OEM Applications In Hydrologic Modeling
1OEM Applications in Hydrologic ModelingUzair M. ShamsiThis chapter shows how digital elevation models (DEM) can be used to develophydrologic models using off-the-shelf GIS sofuvare. A review of DEManalysis software is presented. The chapter shows how to automaticallydelineate watershed subbasins and streams using DEMs. A case study ispresented to compare the DEM results with the conventional manual delineationapproach. It is found that for rural and moderately hilly watersheds, 30 mresolution DEMs are approptiate for automatic delineation of watershedsubbasins and streams. For 30m DEMs a cell threshold of 500 is appropriate.12.1 IntroductionCell-based raster data sets, or grids, are especially suited to representingtraditional geographic phenomena that vary continuously over space such aselevation, slope, precipitation, etc. Grids are also ideal for spatial modeling andanalysis of data trends represented as continuous surfaces, such as inhydrologic modeling. DEM are data files containing information for the digitalrepresentations of elevation information in a raster form. DEMs are generallystored using one of the following three data structures:grid structures, triangular irregular network (TIN) structures, or contour-based structuresRegardless of the tmderlying data structure, most DEMs can be defmedin terms of (X, Y, Z) data values, where (X, Y) represents the locationcoordinates and Z represents the elevation. The focus of this chapter is on gridShamsi, U.M. 2001. "DEM Applications in Hydrologic Modeling." Journal of Water ManagementModelingR207-12. doi: 10.14796/JWMM.R207-12. CHI 2001 www.chijournal.org ISSN: 2292-6062 (Formerly in Models and applications toUrban Water Systems. ISBN: 0-9683681-4-X)175
176DEAf Applications in Hydrologic ModelingDEMs. Grid DEMs consist of a sampled array of elevations for a number ofground positions at regularly spaced intervals. This data structure creates asquare grid matrix with the elevation of each grid square, called a pixel, storedin a matrix node (ASCE, 1999).The size of a DEM file depends on the OEM resolution, i.e., the fmer theOEM resolution, the smaHerthe grid, and the larger the OEM file. For example,if the grid size is reduced by one-third, the file size will increase nine times.Plotting and analysis of high resolution OEM files is slower because of theirlarge file sizes.OEMs can be used as source data for digital orthophotos and as layers ingeographic information systems (GIS). DEMs can also serve as tools for manyactivities including volumetric analysis, site location of towers, or for drainagebasin delineation. The DEM files may be used in the generation ofgraphics suchas isometric projections displaying slope, direction ofslope (aspect), and terrainprofiles between designated points. They may also be combined with other datatypes such as stream location data and weather data to assist in forest firecontrol or they may be combined with remote sensing data to aid in theclassification of vegetation. DEM applications include (USGS, 2000): modeling tenain gravity data fmuse in locating energy resources, detennining the volume of proposed reservoirs,calculating the amount of matelial removed during strip mining,determining landslide probability, anddeveloping parameters for hydrologic models.This chapter focuses on the hydrologic modeling applications of OEMs.12.2 DEMs12.2.1 USGS DEMsIn the United States, the U.S. Geological Survey (USGS) provides OEM datafor the entire country as part of the National Mapping Program. The USGSOEMs are available in 7.5-minute, IS-minute, 2-arc-second (also known as 30minute), and I-degree units. The 7.5- and 15-minuteDEMs are included in thelarge scale category while 2-arc-secondDEMs fall \vithin the intennediate scalecategory and I-degree OEMs fall within the small scale category. This chapteris based on the applications of7.5 minute USGS DEMs. The OEM data for 7.5minute units correspond to the USGS 1:24,000 scale topographic quadranglemap series for aU ofthe United States and its territories. Each 7 .S-minute OEMis based on 30-meter by 30-meter data spacing with the lmiversal transversemercator (UTM) projection. Each 7.S-minute by 7 .S-minute block provides the
