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Integration of Climate Change Analysis and TMDL Development

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Climate change was identified as one of USEPA's top priorities in the USEPA FY 2011–2015 Strategic Plan. Most major initiatives to date have focused on greenhouse gas emissions and related air impacts. While the potential impact of climate change on water resources has been widely acknowledged, limited progress has been made evaluating effects on water quality. Assessments have generally focused on water supply. Less is known about the potential effects of climate change on water quality and aquatic ecosystems.

A project recently conducted for USEPA's Global Change Research Program (GCRP) has assessed the sensitivity of hydrologic and water quality endpoints under a range of alternative 21st century climate conditions across the country, and evaluated the effects of different methodological choices such as different hydrologic models and method of downscaling climate change information on the variability of outcomes. The approach used in this study provides insights regarding ways to quantitatively evaluate the potential effects of climate change on hydrology and water quality. These results are also relevant in the context of assessing the potential implications of climate change for existing or planned TMDLs.

The approach consists of three major steps: developing and calibrating a dynamic watershed model, accessing and processing an ensemble of global climate/general circulation model (GCM) data, and simulating the scenario ensemble. Through the GCRP project, the approach has already been applied to 5 pilot watersheds located throughout the country, and is currently being applied to 15 additional watersheds. For the five pilot watersheds, two distinct dynamic watershed models were applied, Hydrologic Simulation Program Fortran (HSPF) and Soil and Water Assessment Tool (SWAT). The models were calibrated to historical conditions for a 30 year period. Particular attention was paid to representation of hydro-meteorological factors, such as potential evapotranspiration. Climate ensemble data was compiled and processed to represent a range of potential future climate outcomes. Non-downscaled, dynamically-downscaled, and statistically-downscaled datasets from national and international programs were all used in order to comprehensively consider effects. Data processing involved quantifying change statistics between historical and future meteorological conditions using a suite of tools, including USEPA's Better Assessment Science Integrating point and Nonpoint Sources (BASINS) Climate Assessment Tool (CAT). The climate scenario data was then used within the calibrated watershed modeling framework to generate and evaluate various outputs for comparison.

Endpoints include mean monthly and annual stream flow, flood events, low flow events, flow duration curves, shift in timing of annual runoff center of mass, and monthly and annual nitrogen, phosphorus, and sediment loads. Comparisons made for the pilot watersheds demonstrated the highly variable spatial nature of climate change effects. They also demonstrated the importance of precipitation timing and intensity on delivered pollutant loads. And, perhaps most significantly, they highlighted the variability in GCM and downscaled GCM climate data predictions and the importance of implementing an ensemble approach for water quality evaluation.
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Keywords: BASINS; BASINS CAT; GCM; GCRP; HSPF; NARCCAP; NCAR; RCM; SWAT; TMDL; climate change; hydrology; nutrients; sediment; water quality model; watershed model

Document Type: Research Article

Publication date: 2011-01-01

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