Chapter 5 Design and Use of GIS-based Water Resources Database Models

Transcription

1 Chapter 5 Design and Use of GIS-based Water Resources Database Models Abstract This chapter is about water resource database models for urban and regional environmental concerns. GIS database analysts face many choices to identify the most appropriate water resource database model design for a GIS project. The ArcHydro database model is used as the basis for discussion of the water resource database model. Several components are presented. In each of the sections we differentiate database model needs among planning, improvement programming, and implementation decision problems as a way of dealing with the issue that database design depends on the context of the decision situation. This chapter describes water resource database models suitable for use in urban-regional applications of GIS. Remember that a data model language implemented in a particular context results in a database model the (schema) model of a particular database specified in terms of entity-object classes. The database models in this chapter focus on water resource issues for three decision situations planning, improvement programming, and project implementation. 5.1 Motivations for Water Resource Planning, Programming, and Implementation Databases Several motivations exist for water resource database development. A variety of features are part of the water resource database models. A number of interesting database model designs have been developed for water resource applications. Water resources are one type of critical resource protected in the US under various laws, e.g. National Environmental Policy Act of 1970 and Clean Water Act of Such laws motivate various types of data categories to be considered in critical resource databases. As mentioned earlier (chapter 3), the National Environmental Policy Act (NEPA 1970) constrains federal action to protect critical resource areas when federal money is spent for projects. NEPA mandates that an environmental assessment (EA) or an environmental impact statement (EIS) be developed depending on the severity of the impacts anticipated. When necessary, project permits are not issued until an EA or EIS is conducted. Due to the expense and time involved, there are 10 times as many EA s than EIS s performed. EA s and EIS are project based, not ecosystem based. Studying a project would only partially identify problems that might be created for an ecosystem. The Clean Water Act (US Environmental Protection Agency 1972) calls for restoring and maintaining chemical, physical, and biological integrity of Nation s waters. CWA seeks to reduce discharge of pollutants by allocating pollution permits as part of the National Pollution Discharge Elimination System (NPDES). Every storm sewer intake is now part of NPDES database. These laws in part motivate organizations to address several dimensions of watershed management and planning using a watershed planning process (Center for Watershed Protection 1998). A watershed is the land area that contributes runoff to a particular point along a watercourse. 5-1

2 There are a number of dimensions to watershed management and planning. Management includes a number of activities in the development and implementation of a watershed plan. Eight steps describe the development of a watershed plan (Table 5.1). Among these steps are a number of choices about mapping, modeling, monitoring during development of a plan, each with decisions about database design. Table 5.1 Steps in Development of a Watershed Plan (Adapted from Center for Watershed Protection 1998) Step 1: Establish a Watershed Baseline Step 2: Set Up a Watershed Management Structure Step 3: Determine Budgetary Resources Available for Watershed Planning Step 4: Project Future Land Use Change in the Watershed and Each Subwatershed Step 5: Determine Goals for the Watershed and Its Subwatersheds Step 6: Develop the Watershed and Subwatershed Plans Step 7: Adopt and Implement the Watershed and Subwatershed Plans Step 8: Revisit and Update the Watershed and Subwatershed Plans A watershed plan calls for a broad consensus among stakeholders in watershed management process (Table 5.2). Consensus stems from a number of perspectives. Table 5.2 Consensus in Planning. (Adapted from Center for Watershed Protection 1998) Typical Non-Agency Stakeholders: Citizen Associations Developers Property Owners Outdoor Recreation Clubs Water Resource Conservation Groups Local Planner Individual Citizens Farmers Business Interests (industrial, commercial business owners) Utility Companies Environmental Advocates Typical Agency Stakeholders: Regional Council of Government Planning Board Health Department 5.2 Feature Classes in Water Resource Database Models For all of the applications mentioned in the previous subsection, the fundamental unit in water resource planning and management is the drainage unit. Drainage units characterize natural health of urban areas, because they are the connections between land use and water resources. A drainage unit is the land area that contributes runoff to a particular point along a watercourse; and thus is a general name for all such areas that drain. Drainage units (given different names) nest within each other as a way of dealing with spatial scale concerns in management (Figure 5.1). Because drainage units are so fundamental to water resource management, they are fundamental units in database models. 5-2