12.2 DEMs177same coverage as the standard USGS 7.S-minute map series. Table 12.1summarizes the USGS DEM data types.Table 12.1 USGS DEM data fOlmats.DEM TypeScaleBlock Size Glid SpacingLarge1:24,0007.5' x 7.5'IntermediateBetween large and small 30' x 30'2 secondsSmall1:250,0003 seconds]OxI"30 ill12.2.2 OEM AccuracyThe accuracy of a DEM is dependent upon its source and the spatial resolution(grid spacing). The vertical accuracy on .S-minute DEMs is equal to or betterthan 15 meters. A minimum of28 test points per DEM is required (20 interiorpoints and 8 edge points). The accuracy of the 7.S-minute DEM data, togetherwith the data spacing, adequately support computer applications that analyzehypsographic features to a level of detail similar to manual interpretations ofinformation as printed at map scales not larger than 1:24,000 scale.12.2.3 OEM FormatsUSGS DEMs are available in two formats:1. DEM File format: This older file format stores DEM data asASCII text and have an extension ofdem (e.g.lewisburK.:PA.dem).2. Spatial Data Transfer Standard (SDTS): This is the latest DEMfile format. SDTS is a robust way of transferring geo-referencedspatial data between dissimilar computer systems and has thepotential for transfer with no information loss. It is a transferstandard that embraces the philosophy of self-contained transfers, i.e. spatial data, attribute, georeferencing, data qualityreport, data dictionary, and other supporting metadata aU areincluded in the transfer. SDTS DEM data are available as tar.gzcompressed files. Each compressed file contains 18 *.ddf filesand two readme text files.Some DEM analysis software may 110t read the new SDTS data. For suchprograms, the user should translate the SDTS data to a DEM file format. SDTStranslator utilities, like SDTS2DEM or MicroDEM, are available from the USGSSDTS web site ) to convert the SDTS data to other file fonnats.
178DEAf Applications in Hydrologic Modeling12.2.4 DEM AvailabilityUSGS DEMs can be downloaded gratis from the USGS Global Land Infonnation System web site (http://edc.usgs.gov/webglis). DEM data on CD-ROMcan also be purchased for an entire county or state for a small fee to cover theshipping and handling cost. DEM data for other parts of the world are alsoavailable. The 30 arc-second DEMs (approximately 1 km2 square cells) for theentire world have been developed by the USGS Earth Resources ObservationSystems (EROS) Data Center and can be downloaded from the abovementioned website. More information can be found on the W orId Wide Webat the USGS node of the National Geospatial Data Clearinghouse site (http://nsdi.usgs.gov/nsdi/).12.3 Software ToolsSome sample DEM analysis software tools and utilities are listed below: ArcView Spatial Analyst, AreView 3-D Analyst, and Hydroextensions (ESRI, Redlands, California; www.esri.com) IDRISI (Clark University, Worcester, Massachusetts,\v"vw. clarklabs. org) TOPAZ (US Department of Agriculture, Agticulmral ResearchService, El Reno, Oklahoma, grLars.usda.gov) MIKE 11 GIS (Danish Hydraulic Institute, Denmark,www.dhLdk) The Soil and Water Assessment Tool (SWAT) (US Departmentof Agriculture, Agricultural Research Service, Temple, Texas,www.brc.tamw;;.edu/swat/) Watershed Modeling System (WMS) (Engineering ComputerGraphics Lab, Brigham Young University (BYU), Provo, Utah,\V\'{w .ecgL byu.edu/sofh'lare/wmsi) RiverTools (Research Systems, Boulder, Colorado,\v\v\v.rsinc.com) MicroDEM (Developed by Professor Peter Guth of the Oceanography Department, U.S. Naval Academy, www.usna.edulU sers/oeeano/pguthl\vebsite/microdem.htm)Some programs such as Spatial Analyst, Mike 11 GIS, and WMS provideboth the DEM analysis and hydrologic modeling capabilities. ASCE (1999) hascompiled an excellent review of the above and other hydrologic modelingsystems that use DEMs. Major DEM software programs are discussed in thefollowing pages.
