampling points sections we can see that only Baisha and Gufu River sampling points sections are in a healthy state, the rest of the 10 sampling points section are in a sub-healthy state; the proportion of "sub-healthy" sampling points sections is as high as 83.3%. It shows that Xiangxi River watershed is generally in a sub-healthy state.