3 Figure 5.1 Nesting of drainage units, each having a different label to help with water management scale (Center for Watershed Protection 1998) In an urban setting, impervious surfaces have largest influence on water quality in lakes and streams. Impervious surface coverages are more influential at some drainage scales than others (Table 5.3). Table 5.3 Impervious cover influence on unit size (Adapted from Center for Watershed Protection 1998) Watershed Management Unit Typical Area (square miles) Influence of Impervious Cover Sample Management Measures Catchment 0.05 to 0.50 very strong practices and site design Subwatershed 1 to 10 strong stream classification and management Watershed 10 to 100 moderate watershed-based zoning Subbasin 100 to 1,000 weak basin planning Basin 1,000 to 10,000 very weak basin planning Key changes occur in urban streams due to increases in impervious cover. Many of them are the following (adapted from Center for Watershed Protection 1998): - bankfull and sub-bankfull floods increase in magnitude and frequency - dimensions of stream channel are no longer in equilibrium with its hydrologic regime higher flow rates - channels enlarge 5-3

4 - stream channels are highly modified by human activity - upstream channel erosion contributes greater sediment load to the stream - dry weather flow in the stream declines - wetted perimeter of the stream during low flow declines - instream habitat structure degrades - large woody debris is reduced - stream crossings and potential dish barriers increase - riparian forests become fragmented, narrower and less diverse - water quality declines - summer stream temperatures increase - reduced aquatic diversity One of the most important drainage unit scales is subwatershed. Subwatershed is a useful planning unit because of: - influence of impervious cover on hydrology, water quality, biodiversity is most evident - just a few political jurisdictions are likely to cover area - limited number of confounding pollutant sources are present - a map of a reasonable size can be used to depict area - small enough for management plan to be developed within a year s time There are a number of issues for which plans can be developed to improve subwatershed management (Table 5.4). Subwatershed Category Sensitive Stream Impacted Stream Non-Supporting Stream Restorable Stream Urban Lake Water Supply Reservoir Coastal Estuarine Waters Aquifer Protection Table 5.4 Subwatershed Management Applications (Adapted from Center for Watershed Protection 1998) Description Less than 10 % impervious cover. High habitat/water quality rating 10% to 25% impervious cover. Some decline in habitat and water quality Watershed has greater than 25% impervious cover. Not a candidate for stream restoration Classified as Impacted or Non-Supporting. High retrofit or stream restoration potential Subwatershed drains to a lake that is subject to degradation Reservoir managed to protect drinking water supply Subwatershed drains to an estuary or near-shore ocean Surface water has a strong interaction with groundwater. Groundwater is a primary source of potable water A subwatershed contains a network of small stream channels known as headwaters. Stream order is used to characterize stream-branching (Table 5.5). Small stream channels with no connections are order 1. Streams with 1 connecting channel are order 2. Streams of order 1 are the most common. 5-4

5 Table 5.5 US National Stream and River Mileage (Adapted from Strahler 1957) Stream Order* Number of Total Length of Stream Mean Drainage Area Streams (miles) (square miles)** 1 1,570,000 1,570, , , , , , , , , ,000 2, ,000 11, ,000 55, , , ,800 1,250,000 Total 2,023,400 3,250,000 N/A * stream order based on Strahler (1957) method, analyzing maps at a scale of 1:24,000 ** cumulative drainage area, including tributaries 5.3 Example Water Resource Database Models The water resource planning and management issues described above motivate the content and structure of water resource database models. David Maidment and colleagues at the University of Texas, Austin created a book about hydrologic database models, that in particular presents the ArcHydro database model (Whiteaker, Schneider, and Maidment 2001). The schema tutorial is particularly enlightening for the ArcHydro database model, by the Center for Research in Water Resources (2001). An ArcHydro Schema contains four types of hydro features: drainage, network, hydrography, and channel. Each of these is a part of the ArcHydro Schema (subschema), but they all work together (Figure 5.2a-d). Figure 5.2a Drainage boundary representation (Whiteaker, Schneider, and Maidment, 2001). 5-5

6 Figure 5.2b Network functional water flow representation (Whiteaker, Schneider, and Maidment, 2001). Figure 5.2c Hydrography graphic representation of network (Whiteaker, Schneider, and Maidment, 2001). 5-6

7 Figure 5.2d Channel 3D representation of network (Whiteaker, Schneider, and Maidment, 2001). The geodatabase synthesis diagram across Figures 2.44a-d is presented in Figure

8 Figure 2.45 ArcHydro database model template (ESRI 2006a) The water resource portion of the urban database model generalizes the database design requirements relative to the ArcHydro database model, and this generalization is depicted in Figure 5.3. Figure 5.3 Water resource portion of urban data model (ESRI 2006b) 5-8