12.3 Software Tools17912.3.1 Spatial Analyst and Hydro ExtensionSpatial Analyst is an optional extension (add-on program) for ESRl' s Arc ViewGIS software, which must be purchased separately. The Spatial Analystextension adds raster GIS capability to the vector based ArcView GIS. TheSpatial Analyst enables desktop GIS users to create, query, and analyze cellbased raster maps; derive new information from existing data; query information across multiple data layers; and fully integrate cell-based raster data withtraditional vector data sources.Spatial Analyst is supplied with a hydro (or hydrology) extension whichfurther extends the Spatial Analyst user interface for creating input data forhydrologic models. This extension provides functionality to create watershedsand stream networks from a DEM, calculate physical and geometric propertiesofthe watersheds, and aggregate these properties into a single attribute table thatcan be attached to a gtid or shapefile.Before starting any DEM analysis, the user must define the minimumnumber of upstream cells contributing flow into a cell to classify that cell as theorigin ofa stream. This number, also referred to as the cell "threshold", definesthe minimum upstream drainage area necessmy to start and maintain a stream.The smaller the threshold, the smaller the subbasin size, the larger the numberof subbasins, and hence the slower the computations. This is an important partof the DEM analysis where user judgement is required.Depending upon the user needs, there are two approaches to use the hydroextension:1. Hydro Pulldown Menu Options: To create watershed subbasins or thestream network, work directly with the Hydro pulldmvn menu options. Table12.2 provides a brief description of each ofthese menu options. The follmvingsteps should be perfonned to create watersheds using the Hydro pulldo\,mmenu options. The output grid from each step serves as the input grid for thenext step. Import the raw USGS DEM Fill the simes using the "Fill Sinks" menu option (input rawUSGS DEM). This is a very impoliant intennediate step. Thoughsome sinks are valid, most are data errors and should be fined.Compute flow directions using the "Flow Direction" menu option(input filled DEM grid).Compute flow accumulation using the "Flow Accumulation"menu option (input flow directions grid). Delineate streams using the "Stream Network" menu option(input flow accumulation grid). Delineate watersheds using the "Watershed" menu option.
180DEM Applications in If.vdrologic ModelingTable 12.2 Hydro Extension Menu Options.FunctionHydro Menu OptionHydrologic ModelingCreates watersheds and calculates their attributesFlow DirectionCompntes the direction of flow for each cell in aDEMIdentify SinksCreates a grid showing the location of sinks or areas ofFill SinksFills the sinks in a DEM, creating a new DEMFlow AccumulationCalculates the accumulated flow, or number of up-slopeinternal drainage in aDEMcells, based on a flow direction gridWatershedCreates watersheds based upon a user specified flowaccumulation tm:esholdAreaCalculates the area of each watershed in a watershed gridPerimeterCalculates the perimeter of each watershed in a watershedLengthCalculates the straight-line distance from the pour point toflow LengthCalculates the length of flow path for each cell to the pourgridthe furthest perimeter point for each watershedpoint for each watershedFlow Length by \YatcrshedCalculates the maximum distance along the flow path withineach watershedShape Factor by WatershedCalculates a shape factor for each watershed. Shape factor iscalculated as watershed length squared, divided bywatershed areaStream Network As Line ShapeCreates a vector stream network from a flow accumulationgrid based on a user specified thresholdCentroid as Point ShapeCreates a point shape file of watershed centroidsPour Points as Point ShapeCreates a point shape file of watershed pour pointsMean ElevationCalculates the mean elevation within each watershedMean SlopeCalculates the mean slope within each watershed11,Iean PrecipitationCalculates the mean precipitation in each watershedMean Curve NumberCalculates the mean curve number for each watershed2. Hydrologic lvfodeling Dialogue: To create subbasins and calculate manyadditional attributes for them, use the hydrologic modeling dialogue, which isthe first choice under the Hydro pulldown menu. The hydrologic modelingdialog is designed to be a quick one-step method for calculating and thenaggregating a set of watershed attributes into a single file. This file can then be
