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L-THIA - Three Versions, Three Groups, Three Projects

Conventional methods of assessing hydrologic impacts evaluate how land use changes alter peak runoff rates. Peak runoff results produced by these models are used to design runoff and erosion control structures but do little to address water quality issues. The Long-Term Hydrologic Impact Assessment (L-THIA) model was developed to address the long-term, cumulative impacts that human activities have on stormwater runoff volumes and water quality. L-THIA is an ideal tool to assist in the evaluation of potential effects of land use change and to identify the best location of a particular land use so as to produce the minimum amount of negative impact on the local natural environment. Since L-THIA is available on the Internet it is accessible to anyone concerned about what impact a particular land use change might impose.

Based on community-specific climate data, L-THIA estimates changes in recharge, runoff, and nonpoint source (NPS) pollution resulting from past or proposed development. It estimates long-term average annual runoff for land use and soil combinations, based on actual long-term climate data for that area. By using many years of climate data in the analysis, L-THIA focuses on the average impact, rather than an extreme year or storm (Purdue Research Foundation, 2004).

There are multiple versions of L-THIA available. Basic Input L-THIA was developed for those new to land use planning. Detailed Input L-THIA gives the user more functionality and therefore is geared towards those with land use planning experience. GIS L-THIA runs within ESRI ArcView GIS software for those with GIS capabilities and skills.

To demonstrate the different versions of L-THIA, this article will focus on three specific case studies. Each example explains how a specific group used a different version of L-THIA to assist with local planning projects.

BASIC INPUT L-THIA
Using web-based tools and photo visioning in the classroom to identify and map urban development and its contribution to stormwater runoff.

Three high schools in Washington County, Wisconsin, participated in a two-part stormwater and NPS pollution lesson as part of the stormwater pollution education campaign required by their municipality�s NPDES Phase II permit. The University of Wisconsin-Extension and MSA Professional Services, a Madison-based engineering and planning firm, collaborated to write and teach the two-part lesson.

In part one of the lesson, students learned about construction site erosion and other sources of urban runoff pollution. They learned specifically how excess soil in streams and rivers degrades fish habitat and overall water quality. Students learned that in urban areas, construction site erosion is a major contributor of sediment to local waterways. Using one of two Internet-based tools, the West Bend GIS Map Server or Wisconsin DNR WebView, students mapped four specific sites within their community where urban development is occurring. Using aerial photos, they estimated the area of these four sites. Students then headed into the field and used disposable cameras to photograph evidence of erosion or examples of best management practices at the sites they mapped.

In part two of the lesson, students compared the aerial photos of their sites (taken 5 years earlier) with the photos taken at each site the previous week. Students then placed their photographs on a larger map of their community, and identified patterns of urban development taking place within the city or village. They grouped the remainder of the photographs into categories and evaluated the success of the best management practices being utilized at the photographed construction sites.

Students utilized Basic Input L-THIA to estimate the impact of urban development identified in the last five years. With Basic Input L-THIA, students only needed to input their location, the hydrologic soil type found in the area of land use change, and the type and size of land use change that occurred. The model then generated estimated changes in runoff volumes and depths, and expected changes in NPS pollution loadings. Students produced these results as tables, bar graphs, and pie charts. Through this lesson, students received a better understanding of how local land use decisions have affected stormwater quantity and quality.

DETAILED INPUT L-THIA
Using web-based tools to raise community awareness and understanding of local land use issues.

The Friends of Pheasant Branch (FOPB) is a dedicated group working to protect ecologically significant lands in the Middleton area of Dane County, Wisconsin. With a mission to �restore, preserve and understand the value of conservancy lands, other natural habitats, and cultural sites in the Pheasant Branch watershed for today and tomorrow� the FOPB has made it their priority that all land use decisions in the area are given the full attention of the group.

During the past year, the Middleton-Cross Plains Area School District made the decision to sell a 19.27-acre parcel of land in order to procure land in areas where the district is experiencing higher rates of growth. This parcel, the Highland Way parcel, is a partially wooded tract surrounded by high-density residential development on two sides. The remaining two sides are adjacent to the existing Pheasant Branch Conservancy, located just inland of the northwest shore of Lake Mendota.

The Middleton-Cross Plains School Board has granted an option to the FOPB for its fair market value, $3 million. Members of the group have been working on raising the funds needed to purchase the land. To be convincing, their grant applications and fund raising publications require the inclusion of hard data�facts that would make it obvious that preserving this parcel would be best for the community for both ecological and economical reasons.

The FOPB is utilizing Detailed Input L-THIA to help support their efforts in preserving the Highland Way parcel. Detailed Input L-THIA is similar to the Basic Input version but gives users the added ability to enter detailed and customized land uses. Basic Input L-THIA allows users to select from only eight land use choices.

The soils found in the Highland Way parcel are composed of Hydrologic Soil Group B � silt loams and loams with moderate infiltration rates. Preservation of this parcel would preserve the soils� ability to infiltrate water. Conversely, development of the forested parcel into high-density residential use would significantly increase runoff and non-point source pollution loadings into Conservancy and Lake Mendota waters. L-THIA makes it apparent that preservation of this parcel in its current state would be beneficial to the watershed. Not only would NPS pollutant loadings be less but the economic costs of handling the increased volume of runoff would be avoided.

