One of the key challenges in the total maximum daily load(TMDL)development process is how to define the critical condition for a receiving waterbody.The main concern in using a continuous simulation approach is the ab...One of the key challenges in the total maximum daily load(TMDL)development process is how to define the critical condition for a receiving waterbody.The main concern in using a continuous simulation approach is the absence of any guarantee that the most critical condition will be captured during the selected representative hydrologic period,given the scarcity of long-term continuous data.The objectives of this paper are to clearly address the critical condition in the TMDL development process and to compare continuous and event-based approaches in defining critical condition during TMDL development for a waterbody impacted by both point and nonpoint source pollution.A practical,event-based critical flow-storm(CFS)approach was developed to explicitly addresses the critical condition as a combination of a low stream flow and a storm event of a selected magnitude,both having certain frequencies of occurrence.This paper illustrated the CFS concept and provided its theoretical basis using a derived analytical conceptual model.The CFS approach clearly defined a critical condition,obtained reasonable results and could be considered as an alternative method in TMDL development.展开更多
Lake Fuxian is the largest deep freshwater lake in China. Although its average water quality meets Class I of the China National Water Quality Standard （CNWQS）, i.e., GB3838-2002, monitoring data indicate that the w...Lake Fuxian is the largest deep freshwater lake in China. Although its average water quality meets Class I of the China National Water Quality Standard （CNWQS）, i.e., GB3838-2002, monitoring data indicate that the water quality approaches the Class II threshold in some areas. Thus it is urgent to reduce the watershed load through the total maximum daily load （TMDL） program. A three-dimensional hydrodynamic and water quality model was developed for Lake Fuxian, simulating flow circulation and pollutant fate and transport. The model development process consists of several steps, including grid generation, initial and boundary condition configurations, and model calibration processes. The model accurately reproduced the observed water surface elevation, spatiotemporal variations in temperature, and total nitrogen （TN）, total phosphorus （TP）, and chemical oxygen demand （COD） concentrations, suggesting a reasonable numerical representation of the prototype system for further TMDL analyses. The TMDL was calculated using two interpretations of the water quality standards for Class I of the CNWQS based on the maximum instantaneous surface and annual average surface water concentrations. Analysis of the first scenario indicated that the TN, TP and COD loads should be reduced by 66%, 68% and 57%, respectively. Water quality was the highest priority; however, local economic development and cost feasibility for load reduction can pose significant issues. In the second interpretation, the model results showed that, under the existing conditions, the average water quality meets the Class I standard and therefore load reduction is unnecessary. Future studies are needed to conduct risk and cost assessments for realistic decision-making.展开更多
Economic valuation of ecological restoration most often encompasses only the most tangible ecosystem service benefits,thereby omitting many difficult-to measure benefits,including those derived from enhanced reliabili...Economic valuation of ecological restoration most often encompasses only the most tangible ecosystem service benefits,thereby omitting many difficult-to measure benefits,including those derived from enhanced reliability of ecosystem services.Because climate change is likely to impose novel ecosystem stressors,a typical approach to valuing benefits may fail to capture the contribution of ecosystem resilience to sustaining long-term benefits.Unfortunately,we generally lack predictive probabilistic models that would enable measurement and valuation of resilience.Therefore,alternative measures are needed to complement monetary values and broaden understanding of restoration benefits.We use a case study of Chesapeake Bay restoration(total maximum daily load)to show that ecosystem service benefits that are typically monetized leave critical information gaps.To address these gaps,we review evidence for ecosystem services that can be quantified or described,including changes in harmful algal bloom risks.We further propose two integrative indicators of estuarine resilience-the extent of submerged aquatic vegetation and spatial distribution of fish.Submerged aquatic vegetation extent is indicative of qualities of ecosystems that promote positive feedbacks to water quality.Broadly distributed fish populations reduce risk by promoting diverse responses to spatially heterogeneous stresses.Our synthesis and new analyses for the Chesapeake Bay suggest that resilience metrics improve understanding of restoration benefits by demonstrating how nutrient and sediment load reductions will alleviate multiple sources of stress,thereby enhancing the system’s capacity to absorb or adapt to extreme events or novel stresses.展开更多
基金This work was supported in part by the Virginia Department of Environmental Quality and the Virginia Environmental Endowment,Richmond,Virginia,USA.