12.3 Software Tools181used directly in a hydrologic model such as BYU's WMS, or it can bereformatted for input into HEC's HMS model, or others. The following stepsshould be performed to create watersheds using the hydrologic modelingdialogue: Choose Delineate from DEM and select an elevation surface. Fill the sinks when prompted.Specify the cell threshold value when prompted. This will createwatersheds based on the number of cells, or up-slope areadefmed by the user as being the smallest watershed the userwants.DetailedDEM analysis instructions are providedatthe following web sites:1. ESRI web site at http://www.esri.com/arcuser/ (click on the"Terrain Modeling with ArcView GIS")2. University of Texas at Austin (Center for Research in WaterResources) web site at http://vlv\lw.crwr.utexas.eduigis! (clickon GIS Hydm button)The last four hydro options (Table 12.2) work ''lith existing data layers.They do not create elevation, slope, precipitation, and runoff curve munbercoverages. They simply compute mean areal values of these four parametersfor the subbasins using the existing GIS coverages ofthese parameters. Thus,the GIS coverages of elevation, slope, precipitation, and runoff curve numbermust be available for the mean functions of the hydro extension to work.Hydro also has a raindrop or pourpoint feature which helps the user totrace the flow path from a specified point to the watershed outlet. Hydro alsocalculates a flow length as the maximum distance along the flow path withineach watershed. The subbasin area can be divided by flow length to estimatethe overland flow width for input to a rainfall-runoff model like EPA's StormWater Management Model (SWMM).12.3.2 IDRISIlDRISI is not an acronym; it is named after a cartographer born in 1099 AD inMorocco, N. Africa. IDRISI was developed by the Graduate School ofGeography at Clark University. IDRISI covers the full spectrum of GIS andRemote Sensing needs from database query, to spatial modeling, to imageenhancement and cla.'lsification. Special facilities are included for environmental monitoring and natural resource management, including change and timeseries analysis, multi-criteria and multi-objective decision support, uncertaintyanalysis (including Bayesian and Fuzzy Set analysis) and simulation modeling(including force modeling and anisotropic friction analysis). TIN interpolation,Kriging and conditional simulation are also offered.
182DEl',,! Applications in Hydrologic ModelingIDRISI is basically a raster GIS. IDRISI includes tools for manipulatingDEM data to extract streams and watershed boundmies. IDRISI GIS data hasan open format and can be manipulated by external computer programs writtenby users. This capability makes IDRISI a suitable tool to develop hydrologicapplications. For example, Quimpo and Al-Medeij (1998) wrote FORTRANprograms to model surface runoff using IDRISI. Their approach consisted ofdelineating watershed subbasins from DEM data and estimating subbasin SCSrunoff curve numbers from soils and land use data.J!'igure 12.1 IDRISI's DEM analysis features.Figure 12.1 shows IDRISI's DEM analysis capabilities. The upper leftwindow shows a Triangulated Irregular Netw'ork (TIN) model created fromdigital contour data. The upper right window shows a DEM created from theTIN with original contours overlayed. The lower right window shows anilluminated DEM emphasizing relief. The lower left window shows a false colorcomposite image (TM bands 234) draped over the DEM (IDRISI, 2000).
