Using GIS to Delineate Watersheds Ed Poyer NRS 509, Fall 2010 A watershed is an area that contributes flow to a point on the landscape. (Bolstad, 2005). Watersheds are an important focus of study because management of water volume and quality, soil conservation, flood control, wild life habitat and forests require understanding the many features of relevant watersheds. Thus, watershed modeling is an ideal application for GIS. A key component of watershed modeling is determining the drainage area that contributes flow to that point on the landscape, doing so requires identifying channels and divides and delineating watersheds. This paper traces the development of automated techniques to delineate watersheds from vector and DEM raster datasets and Triangulated Irregular Networks. The delineated watersheds are integrated with other GIS data to enhance watershed-based research. Steady progress has been made in terms of precision, speed, accessibility and ease of use. Watershed delineation techniques are an important tool for GIS analysts and consumers because germane watershed boundary datasets with suitable resolutions for a specific study area may be unavailable. Study areas may be too small for existing datasets, requiring a greater level of detail than is available in existing watershed models. For example, although watershed boundaries are available from the USGS Hydrologic Unit Code (HUC) dataset, the 1:250,000 scale may not be suitable for many studies. Drainage density may be high in the study area. Sample points may not be suitably located or conveniently spaced to allow use of datasets with previously delineated and labeled watersheds. In these and other cases, delineating the watersheds in question becomes an early and critical task in studying and managing water resources. Historically watersheds were delineated manually. This process is labor intensive, slow, tedious, inconsistent and error-prone. Early automated techniques required custom software to delineate watershed components using DEM raster datasets spanning the drainage area. These datasets are readily available from large cell size down to 30 meter cell size. Automated approaches are considered to be preferable to costly, tedious and error-prone manual delineation. The basic steps are to extract local stream channel networks, construct the associated divide networks, and topologically code and label the drainage system. DEM- based techniques can efficiently form the basis of a geographic information system designed to address watershed based analysis. Applications to benefit from these techniques include evaporation modeling, tracing drainage paths and hydrologic simulation modeling. (Band, 1986). These techniques were incorporated into commercially viable PC-based GIS systems. While DEM-based algorithms are well suited for working with fairly large areas, they are less well suited for areas with high drainage density or for engineering projects or small study areas. These areas are usually more limited in scope. For these areas, substantial topological information is not captured on standard USGS 1:24,000 topographic maps. Triangulated Irregular Networks (TINs) are well suited to these smaller, more detailed sites (Jones et al. 1990). These smaller areas may be based upon a site survey which provides a scattered set of three dimensional high resolution observations. Alternatively, data points may Ed Poyer, NRS509 Page 1 of 7 November 24, 2010
originate in gridded data or may result from digitizing a contour map. TINs accurately reflect the original survey data whereas grid based approaches require interpolation of the survey points. TIN network boundaries can more closely align with those of the target parcel, and TINs are computationally efficient and flexible. The TINs delineation approach begins with constructing triangles from the survey points according to precise topological criteria. The next steps are to calculate the channel network, delineate drainage boundaries and finally group the resulting triangles into source areas. The source areas are then used to delineate watersheds. Benchmarks show that computing watersheds from delimited nodes is accomplished in 1-2 seconds. Thus, TIN models offer effective and efficient watershed analysis support to interactive GIS. Artificial drainage systems present the next challenge to automating watershed delineation. Developed landscapes require that the manmade drainage network be mapped to the TIN of the natural terrain. This addition required that algorithms be enhanced (Nelson et al. 1994). Geometric parameters required for hydrologic modeling are calculated, and the results interfaced into the U. S. Army Corps of Engineers HEC-1 Flood Hydrograph Package. The delineated basins can be exported as polygons to GIS software. In comparing the raster, TIN and vector-based models, grid-based models advantages include a simpler data structure, the abundance of grid-based data, and the ease in which the gridbased model is transportable across systems. TIN based models have advantages in that they require less memory than grids, and linear features can be accurately represented using triangle edges. Map-based polygon models also require less storage than grids, linear features can again be accurately represented. However, map-based models lack elevation data within the polygons, so sub-basin delineation is not available. Thus, if the study area is below the smallest available HUC level for the locale, then another model must be used (Nelson et al. 1999). A Watershed Modeling System (WMS) integrated watershed delineation methods, a flood hydrograph model and Arc GIS capabilities into a rainfall-runoff model with a graphical user interface. This system was applied to a modeling study conducted in arid Mariposa county, AZ (Nelson et al. 1999). The WMS system calculates geometric parameters from the watershed model. The system extracts land use and soils data from the Mariposa county GIS. This is supplied as input into HEC-1. The resulting computed hydrographs and peak flow data are exported to a GIS using ArcView shapefile format. This data is combined with various other data within the GIS to support decision making. The results were compared to results using conventional methods of watershed modeling, and were found to be in excellent agreement. The integrated nature and graphical user interface of this system represents improvement in both accessibility and ease of use. Advancements in GIS have been used to investigate the lower Cheat River basin, a heavily impacted watershed in the coal mining regions of West Virginia (Strager et al. 2009). This watershed delineation process focused on identifying stream segment level watersheds. Stream segments between stream junctions identified on the 1:24,000 scale NHD were used. An ArcGIS Spatial Analyst extension was used to calculate a flow direction grid, from which watersheds were delineated for each stream segment. These watersheds were linked to enable cumulative analysis. A weighted watershed Mining Index was developed for each stream segment watershed, and aggregated up to the NRCS 12 digit HUC scale within the lower Cheat River watershed. The index was used to establish priorities for reclamation. This approach was recommended to quantify both local stream segment and cumulative impacts to aid in watershed management. Ed Poyer, NRS509 Page 2 of 7 November 24, 2010
Web-based real-time GIS watershed delineation is the next step in the progression. A webbased GIS user interface was developed which obtains the outlet point s position and passes it to a server-based watershed delineation algorithm. The system is based on a raster dataset and uses neighborhood functions to construct a flow direction grid. The system offers ease of use, accessibility and reasonable speed, but is limited in that it cannot handle developed areas, lacking a mechanism to handle artificial drainages (Choi et al. 2003). The need for standards becomes apparent as web-based systems become more popular. The Open Geospatial Consortium (OGC) developed a Web Processing Service (WPS) specification. The OGC is a standards organization focusing on developing non-proprietary GIS standards to promote interoperability. Many GIS developers have actively embraced the standards as a way to promote the technology (Michaelis et al. 2009). The WPS specification is a thin client / fat server model. Minimal software and data reside on the client. Large datasets and the core software engines reside on the server. This simplifies software installation and maintenance: as algorithms improve or new data is available, only the server need be updated. XML is used for almost all communication. This model is most useful in situations where the bulk of the data is on the server so that minimal data need be transmitted, or when the significant processing power of the server is required. When large volumes of data need to be uploaded before processing, or when relatively little processing power is needed, the more conventional approach of local processing has the advantage. A watershed delineation tool was included in a test implementation of the WPS specification. The client browses a series of maps and identifies a point of interest. The server side tool hosts a DEM raster dataset from which it builds stream networks for flow patterns and polygons to delineate the watershed boundaries and provide topological calculations. The tests showed that the WPS specification was workable, and that the test version would be useful for existing GIS users. This paper has traced the development of watershed delineation methods using GIS. Some early approaches used specialized, custom software to process DEM raster data models. Other early approaches used custom written software on TINs, while others utilized polygon based data models. The advantages and limitations of each of these approaches were described. Two case studies were presented in which watershed delineation served a key role in watershed analysis and GIS-based decision making. The growth of internet applications led to interactive web-based GIS approaches. Proliferation led to the need for implementation standards, which were described. That watershed delineation was selected as an early test case for the standard underscores the importance of the function. In the future, one can expect more web-based functionality from both open source GIS vendors and flagship vendors. ESRI founder Jack Dangermond demonstrated the free website ArcGIS.com at ESRI s International User Conference in July, 2010. (Bigelow, 1). Although watershed delineation was not specifically mentioned, there is certainly a trend toward greater accessibility of GIS functionality. Ed Poyer, NRS509 Page 3 of 7 November 24, 2010
References not included in annotated bibliography: Bolstad, P., (2005), GIS Fundamentals, a First Text on Geographic Information Systems, 2 nd Edition, Eider Press, White Bear Lake, MN. Bigelow, Bruce. ESRI Reshapes Its Proprietary Mapping System into an Open Crowdsourcing Platform, Raising a Challenge for Google, xconomy.com. 2010. November 11, 2010. <http://www.xconomy.com/san-diego/2010/07/16/esri-reshapes-its-proprietary-mapping-systeminto-an-open-crowdsourcing-platform-raising-a-challenge-for-google/> Ed Poyer, NRS509 Page 4 of 7 November 24, 2010