文摘One of the key challenges in the total maximum daily load(TMDL)development process is how to define the critical condition for a receiving waterbody.The main concern in using a continuous simulation approach is the absence of any guarantee that the most critical condition will be captured during the selected representative hydrologic period,given the scarcity of long-term continuous data.The objectives of this paper are to clearly address the critical condition in the TMDL development process and to compare continuous and event-based approaches in defining critical condition during TMDL development for a waterbody impacted by both point and nonpoint source pollution.A practical,event-based critical flow-storm(CFS)approach was developed to explicitly addresses the critical condition as a combination of a low stream flow and a storm event of a selected magnitude,both having certain frequencies of occurrence.This paper illustrated the CFS concept and provided its theoretical basis using a derived analytical conceptual model.The CFS approach clearly defined a critical condition,obtained reasonable results and could be considered as an alternative method in TMDL development.
基金supported by the National Natural Science Foundation of China (No. 41101180)the China National Water Pollution Control Program (No.2010ZX07102-006)
文摘Lake Fuxian is the largest deep freshwater lake in China. Although its average water quality meets Class I of the China National Water Quality Standard （CNWQS）, i.e., GB3838-2002, monitoring data indicate that the water quality approaches the Class II threshold in some areas. Thus it is urgent to reduce the watershed load through the total maximum daily load （TMDL） program. A three-dimensional hydrodynamic and water quality model was developed for Lake Fuxian, simulating flow circulation and pollutant fate and transport. The model development process consists of several steps, including grid generation, initial and boundary condition configurations, and model calibration processes. The model accurately reproduced the observed water surface elevation, spatiotemporal variations in temperature, and total nitrogen （TN）, total phosphorus （TP）, and chemical oxygen demand （COD） concentrations, suggesting a reasonable numerical representation of the prototype system for further TMDL analyses. The TMDL was calculated using two interpretations of the water quality standards for Class I of the CNWQS based on the maximum instantaneous surface and annual average surface water concentrations. Analysis of the first scenario indicated that the TN, TP and COD loads should be reduced by 66%, 68% and 57%, respectively. Water quality was the highest priority; however, local economic development and cost feasibility for load reduction can pose significant issues. In the second interpretation, the model results showed that, under the existing conditions, the average water quality meets the Class I standard and therefore load reduction is unnecessary. Future studies are needed to conduct risk and cost assessments for realistic decision-making.
基金This work was partially supported by funding from the US EPA Office of Research and Development,and we thank Brenda Rashleigh and Naomi Detenbeck for their project management and useful insights.
文摘Economic valuation of ecological restoration most often encompasses only the most tangible ecosystem service benefits,thereby omitting many difficult-to measure benefits,including those derived from enhanced reliability of ecosystem services.Because climate change is likely to impose novel ecosystem stressors,a typical approach to valuing benefits may fail to capture the contribution of ecosystem resilience to sustaining long-term benefits.Unfortunately,we generally lack predictive probabilistic models that would enable measurement and valuation of resilience.Therefore,alternative measures are needed to complement monetary values and broaden understanding of restoration benefits.We use a case study of Chesapeake Bay restoration(total maximum daily load)to show that ecosystem service benefits that are typically monetized leave critical information gaps.To address these gaps,we review evidence for ecosystem services that can be quantified or described,including changes in harmful algal bloom risks.We further propose two integrative indicators of estuarine resilience-the extent of submerged aquatic vegetation and spatial distribution of fish.Submerged aquatic vegetation extent is indicative of qualities of ecosystems that promote positive feedbacks to water quality.Broadly distributed fish populations reduce risk by promoting diverse responses to spatially heterogeneous stresses.Our synthesis and new analyses for the Chesapeake Bay suggest that resilience metrics improve understanding of restoration benefits by demonstrating how nutrient and sediment load reductions will alleviate multiple sources of stress,thereby enhancing the system’s capacity to absorb or adapt to extreme events or novel stresses.
