The Response Mechanism Of Water Quality To The Use Of Upstream Basin In Dahuofang Reservoir

This study selected Dahuofang reservoir as research area to explore the key pollutants and water quality change process via quantitative evaluation based on long terms of water quality, land use, soil erosion and water transfer input. In the view of to clear the key factors effected on water quality before and after transfer water input, the water quality changes, the relationship between water quality and the three factors of land use, soil erosion and water transfer input are analyzed comprehensively. The main conclusions are as follows:(1) The main pollutants of Dahuofang reservoir water quality are TN, TP, CODMn, Coli-group, BOD and NH3-N. The water quality data collected by 7 fixed observation section which including 3 water inlet sections (Beizamu, Taigou and Gulou),2 sections at the middle of reservoir (Hun No.73 and Hun No.37),2 water oulet section of Hunhe (Hun7 and water-intake poimt of Funshun).The mean values change trend in different years of the indexes of all sections is the same as the trend of each sections, but the water quality of Beizamu section is the most worse, followed by No.73 section, the water quality of other sections in line with class Ⅱ water standard. The key pollutants of Dahuofang reservoir is TN, TP, followed by CODMn, Coli-group, BOD and NH3-N.(2) The changes of land use in the upstream of Dahuofang reservoir in recent 10 years, in addition to the agricultural land, the construction land, forest land, orchard land, grassland and unused land changed obviously, and the areas of forest land and construction land increased 1.3% and 0.93% respectively, the areas of orchard land, grass land and unused land decreased 0.44%,1.99% and 0.8% respectively. The main types of land use structure did not changed in nearly 10 years, the area of forest land> farmland> construction land> grassland> unused land> orchard, and the change rete of unused land> grassland> orchard construction land> forest land> farmland.There have obvious correlation between water quality of each section and the changes of land use of upstream in Dahuofang reservoir before transfer water input and after. The forest land area percentage has significantly negative correlation with water quality of N, P, etc indexes in both before and after water diversion which played an important role on water quality protection and reduced the loose of N, P, etc in soil. Construction land construction land area percentage has significantly positive correlation with water quality of N, P, etc indexes in both before and after water diversion which transmitted the N, P, etc pollutants in to downstream water. The area percentage of grassland and unused land have significant positive correlation with water quality of N, P, etc indexes before water diversion, but after water diversion have significant negative correlation maybe for the reasons of ecological management in the upstream of Dahuofang reservoir. The area percentage of orchard has obvious positive correlation with water quality of N, P, etc indexes in both before and after water diversion which can not held nutrient runoff due to its area is too small.(3) The soil erosion area decreased 36371 hm2 nearly 10 years, almost reduced 23.92% compared to 10 year ago. The areas of mild erosion and moderate erosion showed a trend of decreased, but strong erosion and above strong erosion showed increased in some extent. The annual soil erosion change rate of different degrees is strong erosion> very intensity erosion > mild erosion> moderate erosion> severe erosion.The correlation between water quality and the percentages of soil erosion area and different degrees of soil erosion area is obvious before and after the water diversion of Dahuofang reservoir. The percentages of soil erosion and mild erosion area have significant positive correlation with water quality of N, P, etc. indexes before and after the water diversion of Dahuofang reservoir. This phenomenon showed soil erosion is the key factor affected the downstream water quality deterioration, and also is the main zones of agricultural non-point pollution due to lots of soil loosed in mild erosion areas are rich in nutrients. The percentages of moderate erosion, strong erosion and very intensity erosion area have significant negative correlation with water quality of N, P, etc. indexes, before and after the water diversion of Dahuofang reservoir. The above phenomenon showed the soil loosed in these zones lack in nutrients, and the contribution to water quality deterioration is smaller than mild erosion. The percentage of severe erosion area has no obvious correlation with the indexes of water quality, but has positive correlation with CODvin and NH3-N in some sections before and after the water diversion of Dahuofang reservoir.(4) Since the operation of Dahuofang reservoir water transfer began, the total input of DO is 6739.17t, total input of CODMn is 1735.00t, total input of BOD is 625.83t, and the total input of NH3-N, TP, TN, Coli-group and Chl-a are 12.07t,12.5t,1869.75t,1.38×1011 and 4966.75t respectively. The input indexes of water diversion have a significant correlation with water quality indexes of Dahuofang reservoir. While the input of TP didn’t appeal obvious regularity with water quality indexes of each sections. The input of TN was significantly correlated with the TP of three water quality sections, as well with the TN of two water quality sections. And the input of DO was significantly correlated with the TN of all water quality sections, while have a significant correlation with the TP of three water quality sections, and correlated with permanganate indices of only one sections. At the same time, there was significant positive correlation between permanganate indices and TP of five sections, and TN of four sections.(5) The result of comprehensive analysis of Dahuofang reservoir which was based on entropy weight method shows that the weight sequence of water input factors is TN>Coli-group> NH3-N>BOD>DO> CODMn>TP. The weight sequence of land utilization factors is construction land>other land> garden plot>meadowland>forest land. The weight sequence of soil erosion factors is very intensity erosion>mild erosion> moderate erosion> strong erosion>severe erosion. By analyzing three impact factors, the weight of water input factor is 0.543, and the weight of land utilization factor is 0.299, while the weight of soil erosion is 0.157. According to the result, a targeted water ecological protection system should be built after combine all of impact factors of watersheds with reeds resistance.In summary, the key pollutants like N and P of Dahuofang reservoir increased in recent ten years, TN and TP reached highest value in 2013 after water diversion, the water quality was decreased. Land utilization, soil erosion and water input have significant correlation with water quality of Dahuofang reservoir. The interception capacity to pollutant of meadowland and timberland in Dahuofang reservoir was increased after water diversion, and it have significant negative correlation with key factors like TN and TP. Meanwhile, the area of soil erosion and mild erosion area with high content of nutrient loss were decreased. But the water quality of Dahuofang reservoir was between Ⅱ and Ⅲ level before water diversion, but at Ⅲ level after water diversion. By analysing the relation between water quality and land utilization, soil erosion and other factors, it can be seen that land utilization, soil erosion before water diversion are the impact external cause of water quality of reservoir, and the input after water diversion became the main controlling factor of water quality of reservoir, the influence to water quality of land utilization and soil erosion was decreased. The weight sequence of impact factors of water quality of Dahuofang reservoir after water diversion is water input factor>land utilization factor> soil erosion factor. According to the results of the analysis, this research suggest build artificial wetland at water inlet and river flow period, put forward the improvement measures for important impact factors, establish a comprehensive ecological protection system to secure the water quality of Dahuofang reservoir.