John Riverson

A watershed-scale design optimization model for stormwater best management practices

U.S. Environmental Protection Agency developed a decision-support system, System for Urban Stormwater Treatment and Analysis Integration (SUSTAIN), to evaluate alternative plans for stormwater quality management and flow abatement techniques in urban and developing areas. SUSTAIN provides a public domain tool capable of evaluating the optimal location, type, and cost of stormwater best management practices (BMPs) needed to meet water quality and quantity goals. It is a tool designed to provide critically needed support to watershed practitioners in evaluating stormwater management options based on effectiveness and cost to meet their existing program needs. SUSTAIN is intended for users who have a fundamental understanding of watershed and BMP modeling processes. How SUSTAIN is setup described here using a case study, conducted by actual data from an existing urban watershed. The developed SUSTAIN model was calibrated by observed rainfall and flow data, representing the existing conditions. The SUSTAIN model developed two BMP cost-effectiveness curves for flow volume and pollutant load reductions. A sensitivity analysis was also conducted by varying important BMP implementation specifications.

BMP Analysis System for Watershed-Based Stormwater Management

Best Management Practices (BMPs) are measures for mitigating nonpoint source (NPS) pollution caused mainly by stormwater runoff. Established urban and newly developing areas must develop cost effective means for restoring or minimizing impacts, and planning future growth. Prince Georges County in Maryland, USA, a fast-growing region in the Washington, DC metropolitan area, has developed a number of tools to support analysis and decision making for stormwater management planning and design at the watershed level. These tools support watershed analysis, innovative BMPs, and optimization. Application of these tools can help achieve environmental goals and lead to significant cost savings. This project includes software development that utilizes GIS information and technology, integrates BMP processes simulation models, and applies system optimization techniques for BMP planning and selection. The system employs the ESRI ArcGIS© as the platform, and provides GIS-based visualization and support for developing networks including sequences of land uses, BMPs, and stream reaches. The system also provides interfaces for BMP placement, BMP attribute data input, and decision optimization management. The system includes a stand-alone BMP simulation and evaluation module, which complements both research and regulatory nonpoint source control assessment efforts, and allows flexibility in the examining various BMP design alternatives. Process based simulation of BMPs provides a technique that is sensitive to local climate and rainfall patterns. The system incorporates a meta-heuristic optimization technique to find the most cost-effective BMP placement and implementation plan given a control target, or a fixed cost. A case study is presented to demonstrate the application of the Prince Georges County system. The case study involves a highly urbanized area in the Anacostia River (a tributary to Potomac River) watershed southeast of Washington, DC. An innovative system of management practices is proposed to minimize runoff, improve water quality, and provide water reuse opportunities. Proposed management techniques include bioretention, green roof, and rooftop runoff collection (rain barrel) systems. The modeling system was used to identify the most cost-effective combinations of management practices to help minimize frequency and size of runoff events and resulting combined sewer overflows to the Anacostia River.

Projected 21st century trends in hydroclimatology of the Tahoe basin

With down-scaled output from two General Circulation Models (the Geophysical Fluid Dynamics Laboratory, or GFDL, and the Parallel Climate Model, or PCM) and two emissions scenarios (A2 and B1), we project future trends in temperature and precipitation for the Tahoe basin. With the GFDL, we also project drought conditions and (through the use of a distributed hydrologic model) flood frequency. The steepest trend (GFDL with A2) indicates a 4–5°C warming by the end of the 21st century. Trends in annual precipitation are more modest with a dip in the latter half of the 21st century indicated by the GFDL/A2 case, but not the others. Comparisons with the Palmer Drought Severity Index show that drought will increase, in part due to the declining role of the snowpack as a reservoir for soil moisture replenishment. Analysis of flood frequency for the largest watershed in the basin indicates that the magnitude of the 100-yr flood could increase up to 2.5-fold for the middle third of the century, but decline thereafter as the climate warms and dries. These trends have major implications for the management of land and water resources in the Tahoe basin, as well as for design and maintenance of infrastructure.

Nutrient and particle load estimates to Lake Tahoe (CA-NV, USA) for Total Maximum Daily Load establishment.

