Urban Best Management Practices Research Articles

Evaluating the Hydrologic and Water Quality Impacts of the Revised SCS CN Method and its Implications on Decision-Making

Highlights The revised CN equation uses decreased CN values, causing decreased initial abstraction values and increased runoff. SWAT’s water balance approach simulated increased runoff to decrease groundwater flow and associated baseflow with the revised SCS CN method. Landscape analysis showed minor changes in runoff and nutrient loadings, except in row cropping areas in high runoff potential soils. Reduced mitigation occurs if post-development runoff increases by a greater margin than the pre-development runoff. Abstract. The Soil Conservation Service (SCS) Curve Number (CN) method is a widely used model developed by the Natural Resources Conservation Service (NRCS) that estimates surface runoff generated from a land area using factors such as land cover, soil type, and precipitation depth. However, because this empirical model was developed over 60 years ago with limited data, the NRCS is considering revising it with the help of an ASCE-ASABE task group. The proposed revisions include updating the initial abstraction (Ia) ratio in the CN equation and modifying the curve number (CN) for all land use and soil hydrologic group combinations. The proposed revisions are expected to alter the hydrologic response estimation, especially for low flow events. Since surface runoff is driving sediment, nutrients, and the transport of chemicals in natural systems, this study aims to understand how predicted hydrologic and water quality variables vary between the original and the proposed revised versions of the SCS CN method using the Soil and Water Assessment Tool (SWAT). Additionally, a case study site was selected to understand how the revisions alter the design storm runoff analysis. Results from the simulation case study on a 36,900 ha mixed land-use watershed located in central Pennsylvania, USA indicate a 5% to 14% increase in annual runoff predictions with the revised equation compared to the original equation. Though most average monthly runoff depths increased by 1% to 25% using the revised equation, winter months generally simulated runoff depths that were 1% to 5% lower than the original equation. Average daily loadings at the watershed outlet increased by 11% to 19% and 4% to 11% for nitrate and mineral phosphorus, respectively. The SWAT simulation results suggest that current agricultural and urban best management practices (BMPs) will experience higher runoff volumes on a yearly time scale due to the decrease in the Ia term, while the design storm case study results suggest that urban BMPs can be smaller in size when rural lands are urbanized due to the reduction in runoff estimation from the initial land use with the revised equation. Additional detailed evaluations of the proposed SCS CN method are required for water quality simulations, stormwater management, and BMP designs. Keywords: Hydrology, Revised curve number method, SCS Curve Number Method, Soil and Water Assessment Tool, Water quality.

Evaluation of selected watershed characteristics to identify best management practices to reduce Nebraskan nitrate loads from Nebraska to the Mississippi/Atchafalaya River basin

Nebraskan streams contribute excess nitrogen to the Mississippi/Atchafalaya River Basin and Gulf of Mexico, which results in major water-quality impairments. Reducing the amount of nitrogen (N) exported in these streams requires the use of best management practices (BMPs) within the landscape. However, proper BMP utilization has rarely been statistically connected to potential controls of N export within watersheds, particularly precipitation and soil characteristics. In this study, 19 watershed variables were evaluated in five categories (hydrological, physiographic, point sources, land use, and soil properties) to determine the characteristics that influenced variable nitrate nitrogen (NO3-N) concentrations in 17 Nebraska watersheds with known high NO3-N export rates. Each characteristic was derived from publicly-available datasets in an effort to develop a multiregional method. Of the 19 variables evaluated, 10 variables (developed, cropland, herbaceous, forest, excessively-drained soils, precipitation, base-flow index, slope, organic matter and point sources) were identified to statistically influence stream NO3-N concentrations. The 17 watersheds were divided into five subset groups using principal component analysis. Distributions of the 10 watershed variables were then used to determine the most applicable BMPs for NO3-N reductions for each stream subset: excessively drained with high baseflow index (Groups 1 and 2), dominantely row crop land usage with well-drained soils, higher precipitation, and an increased tendency for surface runoff concerns (Group 3), highly developed watersheds (Group 4), and single river dominated by wastewater treatment plant discharge (Group 5). Based on the most influential variables a variety of BMPs were recommended, including N fertilizer application management and accounting for N credit from mineralization and NO3-N in irrigation water (Groups 1 and 2), installation of riparian buffers and wetlands (Group 3), urban BMPs such as bioretention cells and permeable pavement (Group 4), and upgrades to the wastewater treatment plant (Group 5). This study provides an improved technique for facilitating watershed management by linking BMPs directly to the characteristics of each watershed to reduce current nitrate export.

