Research On Water Environmental Carrying Capacity Of Qiantang River Watershed

In China, water resources shortage because of water pollution becomes the vital problem to economical development and it is the key influential factor of the carrying capacity of water environmental in watershed. How to evaluate water environmental carrying capacity exactly has become a hot topic in watershed water-quality management. In order to study water environmental carrying capacity of watershed, taking Qiantang River watershed in Zhejiang province as an example. This dissertation aimed to develop methods of calculating the capacities to assimilative chemical oxygen demand(COD), ammonium(NH₄⁺-N) and biochemical oxygen demand(BOD) using different water quality models. According to the results of assimilative capacity, the load reduction and allocation in typical river segment of Qiantang River watershed were studied on the basis of the principles of Total Maximum Daily Load. And the water environmental carrying capacity of Qiantang river was evaluated based on index system under different social-economic development scenarios. The main results are summarized as follows.1. The quantitative method of COD and NH₄⁺-N assimilative capacities were established. The capacity of the main stream of the Qiantang River and its primary tributary to assimilative COD and NH₄⁺-N were calculated using the 1-D and 2-D water quality model. The results indicated that the total COD and NH₄⁺-N assimilative capacity calculated based on 75% confidence instream flow condition is 252239 t·a^-1 and 15369 t·a^-1 respectively. The total COD and NH₄⁺-N assimilative capacity of mainstream in Qiantang River watershed decreased in the following order: Xinan > Fuchun Segment > Qiantang Segment > Lan Segment > Qu Segment. The total COD and NH₄⁺-N assimilative capacity, calculated according to district in Qiantang River watershed, decreased in the following order: Hangzhou> Quzhou > Jinhua > Shaoxing > Lishui.2. The BOD assimilative capacity calculation model and quantitative method were developed. The capacities of the main stream of non-tidal Qiantang River and its primary tributary to assimilate BOD were studied using two theoretical scenarios. Scenario 1 was based on a QUAL2K model, while Scenario 2 was based on a 1-D model. The BOD assimilative capacity calculated in the basis of the QUAL2K model was larger than that of the 1-D model. The m value assimilative capacity method based on the QUAL2K model is suitable for Qiantang River watershed, as it integrated Total Maximum Daily Load with pollutant concentration control. The total BOD assimilative capacity of Qiantang River watershed is 105809 t·a^-1 under 75% confidence instream flow-condition, and it decreased in the following order: Fuchun Segment > Qiantang Segment > Xinan Segment > Qu Segment > Lan Segment. The total BOD assimilative capacity, calculated according to district in Qiantang River watershed, decreased in the following order: Hangzhou > Jinhua > Quzhou > Shaoxing > Lishui.3. The exploratory study for the technique of water-quality target management was carried out. In order to address the imbalance between water quality and designated use in Yiwu Segment, Jinhua River watershed, a environmental management mode for the watershed was developed based on assimilative capacity. The assimilative capacity of BOD and NH₄⁺-N of the Yiwu Segment, Jinhua River watershed, was estimated on the basis of QUAL2K model, the assimilative capacity of COD was estimated on the basis of 1-D model. The COD, NH₄⁺-N and BOD Total maximum daily load were developed while considering both point and non-point sources. The COD, NH₄⁺-N and BOD assimilative capacity of Yiwu segment were 7264.6 kg·d^-1, 431.20 kg·d^-1 and 4865.5 kg·d^-1 respectively. Under present condition, pollutant load reduction for COD, NH₄⁺-N and BOD reached 79.38%, 88.71% and 49.63% respectively, and the water quality of Yiwu River watershed matched its designated use. The Best Management Practices such as ecological agriculture, riverfront vegetated buffer zone were suggested to reduce load from non-point sources. On the other hand, the load from point sources can not be ignored. 4. The evaluation method of water environmental carrying capacity under different social-economic development scenarios was constructed. According to the socioeconomic development planning demand and water pollution state in Qiantang River watershed, an index system to evaluate the carrying capacity of watershed is designed from the aspects of economy, society, resources, environment, and technology, and a three-layer hierarchy-structured water environmental carrying capacity evaluation model was proposed. The current situation of water environmental carrying capacity was evaluated and different scenarios of medium or long period were analyzed using the water environmental carrying capacity evaluation model. The result of current situation evaluation showed that the carrying capacity of water environment in three regions from year 2000 to 2005, decreased in the following order: Hangzhou > Jinhua > Quzhou, except 2001. The scenarios analysis indicated that all of the four kinds measure, such as maintain current situation, save water, increase investment of environmental protection, reduce amount of COD per unit discharge, could improve the water environmental carrying capacity of Qiantang River watershed in 2010 and 2015 respectively.The quantitative technique research of water environmental carrying capacity will be useful for industrial structure adjustment and developmental pattern optimization, and can provide scientific basis for pollutant total amount control implementation and water quality improvement in watershed.