Band, L. E., (1986). Topographic partition of watersheds with digital elevation models, Water Resources Research, 22:15-24. Band reviews essential automated techniques to delineate watershed components using DEM raster datasets spanning the drainage area. He cites these as preferable to costly, tedious and error-prone manual delineation. The basic steps are to extract local stream channel networks, construct the associated divide networks, and topologically code and label the drainage system. He describes algorithms and addresses problems presented by artificial pits, in which cells present local minima which may confound the model. These handled by either pre-processing, or dynamically filling the pits by examining adjacent cells and gradually adjusting elevations to smooth the surface. Thinning techniques are used to reduce the dataset and to ensure that the stream flows along the valley floor. Band concludes that the resulting dataset can efficiently form the basis of a geographic information system designed to address watershed based analysis. Band suggests that areas of watershed research that may benefit from these techniques include evaporation modeling, tracing drainage paths and hydrologic simulation modeling. Jones, N. L., Wright, S. G., and Maidment, D. R., (1990). Watershed delineation with triangle-based terrain models, Journal of Hydraulic Engineering, 116:1232-1251. This paper describes delineating watersheds from Triangulated Irregular Networks (TINs). According to the authors, TINS provide advantages over grid-based models. TINs accurately reflect the original survey data whereas grid based approaches require interpolation of the survey points. TINs boundaries can more closely align with those of the target parcel. Finally, they argue that TINs are computationally efficient and flexible. The authors review procedures to construct triangles from survey points, and describe useful data structures and algorithms to identify paths of steepest descent and flow lines. The authors review pre-processing tasks for dealing with problem data such as artificial pits, diverging streams and flat triangles. An essential topological criterion is that the triangles meet the Delauney tessellation criteria: the circle defined by the triangle vertices shall encompass no other vertices. This avoids problems fostered by long, thin triangles and ensures that triangles are as equiangular as possible. The authors provide the algorithms and formulae for each step to delineate source areas: calculate the channel network, delineate drainage boundaries and sub-divide triangles, and group the resulting triangles into source areas. The source areas are then used to delineate watersheds. The authors summarize benchmarks run on a Macintosh II computer, resulting in computing watersheds from delimited nodes in 1-2 seconds. The authors conclude that TIN models offer effective and efficient watershed analysis support to interactive GIS, to address problems such as hazardous liquid spills and site grading. Nelson, E. J., Jones, N. L., and Miller, A. W. (1994). "An algorithm for precise drainage basin delineation." Journal of Hydraulic Engineering, ASCE, 120(3), 298-312. This paper refines drainage basin delineation using TIN models. The algorithms were extended to address the issues which streets and other artificial drainages introduce in developed terrain. The extended algorithm requires maps of these artificial drainages to be added to the TIN network, which can be accomplished by interactive editing. The introduction of these arbitrary stream segments required further enhancements to the algorithm. For thousands of data points the entire process executes on a PC in a minutes to a few hours, depending on the amount of editing. With these enhancements, TIN models can be applied to urban as well as rural areas. Ed Poyer, NRS509 Page 5 of 7 November 24, 2010
Geometric parameters required for hydrologic modeling can be calculated, and the results have been interfaced into HEC-1 for surface runoff analysis. HEC-1 is an industry-standard program for hydrologic analysis, developed by the Hydrologic Engineering Center in Davis, California. The polygons representing drainage basin boundaries can be used with GIS software. Some limitations remain, such as the assumption that no flat triangles exist. As of the writing of the article, techniques were being researched for automatic editing of flat triangles, and preprocessing of data extracted from gridded DEM models and contour data. Nelson, E. J., Smemoe, C. M., and Zhao, B., (1999), A GIS approach to watershed modeling in Maricopa county, Arizona, American Society of Civil Engineers 29 th Annual Water Resources Planning and Management Conference Environmental Engineering, Tempe, AZ Nelson et al. describe an integrated Watershed Modeling System (WMS) developed for Maricopa county Arizona. The system leverages existing watershed delineation methods, a flood hydrograph model, and tight integration with Arc GIS capabilities into a rainfall-runoff model with a graphical user interface. The WMS is flexible in that it automates watershed delineation from a TIN model, a grid based model such as DEM, or from maps using a vector based model. The WMS system calculates geometric parameters from the watershed model. The system is customized to extract land use and