Modeling framework for representing long-term effectiveness of best management practices in addressing hydrology and water quality problems: Framework development and demonstration using a Bayesian method

Best management practices (BMPs) are popular approaches used to improve hydrology and water quality. Uncertainties in BMP effectiveness over time may result in overestimating long-term efficiency in watershed planning strategies. To represent varying long-term BMP effectiveness in hydrologic/water quality models, a high level and forward-looking modeling framework was developed. The components in the framework consist of establishment period efficiency, starting efficiency, efficiency for each storm event, efficiency between maintenance, and efficiency over the life cycle. Combined, they represent long-term efficiency for a specific type of practice and specific environmental concern (runoff/pollutant). An approach for possible implementation of the framework was discussed. The long-term impacts of grass buffer strips (agricultural BMP) and bioretention systems (urban BMP) in reducing total phosphorus were simulated to demonstrate the framework. Data gaps were captured in estimating the long-term performance of the BMPs. A Bayesian method was used to match the simulated distribution of long-term BMP efficiencies with the observed distribution with the assumption that the observed data represented long-term BMP efficiencies. The simulated distribution matched the observed distribution well with only small total predictive uncertainties. With additional data, the same method can be used to further improve the simulation results. The modeling framework and results of this study, which can be adopted in hydrologic/water quality models to better represent long-term BMP effectiveness, can help improve decision support systems for creating long-term stormwater management strategies for watershed management projects.

Mosquito Production in Stormwater Treatment Devices in the Lake Tahoe Basin, California

In response to increasing evidence of mosquito production in structural stormwater Best Management Practices (BMPs), a collaborative project was developed to document the occurrence, species composition, and seasonal abundance of mosquitoes from selected urban and highway BMPs in the Lake Tahoe Basin, California. Structural and environmental factors associated with mosquito production in highway BMPs were identified and analyzed. Ten species of mosquitoes were collected from 47 BMPs, including Culex tarsalis, Culiseta incidens, Cs. inornata, and 7 species of Aedes. In and around South Lake Tahoe, immatures were most abundant in urban BMPs during the warmer summer and fall months, whereas natural water sources in the surrounding area harbored mosquitoes more often during the colder months of early spring. In BMPs installed along Lake Tahoe's perimeter highways, mosquitoes were observed in 11% of site visits conducted during a single season. Larval presence in highway BMPs was positively associated with water temperature and negatively associated with precipitation, sand, and unspecified organic matter. The significance of mosquito production in BMPs of the Tahoe Basin and the potential for increased transmission of mosquito-borne disease are discussed.

Assessing the performance of urban BMPs in Scotland

Best Management Practices (BMPs) in urban drainage have been promoted in Scotland for a number of years in response to the need to combat pollution arising from diffuse sources in urban areas. This has led to the construction of around one hundred BMP installations using both source and end of pipe control. These are principally in the East of Scotland, and the number is growing rapidly as education and knowledge increases. A programme of investigations into the factors which influence the performance of the systems commenced in 1997. A range of types of BMP are under study including both source control and end of pipe systems. The sites have been divided into groups for which different depths of investigation are being undertaken. The programme involves a number of parallel investigations requiring field studies, data reviews and enquiries to confirm applicability and performance in situ. Many of the factors which influence the selection, installation and operation of a particular system are social, legal and administrative in addition to those which relate to its design and construction. The different strands of the study have been developed to ensure that all influences are identified and evaluated.