The Lake Tahoe Total Maximum Daily Load (TMDL) requires detailed methodologies to identify sources of flows and pollutants (particles and nutrients) for estimating time-variant loads as input data for the Lake Tahoe clarity model. Based on field data and a modeling study, the major sources of pollutant loads include streams (three subdivisions of this category are urban, nonurban, and stream channel erosion), intervening zones (IZs) (two subdivisions of this category are urban and nonurban), atmosphere (wet and dry), groundwater and shoreline erosion. As Lake Tahoe remains well oxygenated year-round, the contribution of internal loading from the bottom sediments was considered minor. A comprehensive quantitative estimate for fine particle number (< 16 μm diameter) and nutrient (nitrogen and phosphorus) loading is presented. Uncertainties in the estimation of fine particle numbers and nutrients for different sources are discussed. Biologically available phosphorus and nitrogen were also evaluated. Urban runoff accounted for 67% of the total fine particle load for all sources making it the most significant contributor although total urban runoff was only 6%. Non-urban flows accounted for 94% of total upland runoff, but the nitrogen, phosphorus and fine sediment loadings were 18%, 47% and 12%, respectively of the total loadings. Atmospheric nitrogen, phosphorus, and fine particle loadings were approximately 57%, 20%, and 16%, respectively of the total loading. Among streams and IZs, IZ 8000, Upper Truckee River, Trout Creek, Blackwood Creek, and Ward Creek are the top fine particle, nitrogen and phosphorus contributors. The relative percentage contribution of inorganic fine particles from all sources based on annual average for the period 1994-2008 on size classes 0.5-1, 1-2, 2-4, 4-8, and 8-16 μm are 73%, 19%, 5%, 2%, and 1%, respectively. These results suggest clear priorities for resource managers to establish TMDL on sources and incoming pollutants and preserving lake clarity.

Erratum to: Climate variability and change in mountain environments: some implications for water resources and water quality in the Sierra Nevada (USA)

This article introduces this special journal issue on climate change impacts on Sierra Nevada water resources and provides a critical summary of major findings and questions that remain open, representing future research opportunities. Some of these questions are long standing, while others emerge from the new research reported in the eight research papers in this special issue. Six of the papers study Eastern Sierra watersheds, which have been under-represented in the recent literature. One of those papers presents hydrologic projections for Owens Valley, benefiting from multi-decadal streamflow records made available by the Los Angeles Department of Water and Power for hydrologic model calibration. Taken together, the eight research papers present an image of localized climatic and hydrologic specificity that allows few region-wide conclusions. A source of uncertainty across these studies concerns the inability of the (statistically downscaled) global climate model results that were used to adequately project future changes in key processes including (among others) the precipitation distribution with altitude. Greater availability of regional climate model results in the future will provide research opportunities to project altitudinal shifts in snowfall and rainfall, with important implications to snowmelt timing, streamflow temperatures, and the Eastern Sierra’s precipitation-shadow effect.

Guided adaptive optimal decision making approach for uncertainty based watershed scale load reduction.

Previous optimization-based watershed decision making approaches suffer two major limitations. First of all, these approaches generally do not provide a systematic way to prioritize the implementation schemes with consideration of uncertainties in the watershed systems and the optimization models. Furthermore, with adaptive management, both the decision environment and the uncertainty space evolve (1) during the implementation processes and (2) as new data become available. No efficient method exists to guide optimal adaptive decision making, particularly at a watershed scale. This paper presents a guided adaptive optimal (GAO) decision making approach to overcome the limitations of the previous methods for more efficient and reliable decision making at the watershed scale. The GAO approach is built upon a modeling framework that explicitly addresses system optimality and uncertainty in a time variable manner, hence mimicking the real-world decision environment where information availability and uncertainty evolve with time. The GAO approach consists of multiple components, including the risk explicit interval linear programming (REILP) modeling framework, the systematic method for prioritizing implementation schemes, and an iterative process for adapting the core optimization model for updated optimal solutions. The proposed approach was illustrated through a case study dealing with the uncertainty based optimal adaptive environmental management of the Lake Qionghai Watershed in China. The results demonstrated that the proposed GAO approach is able to (1) efficiently incorporate uncertainty into the formulation and solution of the optimization model, and (2) prioritize implementation schemes based on the risk and return tradeoff. As a result the GAO produces more reliable and efficient management outcomes than traditional non-adaptive optimization approaches.

Modeling the transport of nutrients and sediment loads into Lake Tahoe under projected climatic changes

The outputs from two General Circulation Models (GCMs) with two emissions scenarios were downscaled and bias-corrected to develop regional climate change projections for the Tahoe Basin. For one model—the Geophysical Fluid Dynamics Laboratory or GFDL model—the daily model results were used to drive a distributed hydrologic model. The watershed model used an energy balance approach for computing evapotranspiration and snowpack dynamics so that the processes remain a function of the climate change projections. For this study, all other aspects of the model (i.e. land use distribution, routing configuration, and parameterization) were held constant to isolate impacts of climate change projections. The results indicate that (1) precipitation falling as rain rather than snow will increase, starting at the current mean snowline, and moving towards higher elevations over time; (2) annual accumulated snowpack will be reduced; (3) snowpack accumulation will start later; and (4) snowmelt will start earlier in the year. Certain changes were masked (or counter-balanced) when summarized as basin-wide averages; however, spatial evaluation added notable resolution. While rainfall runoff increased at higher elevations, a drop in total precipitation volume decreased runoff and fine sediment load from the lower elevation meadow areas and also decreased baseflow and nitrogen loads basin-wide. This finding also highlights the important role that the meadow areas could play as high-flow buffers under climatic change. Because the watershed model accounts for elevation change and variable meteorological patterns, it provided a robust platform for evaluating the impacts of projected climate change on hydrology and water quality.