soils data from the Mariposa GIS. This is supplied as input into the U. S. Army Corps of Engineers HEC-1 Flood Hydrograph Package. The resulting computed hydrographs and peak flow data are then exported to a GIS using ArcView shapefile format. This data can then be combined with various other data within the GIS to support decision making. A central purpose of the study was to reconcile WMS test results to results from conventional practices. The authors characterize the results as reconciling with excellent agreement. This automated system produces results much more quickly than, and as accurately as, conventional methods. Choi, J. and Engel, B. A., (2003).Real-time watershed delineation system using Web-GIS, Journal of Computing in Civil Engineering, 17:189-196. This work describes a web-based GIS system developed to delineate watersheds in real time. They observe that watershed delineation functionality in commercial and in public domain GIS was too difficult to use in a web-based development environment, and that real time functionality was needed. The authors reviewed user interface requirements and technical design considerations, then provided a system overview. The system used a web-based GIS user interface to obtain the outlet point s UTM coordinates to pass to the server-based watershed delineation algorithm. The algorithm uses a grid data format proposed by ESRI in 1992, and examines the eight adjacent cells to build a flow direction grid, building upward from the user specified outlet point. The system takes from a five to fifteen seconds to build a 2,000 km 2 watershed using 30 m cell size. The calculated watershed areas were compared to those of reference watersheds delineated by a specialist using more extensive data sets, yielding errors from 0.9 to 12.5 percent. Comparing areas is a very limited way to assess the results, but watershed delineation results are by their nature difficult to compare. The system promises the advantages of ease of use, accessibility and speed. The system is limited in that it cannot handle developed areas, because there is no mechanism for handling artificial drainages. Ed Poyer, NRS509 Page 6 of 7 November 24, 2010
Michaelis, C. D., and Ames, D. P., (2009). Evaluation and Implementation of the OGC web processing service for use in Client-side GIS, Geoinformatica, 13:109-120. This paper describes and evaluates the Open Geospatial Consortium (OGC) Web Processing Service (WPS) specification. The OGC is a standards organization focusing on developing non-proprietary GIS standards to promote interoperability. Many GIS developers have actively embraced the standards as a way to promote the technology. The WPS specification is a thin client / fat server model. Minimal software and data resides on the client. Large datasets and the core software engines reside on the server. This simplifies software installation and maintenance: as algorithms improve or new data is available, only the server need be updated. Tasks are sent via XML documents from a client to a server to process the request. Communication from the server back to the client is also accomplished via XML, which may include URL links for complex output. The authors explore the conditions under which processing is most effectively handled locally (on the client) versus on the server. These conditions include dataset size and computational complexity, so that the criteria are based on the time to transmit the data, process it and return the results. A need to transmit large datasets shifts the balance toward client side, processing, while a need for high computational power shifts the balance toward server based processing. The authors describe two test implementations, one of which was a watershed delineation tool. The client browses a series of maps and identifies a point of interest. The server side tool hosts a DEM raster dataset from which it builds stream networks for flow patterns and polygons to delineate the watershed boundaries and provide topological calculations. The tests convinced the authors that the WPS specification was workable, and that the test version would be useful for existing GIS users. Strager, M. P., Petty, J. T., Strager, J. M., and Bartker-Fulton, J. (2009), A spatially explicit framework for quantifying downstream hydrologic conditions, Journal of Environmental Management, 90:1854-1861. This paper explores how advancements in GIS have improved investigations into stream ecosystems and their watersheds. The authors applied improved datasets and more detailed and accessible techniques for stream network modeling and watershed delineation to analyze the lower Cheat River basin, a heavily impacted watershed in the coal mining regions of West Virginia. This watershed delineation process focused on identifying stream segment level watersheds using stream segments between stream junctions identified on the 1:24,000 scale NHD. An ArcGIS Spatial Analyst extension was used to calculate a flow direction grid, from which watersheds were delineated for each stream segment. Using the NHDs stream flow data, these watersheds were linked to enable cumulative analysis. A weighted watershed Mining Index was designed and calculated for each stream segment watershed, and aggregated up to the NRCS 12 digit HUC scale within the lower Cheat River watershed. The index was used to establish priorities for reclamation. The authors recommend this approach to quantify both local stream segment and cumulative impacts to aid in watershed management. Ed Poyer, NRS509 Page 7 of 7 November 24, 2010