9 5.4 Comparing and Contrasting Water resource Database Models Planning oriented database model and data model needs are commonly developed to focus on overall flow at watershed scale, with fewer objects in planning than in programming and implementation because of the broad spatial and temporal scales. Because plans incorporate all aspects of water resources for a particular planning region, they are not as detailed as programs that focus on how to budget for the implementation of the projects. Thus, system-wide plans are broad, but not detailed. For an example of a city water plan see City of Seattle (2006a). A good example of a plan that appreciates the needs for capital improvements to address goals for dependable water supply service, a water supply that meets public health and environmental stewardship water for fish and ecosystem is the one for Cedar River Watershed Habitat Conservation Planning (See City of Seattle 2006b). Improvement programming database model and data model needs focus on the elements of plan that need to be financed in the near term. For an example of what is going on in water resource capital improvement at the City of Seattle see (City of Seattle 2006c). 5.5 Summary This chapter describes water resource database models suitable for use in urban-regional GIS applications. Because water resource planning, programming, and project implementation decision situations are motivated by laws, organizational missions, and regulations, the water resource database designs are also motivated by the particular planning, programming, and project implementation applications that address those decision situations. Water demand, supply, quality and wastewater are four fundamental issues that concerns GIS-based applications for water. Nesting of drainage units basins, sub-basins, watersheds, sub-watersheds, catchment areas are ways of addressing scale of water drainage (flow). Each of the units is relevant to particular human-environment processes at work in urban-regional settings. Consequently, water resource planning and projects can take place at all of those scales. The most common however is watershed level planning. It is the size of drainage area large enough, but not too large for addressing budgeting issues to carry out improvement programs and to monitor projects. That is not to say that GIS applications are not undertaken for the others, because indeed they are, but watersheds are human-scale manageable with the way organizations are working these days. Sub-watersheds are commonly the operative unit at work within watersheds. Thus, there are many subwatershed-level monitoring programs that together work to understand the health of watersheds. The development of the ArcHydro data model by David Maidment and his colleagues and students at the University of Texas moved water resource GIS applications forward in significant ways. The data model can be used to jump start the development of particular database models for specialized applications of GIS addressing water flow issues. There are four major components in the ArcHydro data model: drainage, network, hydrography, and channel. These four components work together to create a database model that handles spatio-temporal change in drainage areas. It is up to a GIS analyst to put the schemas to use for particular scales of drainage unit database modeling. 5-9

10 5.6 References Center for Research in Water Resources ArcHydro Data Model Overview, last accessed November 15, Center for Watershed Protection, Rapid Watershed Planning Handbook. Ellicott City, MD. City of Seattle 2006a. City of Seattle Water System Plan, last accessed November 16, City of Seattle 2006b. Cedar River Habitat Conservation Plan (HCP) HCP/Documents/index.asp, last accessed November 16, City of Seattle 2006c. Water Capital Improvement Program last accessed November 16, ESRI (Environmental Systems Research Institute) 2006a ArcHydro Data Model last accessed November 15, ESRI (Environmental Systems Research Institute) 2006b Urban Basemap Database Model, last accessed November 15, NEPA National environmental policy act of 1969, pub. L , 42 usc , january 1, 1970, as amended by pub. L , july 3, Strahler, A. N Quantitative analysis of watershed geomorphology. Transactions of the American Geophysical Union (38): US Environmental Protection Agency Clean Water Act. last accessed November 16, 2006 Whiteaker, T., Schneider, K. and Maidment, D Applying the ArcGIS Hydro Data Model last accessed November 15, Wilcox, D. J Proposed Map Accuracy Standards for a Multipurpose Cadastre. Computers, Environment, and Urban Systems (9):

11 5.7 Review Questions 1. What is the motivation behind water resource database models? 2. What are the feature classes of water resource database models? 3. What are the four main components of the ArcHydro database model? Why does each exist? 4. How can we compare/contrast water resource database models in regards to GIS applications for planning, programming, and implementation situations? 5.8 Glossary ArcHydro a data(base) model describing watershed (resource) information that was created by David Maidment and his associates at the U of Texas Austin database model A schema and data dictionary associated with the outcomes of a particular database design process. drainage unit an area that collects and drains water schema description of data categories plus data fields that characterize features (entities) about some portion of the world in a database model. topology, spatial the study of relationships including connectedness, adjacency and containment among points, lines, polygons. 5-11