Framework Design for BMP Placement in Urban Watersheds

A number of stormwater control strategies, commonly known as best management practices (BMPs), are used to mitigate runoff volumes and associated nonpoint source pollution due to wet-weather flows (WWFs). BMP types include ponds, bioretention facilities, infiltration trenches, grass swales, filter strips, dry wells, and cisterns. Another control option is “low impact development” (LID) – or hydrologic source control – which strives to retain a site’s pre-development hydrologic regime by combining impervious area controls with small scale BMPs, reducing WWFs and the associated nonpoint source pollution and treatment needs. To assist stormwater management professionals in planning for BMP/LID implementation, the U.S. Environmental Protection Agency (EPA) initiated a research project in 2003 to develop a decision support system for selection and placement of BMP/LID at strategic locations in urban watersheds. The BMP/LID assessment tools based on sound science and engineering will help develop, evaluate, select, and place BMP options based on cost and effectiveness. The system is called the Integrated Stormwater Management Decision Support Framework (ISMDSF). The ISMDSF will provide a means for objective analysis of management alternatives among multiple interacting and competing factors. The desired outcome from the system application is a thorough, practical, and informative assessment considering the significant factors in urban watersheds. The ISMDSF will be applied to several diverse urban watersheds to evaluate and demonstrate its capability (Lai et al. 2003, Lai et al. 2004, Riverson et al. 2004). The initial phase of this research is expected to be completed in 2005 and will include a comprehensive design and a functional system with all pieces in place but not all functionalities. The subsequent phase will include enhanced geographical information system (GIS) capabilities for visualization of placement options, more powerful post-processors, expanded cost estimating functions, improved BMP simulation processes, and more importantly, a multiple objective optimization

Development of the Lake Tahoe Watershed Model: Lessons Learned through Modeling in a Subalpine Environment

A comprehensive watershed model has been developed for the Lake Tahoe basin as part of the 2007 Lake Tahoe technical Total Maximum Daily Load (TMDL) initiative. Integral to this effort was the adaptation of the model to include research results from various parallel ongoing studies, as well as unique subalpine environment considerations. The primary reasons for developing a watershed model were (1) to determine basin-wide estimates for watershed loading of sediment and nutrients to Lake Tahoe based on land use type, (2) to provide input to the Lake Clarity TMDL Model (Reuter and Roberts, 2004), (3) to create a platform for load allocation and (4) to project load reductions from BMPs and other management scenarios. No such model had been previously developed for the Lake Tahoe basin. This paper focuses on insights gained through modeling analysis of site-specific watershed and meteorologic features and approach development, and presents a selection of innovative solutions that emerged from the process. The high level of detail involved in compiling, analyzing, and organizing the required data for modeling not only benefits the current TMDL objectives, but also, forms a lasting database of information to support other future scientific and water quality planning studies. Three selected watershed insights gained through the process include: (1) increased understanding of the impact of watershed physical setting, topography, and land use, (2) the effect of watershed features on meteorological spatial variability, evapotranspiration, and temperature lapse rate in a subalpine environment, and (3) the domineering impact of snowfall/snowmelt sequences on hydrology, water quality, and selected management practice alternatives. The lessons learned will help guide TMDL decision makers to more realistic conclusions when employing models in complex mountainous environments.

Design of a Decision Support System for Selection and Placement of BMPs in Urban Watersheds

The U.S. Environmental Protection Agency (USEPA) has funded the development of a decision support system for selection a nd placement of best management practices (BMPs) at strategic locations in urban watersheds. The primary objective of the system is to provide stormwater management professionals with a BMP assessment tool based on sound science and engineering that helps develop, evaluate, select and place BMP options based on cost and effectiveness. The system is called the Integrated Stormwater Management Decision Support Framework (ISMDSF) and is being designed through a systematic review of modeling needs, technical requirements, current and emerging data management technology, and available watershed and BMP models. The ISMDSF will be applied to a real urban watershed to evaluate its ability. There are four major design aspects for the ISMDSF development. First, t he system provides a robust computer platform for BMP selection, sizing, and placement in the context of several integrated watershed factors and influences. Second, it is applicable to mixed land use urban watersheds, and can perform watershed simulation based on watershed size, scale, anthropogenic, and natural characteristics. Third, it incorporates hydrologic/hydraulic and water quality modeling, integrating surface runoff and direct discharges to surface water bodies, based on relevant data collection. Finally, it will have the capability to objectively evaluate multiple solution alternatives based on cost and the desired water -quality objectives. Programs that would benefit from the application of the ISMDSF include Municipal Separate Storm Sewer System (MS4) permits under the NPDES Stormwater Program (Phase I and II), Total Maximum Daily Load (TMDL) evaluations, and source-water protection. The ISMDSF will provide a means for objective analysis of management alternatives among multiple interacting and competing factors. The desired outcome