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Article Excerpt
Abstract. -- The Blanco River of central Texas is supplied through spring flow and tributaries. Land use in the watershed is predominantly rural, but in recent years the rate of urbanization within the watershed has increased. Rates of population growth and urbanization are expected to increase into the future. The SWAT (Soil and Water Assessment Tool) model is used to predict the impact of land management practices on water yields in complex watersheds under varying soils, land use, and management conditions. Using SWAT, the flows in 38 separate sub-basins of the Blanco watershed are predicted under a range of future land use and climate scenarios. Two climate conditions representing current precipitation patterns and drought conditions were modeled. Three land use urbanization scenarios were modeled: current land use conditions of 2% urban area, 13% urbanization of the watershed, and 78% urbanization of the watershed area. Results show a decrease in river flow as the urban area reaches 13% of total land use. This reflects an increase in household water use yet enough rural area to allow for rainfall infiltration to the subsurface. At 78% urbanization of the watershed, flows to the river are increased. Rainfall does not infiltrate due to the extensive impervious cover but rather flows directly to the Blanco River as overland runoff. Baseflow is maintained through increased discharges from municipal wastewater plants. The predicted flows under all scenarios modeled are lowest for the first portion of the watershed, upstream of any major tributary contributions.
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The connection between land use in a watershed and the water quality and quantity of the main channel has become part of the public conscience as populations in the arid and semi-arid regions of the US increase. With the passage and implementation of the Clean Water Act, water quality issues associated with land use were brought into the public eye. Over the past 30 years, this awareness has been extended as the public has become increasingly aware of the possible threats to natural ecosystems posed by land use changes associated with urbanization.
As cities plan for expected growth, they often try to choose a growth strategy that will minimize negative impacts to the environment. A number of methods are exemplified through a sampling of city growth strategies in Texas (Vaughn, pers. comm.). In Austin in the 1970's, the city decided to limit future growth to a north-south corridor along I-35, in part to preserve the recharge area for the Edwards Aquifer as undeveloped land. The plan was unsuccessful, as private developers created Municipal Urban Districts outside the city's planned growth areas. Over the same period of time, San Antonio allowed growth in any direction but tried to influence the growth through zoning laws.
Public involvement in growth planning has increased since the 1970's, both formally and informally. Formal public participation exists through public city council meetings and is not new. Informal participation through events like the Envision Central Texas Project is a new way in which the public hopes to influence policy. Envision Central Texas has been holding public meetings in towns throughout the region between (and inclusive of) Austin and San Antonio where the attendees are asked to draw their desired type of and areas of development on a regional map (White Lion & EnviroMedia 2006). The overwhelming majority of participants placed a high value on environmental integrity and the preservation of rural space on those lands where there is recharge to aquifer and stream systems. Through this exercise, the public demonstrated an awareness and appreciation for the impact of changing land use within a large watershed area on the quality and quantity of water in the main channel of the watershed.
Concerns regarding predicted future growth have prompted interest in the Blanco Watershed of Central Texas. Historic land use in the watershed is predominantly ranching although there are two small urban centers: Kyle and Wimberley. Over the past decade, the growth of these two cities has exploded along with the larger central Texas region. With more developments planned into the foreseeable future, the rate of growth of both Kyle and Wimberley is expected to continue and possibly increase (Rodriguez 2004). As more rural land is converted to urban use, concerns have arisen about the potential impact on the Blanco River.
Computer models that simulate the effects of management decisions over a long time frame are increasingly used to meet the public demand for environmentally responsible growth planning. The Soil and Water Assessment Tool (SWAT) model is a river basin scale model that predicts the impact of land management practices on water yields in complex watersheds with varying soils, land use, and management conditions. It is a physically based model designed to predict the impacts to basins without gages or with limited available data.
This project applies the SWAT model to six possible management and climate scenarios in the Blanco watershed. One of the predictions of climate change for the central Texas region is an increased likelihood of droughts. Thus, two climate periods are modeled. The first represents current weather conditions and the second simulates drought conditions. Three management scenarios are modeled: current conditions, partial watershed urbanization, and majority watershed urbanization. This paper presents the results of the modeling efforts.
STUDY SITE
The Blanco River headwaters are springs in northwestern Kendall County, near the Gillespie County line. The main channel of the Blanco River flows southeast for 140 km (87 mi), where it joins with the San Marcos River (McCord 2006). Although the Blanco is a sub-watershed of the larger Guadalupe River, its watershed area is 1365 [km.sup.2] (527 [mi.sup.2]) (Jasinski 2006). As the river flows downstream, tributaries include Callahan Branch, Flat Creek, the Little Blanco River, Cypress Creek, and Sink Creek. This study examines the length of the Blanco River and its tributaries.
The Texas Hill Country features karst topography where limestone bedrock erodes to form slab benches and a stair-stepped topography. Channel substrate varies between bedrock and thin layers of alluvium. Vegetation in undeveloped areas of the watershed is commonly a mix of oak, juniper, mesquite, and grasses. Riparian areas are dominated by bald cypress, oak, elm, cedar, and conifers. The climate supporting the vegetation is subtropical with short, cool winters and long, hot summers. Annual precipitation averages near 75 cm but falls primarily as brief, localized thunderstorms (Jasinski 2006).
The combination of...
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