why arc hydro? - .why arc hydro? 1 chapter one david maidment, university of texas at austin water
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Why Arc Hydro?
David Maidment, University of Texas at Austin
Water is fundamental to human life and the functioning of the natural environment. The scope and scale of water resources problems makes geo-graphic information system (GIS) software a powerful tool for developing solutions, and the advent of ESRI ArcGIS has created an opportunity to rethink the way that water resources data is represented in GIS. The result is Arc Hydroan ArcGISdata model for water resources. Arc Hydro opens the way to building hydrologic information systems that synthesize geo-spatial and temporal water resources data to support hydrologic analysis and modeling.
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Chapter 1: Why Arc Hydro?
Water and life
The health of a communitys water systems is naturally a concern of its citizens. They expect to be protected from oods, to have a reliable water supply, to swim and sh in their rivers and lakes. But nature can be cruel as well as kind. Floods can devastate communities, destroy-ing in hours the labor of a lifetime. There are always con icts among people who share water resources, con icts that become critical during droughts when there is not enough water to meet everyones needs. Population growth and economic development can degrade water quality and the living environment for wildlife. All these pressures exist in Texas, a state where the recurring cycle of ood and drought is a way of life. The Guadalupe River basin in Texas is used throughout this book as an example application of the Arc Hydro data model.
Rainfall in Texas was lower than normal in the autumn of 1995, the beginning of what would become the 1996 Texas drought. Over the winter and spring, the dearth of rainfall continued, and the rains that normally nourish seedlings and replenish water tables in April and May never arrived. By June, south Texas was already in the midst of drought. In the Guadalupe River basin, experts called the weather an extended dry spell, but admitted there were signs that things could evolve into a disaster. Reserves in reservoirs and aquifers continued to fall, crops with-ered, livestock died, and the 1996 drought ended up costing Texas over $1.5 billion. And this drought wasnt even a particularly extended onethe great drought of the 1950s, which lasted for six years, is etched in the memory of any Texan who lived through it.
As counties across Texas clamored for relief in 1996, then-governor George W. Bush contacted state water of cials. He asked them how much water was available and how long it could last. These intelligent, sensible questions had the potential to solve problems if they could be answered, but state water of cials did not have adequate information to provide answers. Rec-ognizing these shortcomings, the Texas Legislature in 1997 launched an ambitious plan to over-haul water planning in the state, and to construct detailed, statewide data layers of geospatial
U.S. Agriculture Secretary Dan Glickman has called drought an even more insidious natural disaster than hurricanes, fl oods, or tornadoes because it happens very slowly. Its impact is more long range, far reaching.
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Water and life
information on land-surface terrain, soils, land use, and stream hydrography. Water availability in Texas is now managed using this digital geospatial infrastructure instead of stacks of paper maps. The Arc Hydro data model described in this book is used to structure the geospatial infor-mation for Texas water availability modeling.
The 1996 drought was hardly over when a severe storm in October 1998 caused a huge ood in south Texas. Flood ows in the lower Guadalupe basin were more than twice as large as any recorded since stream-gaging stations were installed on the Guadalupe River during the 1930s. The October 1998 ood killed more than 40 people people and caused $2.1 billion in damages, yet numbers and statistics dont tell the human story. When a ood recedes from a home that has been under 23 feet of water, all that can be done is to bring in a bulldozer and clean off the concrete foundation slab; nothing else remains.
The cleanup from the October 1998 ood was still going on when Tropical Storm Allison hit Houston in June 2000. A total of 77,000 buildings were inundated there in what became the most costly urban ood disaster in the history of the United States. The story continues. It is at times like these that understanding and managing water resources becomes so much more than understanding and managing informationit becomes life and death. The question is not whether the next ood will occur, but where and when will it happen?
Accurate oodplain mapping is needed to be able to estimate ood risks precisely. One of the reactions to Tropical Storm Allison was a complete remapping of the oodplains of Houston
Nature is devastating, said Governor George W. Bush. It was only three months ago that we were praying for rain, and now in Texas weve got too
much rain. It happened so quickly and so suddenly.
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Chapter 1: Why Arc Hydro?
using LIDAR (Light Detection and Ranging), a remote sensing technique from aircraft that pro-duces highly detailed and precise maps of land-surface terrain. Chapter 5 in this book shows how Arc Hydro can be used to process terrain information for oodplain mapping. Precise ter-rain information is also being used to design new ood channels and pipelines to carry away ood waters from the next severe storm in Houston.
In the United States, the Nexrad radar rainfall system provides maps of storm precipitation updated every few minutes. There would be a very signi cant bene t for public safety if these radar rainfall maps could be quickly translated into anticipated ood inundation maps during storm events, so that citizens could be better warned to stay away from roads that cross swollen rivers and streams, since being swept away in a vehicle causes many deaths in oods in Texas. Being able to create real-time ood inundation maps requires connecting map data on ground conditions with a time sequence of rainfall maps and with ood simulation models to produce a time sequence of ood maps on the ground. Chapter 7 shows how time sequences of Nexrad radar rainfall map data can be incorporated into Arc Hydro.
Concern about water resources is not limited to the impacts of oods and droughts. Citizens want livable cities, where urban growth and development can coexist with a clean environment and healthy ecosystems. Water-quality management in cities requires solutions that assess how regulation of land development will affect water quality and ecological resources. What combi-nation of pollution prevention structures, education, and regulation best serves to enhance the quality of the urban water environment?
From a larger perspective, in the United States the goal of the 1972 Safe Drinking Water Act was to make the nations waters shable and swimmable. After thirty years of pollution preven-tion efforts, mostly aimed at controlling point sources of pollution, many of the nations waters
Urban hydrologic systems pose many challenges not seen in rural systems.
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are still not shable and swimmable because of nonpoint sources of pollution, such as runoff from farmlands.
To rectify this problem, the U.S. Environmental Protection Agency (EPA) has embarked on an ambitious program called Total Maximum Daily Load, whose goals include setting regulatory standards on the quality of water within natural water bodies, rather than regulating just the out ows of wastewater discharge pipes. Trying to regulate the quality of natural water bodies requires a much more comprehensive understanding of the impact of all types of pollution sources on receiving water quality than has been required in the past. The EPA has created a decision support system called Basins, built using ArcView version 3, to integrate geospatial and temporal water resources information, and water-quality models for improved water-quality management. The Arc Hydro data model presented in this book can be used to extend the Basins system so that it can be implemented in a more robust and effective manner using ESRIs new ArcGIS technology.
These examples of the importance of water issues in Texas and the United States are not unique. The same issues are faced in other countries, and better solutions to information man-agement for water resources are needed throughout the world.
GIS for water resources
Hydrologists use many data sources to assess water quality, determine water supply, prevent ooding, understand environmental issues, and manage water resources. During the 1990s, GIS emerged as a signi cant support tool for hydrologic modeling. In particular, GIS provided a con-sistent method for watershed and stream network delineation using digital elevation models (DEMs) of land-surface terrain. Standardized GIS data sets for land cover, soil properties, gaging station locations, and climatic variables were developed, and many of these data sets are pub-lished on the Internet. GIS data preprocessors were developed to prepare input data for water ow and water-quality models. GIS is now accepted as a useful tool for assembling water resources information, and the community of water resources and GIS specialists who are famil-iar with these tools and data sets is growing.
While much progress has been made with the application of GIS in water resources, and the creation of national, regional, and local data sets, man