With the results from L-THIA, the FOPB has been able to effectively communicate the importance of conserving the Highland Way parcel. The results from L-THIA will be useful for educating the community about alternative land uses and raising the funds necessary to protect the land.

GIS L-THIA
Using web-based tools to guide local decisions within a watershed.

Recently, the Bay-Lake Regional Planning Commission (BLRPC), in partnership with the Baird Creek Preservation Foundation, completed a study evaluating different land use scenarios and their impact on volume and quality of runoff within the rural portion of the Baird Creek watershed.

This was a BLRPC subsidized pilot project that received additional funding from the Baird Creek Preservation Foundation and supporting software from Wisconsin Costal Management Program. BLRPC is looking at how L-THIA can be used in combination with other low-cost models to demonstrate the effects of different land uses on non-point source pollution.

Baird Creek is a subwatershed draining 25 square miles in the northeast corner of the East River watershed, near Green Bay, Wisconsin. The creek originates in agricultural lands and flows west 31.1 miles through wetland, park and urban areas. The lower 3.5 miles are perennial and classified as supporting warm water sport fish communities, with the upper 27.6 miles of intermittent stream designated as supporting warm water forage fish. The predominant land use in the upper reaches of the watershed is agriculture but it is quickly being urbanized as the population of the Green Bay metropolitan area spreads eastward (Reyburn, 2003).

In general, water quality ranges from good in the upper reaches to poor in the lower urban reaches. The stream suffers from NPS pollution from barnyard runoff, stream bank and cropland erosion causing turbidity, nutrient problems, sedimentation and habitat loss (Reyburn, 2003).

The Bay-Lake Regional Planning Commission has been utilizing L-THIA GIS to develop a series of digital maps and corresponding databases for the Baird Creek Preservation Foundation. The map series depicts analysis estimates for the rural portions of the Baird Creek watershed planning area, including the Towns of Humboldt and Eaton in Brown County and a small area in the Village of Bellevue.

L-THIA, as with other hydrologic models, utilizes the United States Department of Agriculture, Soil Conservation Services curve number (CN) method. CN�s are based on soil and land use characteristics for a given area. These values are used to empirically calculate surface runoff volumes for a given rainfall event (Engel, 2005).

Basic and Detailed Input versions of L-THIA use an average of CN values over a large area to calculate runoff, whereas L-THIA GIS uses a distributed method. The distributed method calculates runoff for all unique areas in the watershed, and then sums them, producing a more accurate result (Engel, 2005). L-THIA GIS also gives users the ability to display their results graphically through maps.

The first step in this project was the creation of a GIS base map for the Baird Creek watershed study area. The base map contains all pertinent information such as roads, county boundaries, civil division boundaries, soils, updated land use, streams, swales, and drainage basins. 2002 land use data was updated by conducting on-the-ground visual inventories of current land uses in the study area.

After reformatting the soils and land use data, determining hydrologic soils groups, and converting the data into grids, the runoff scenarios are created. Using this information the model predicts NPS pollutant loadings. The digital maps produced by L-THIA will be used by the Foundation during planning and educational activities concerning the rural portions of the Baird Creek Watershed.

ADDITIONAL APPLICATIONS
L-THIA can also be used to assess the hydrologic impacts of a variety of projects, including:

• Impacts of a proposed land use change on downstream residents.

• Tourism / agriculture transition impacts on a coastal watershed.

• Past and future land use change in an urban-rural fringe watershed, including impact fee assessment.

• Watershed scale implications of land use change for NPS pollution.

L-THIA, along with more information and instructions on how to use the tool, is available at www.ecn.purdue.edu/runoff/lthianew/. n

REFERENCES
Engel, B.A., 2005 Update, L-THIA NPS (Long Term Hydrologic Impact Assessment and Non-Point Source Pollutant Model, Version 2.3), Purdue University and U.S. Environmental Protection Agency. See also: www.ecn.purdue.edu/runoff/lthianew.

Purdue Research Foundation, 2004. L-THIA Model. Available: www.ecn.purdue.edu/runoff. Accessed 5 July 2005.

Reyburn, J., Aquatic Biologist. Wisconsin Department of Natural Resources, Baird Creek Watershed Management Study, 2003.

ACKNOWLEDGEMENTS
Thanks to the following individuals who reviewed or contributed to this article: Amy Workman, UW-Extension Milwaukee Basin Educator; Tony D. Bellovary, and Angela Pierce, Bay-Lake Regional Planning Commission; Dreux Watermolen and Sally Kefer, Wisconsin Department of Natural Resources.

Reviewed by Rebecca Roberts of the Center for Land Use Education.

R. Chris Welch is a Forest Resource Educator/Web Developer with the Department of Natural Resources Division of Forestry. He has done work with the DNR Bureau of Integrated Science Services Computer Tools for Planning, Conservation, and Environmental Protection technical assistance program. This program is an effort to increase awareness of Internet resources available to local governments, citizen planners, educators, and anyone else involved in land use planning. For more information, see: http://dnr.wi.gov/org/es/science/landuse/CompTools/.

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