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Watershed Modeling to Evaluate Potential Impacts of Climate and Land Use Change on the Hydrology and Water Quality of Major U.S. Drainage Basins

Client: Tetra Tech EM Inc.

Background

The EPA Office of Research and Development Global Change Research Program (GCRP) works to build the capacity of EPA program and regional offices, water managers, and other decision-makers to assess and respond to global change impacts on water quality and aquatic ecosystems. Research and assessment activities in the GCRP Water Quality focus area broadly support EPA’s mission and responsibilities as defined by the Clean Water Act and the Safe Drinking Water Act. Water quality is a complex outcome of watershed hydrologic, biogeochemical, and ecological processes coupled with the impacts of human use and disturbance. Accordingly, although the GCRP’s research and assessments have a primary focus on water quality, they also include watershed hydrologic processes (e.g. streamflow), aquatic ecosystems, and linked terrestrial ecosystems.

At present, few studies have quantified the combined potential future impacts of climate and land use change on hydrology and water quality in major U.S. drainage basins. The quantity and quality of water delivered from U.S. drainage basins are an essential resource necessary to support human health, welfare, and settlements. Water quantity and quality are also essential to the structure, functioning, and services provided by aquatic ecosystems. Assessing and managing the risk of harmful future impacts on water and watershed systems will require an improved understanding of how future climate and/or landcover change could impact the hydrology and water quality of major U.S. drainage basins.

The project team for this work consisted of Tetra Tech as the prime contractor, AQUA TERRA for HSPF model support, Texas A&M for SWAT model support, Stratus Consulting and FTN Associates for climate change science and GCM/RCM modeling support.

Objective

This task order called for watershed modeling to assess the potential impacts of climate and land use change on the hydrology and water quality of major U.S. drainage basins. Hydrologic and water quality change “scenarios” reflecting a range of potential future changes in climate and land use were developed for 20 approximately hydrologic unit code (HUC) 4-digit sized drainage basins. Climate and land use change scenarios were provided by EPA GCRP. Watershed modeling was conducted using two watershed models, HSPF and SWAT. The resulting hydrologic and water quality change scenarios will contribute, in part, to a comprehensive Water Quality Assessment Report to be prepared by the EPA GCRP in 2013. The following tasks were completed:

Task 1: Establish communication and develop a Quality Assurance Project Plan

Task 2: Delineate boundaries of 20 study watersheds: 5 to be used as pilot study watersheds and 15 to be used as non-pilot study watersheds.

Task 3: Acquire and format climate and landuse change scenarios.

Task 4: Conduct watershed modeling in the 5 pilot study watersheds. AQUA TERRA set up, calibrated, and validated the HSPF models for 3 of the 5 pilot study watersheds and provided support for a series of Technical Reports.

Task 5: Conduct watershed modeling in the 15 non-pilot study watersheds

Task 6: Technical review and input for preparation of publications in peer-reviewed scientific journals summarizing the results of the watershed modeled.

Task 7: Provide watershed modeling output files and package software for future use by EPA

Figure 1: 20 test basins that were selected

Climate and Landuse Changes

The EPA GCRP has partnered with the North American Regional Climate Change Assessment Project (NARCCAP) to acquire simulations of future climate change over the contiguous U.S. in a form suitable for driving basin-scale hydrologic models. NARCCAP is a multi-institution cooperative effort that uses state-of-the-art Regional Climate Models (RCMs) at a number of institutions to downscale output from the most recent Intergovernmental Panel on Climate Change (IPCC) Global Climate Model (GCM) simulations of climate to a much higher resolution over the U.S. for present-day (1970-2000) and future (2040-2070) decades.

The EPA GCRP has also recently developed seamless, national-scale land use change scenarios compatible with the IPCC emissions storylines underlying NARCCAP and other GCM climate change projections. This project is known as the Integrated Climate and Land Use Scenarios (ICLUS) dataset. ICLUS uses a demographic model, which consists of a cohort-component model and gravity model, to project population to 2100 for each county in the coterminous U.S.

Simulations

Five of the 20 HUC 4-digit sized study watersheds was selected as pilot study watersheds: Apalachicola-Chattahoochee Flint, Central Arizona, Susquehanna, Willamette, and Upper Mississippi River Basins. Pilot study watersheds were assessed for a wider range of climate and land use change scenarios than non-pilot watersheds, and hydrologic and water quality change scenarios will be developed independently using two watershed models: Hydrologic Simulation Program FORTRAN (HSPF) and Soil Water Assessment Tool (SWAT). The following climate/land use change scenarios were assessed in each of the 5 pilot watersheds:

  • baseline conditions (=1 simulation; current climate and current land use),
  • climate change alone (=15 simulations; 15 climate change scenarios with current land use),
  • land use change alone (=2 simulations; 2 land use change scenarios with current climate),
  • combined effects of climate and low-impact land use change (=15 simulations; 15 climate change scenarios with low-impact land use),
  • combined effects of climate and high-impact land use change (=15 simulations; 15 climate change scenarios with high-impact land use change).

Watershed modeling in remaining 15 HUC 4-digit sized watersheds were conducted using a single watershed model using just four of the 15 NARCCAP climate change scenarios. The following climate/land use change scenarios were assessed in each of the 15 non-pilot watersheds:

  • baseline conditions (=1 simulation; current climate and current land use),
  • climate change alone (=4 simulations; 4 climate change scenarios with current land use),
  • land use change alone (=2 simulations; 2 land use change scenarios with current climate),
  • combined effects of climate change and low-impact land use change (4 simulations; 4 climate change scenarios with low-impact land use change),
  • combined effects of climate and high-impact land use change (4 simulations; 4 climate change scenarios with high-impact land use change).
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