12.4 Case Study18312.3.3 TOPAZTOPAZ is a software system for automated analysis oflandscape topographyfrom DEMs. The primary objective of TOPAZ is the rapid and systematicidentification and quantification of topographic features in support of investigations related to land-surface processes, hydrologic and hydraulic modeling,assessment of land resources, and management of watersheds and ecosystems. Typical examples oftopographic features that are evaluated by TOPAZinclude telTain slope and aspect, drainage pattems and divides, channelnetwork, watershed segmentation, subcatchment identification, geometric andtopologic properties of channel links, drainage distances. representativesubcatchmcnt properties, and chalmel network analysis (Garbrecht and Martz,2000).12.4 Case StudyAlthough DEM delineation techniques have been advanced in recent years, theliterature lacks a comparison of manual versus DEM teclmiques. The objectiveof this case study was to test the efficacy ofDEM-based automatic delineationof watershed subbasins and streams. It was assumed that the manualdelineations are more accurate than the DEM delineations. Thus, a comparisonof manual and DEM delineations was made to test the accuracy of DEMdelineations.The case study watershed is the Bull Run watershed located in UnionCounty in north-central Pennsylvania (Shamsi, 1996). This watershed wasselected because of its small size so that the report size GIS maps can be clearlyprinted. However, the proposed teclmique has also been successfully appliedto large watersheds. Bull Run watershed's 21.8 km2 (8.4 mi 2) drainage area istributary to the West Branch Susquehanna River at the eastem boundary ofLewisburg Borough. The 7.5 minute USGS topographic map of the watershedis shown in Figure 12.2. The predominant land use in the watershed is openspace and agricultural. Only 20% of the watershed has residential, commercial,and industrial land uses.Manual watershed subdivision was the first step of the case study. Manualdelineation is done by outlining the watershed boundary on a topographic map,identifying the major drainage paths (streams, rivers, etc.), subdividing thedrainage paths into smaller segments (sewers, swales, etc.) caned reaches, andfinally subdividing the watershed into smaller drainage areas called subbasins.All subbasins must enclose one or more reaches. Each reach must correspond
.00 hJ (J ;:::t,oi;ls· 1}o0-OQ . f}Figure 12.2 The case study watershed showing manual subbasins and streams,i:;'";:s'OQ
12.4 Case Study185to one and only one subbasin. There are two ways of manual watershedsubdivision: (i) multi reach subdivision in which a subbasin may contain severalreaches, and (ii) single reach subdivision in which only one reach per subbasinis allowed. The first approach creates large subbasins for which computationof overland flow width and slope is difficult. The single reach subdivisioncreates the smallest natural subbasins from the available topographic maps.Single reach subbasins are consistent with many modeling packages such asHEC-l, and provide an intuitive nomenclature scheme (same ID for a reach andits tributary subbasin). Note that the db;tinction bet\veen lumped and distributedmodels becomes unclear when the subbasins are made very sman (DeVantierand Feldman 1993). By creating small subbasins, the difference between thelumped and distributed models can be minimized and the inherent inaccuraciesof grossly lumped models can be reduced. Due to its many advantages, thesingle reach subdivision scheme was used in this study. The 7.5- minute USGStopographic map of the study arca was used for manual subbasin delineationwhich resulted in 28 subbasins shown in Fi
Compntes the direction of flow for each cell in aDEM Creates a grid showing the location of sinks or areas of internal drainage in aDEM Fills the sinks in a DEM, creating a new DEM Calculates the accumulated flow, or number of up-slope cells, based on a flow direction grid Creates watersheds based upon a us
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Thus, hydrologic modeling is an essential tool and cost-effective process in understanding the movement of water balance within a watershed (Combalicer et al., 2010). This paper used the soil and water assessment tool (SWAT) model to simulate hydrologic processes influenced by land cover
CHAPTER II LITERATURE REVIEW This chapter focuses on reviewing hydrologic modelling, particularly with regards to the hydrologic cycle, watershed hydrology, watershed modeling, and watershed models. However, the review on watershed models is more exhaustive because this is the main topic of this research project. 2.1 HYDROLOGIC CYCLE .
Section 4.1 - Introduction to Hydrologic Analysis Volume 2, Technical Guidance Page 4 - 1 4.1 Introduction to Hydrologic Analysis This Section addresses surface water hydrology and % TSS Removal Calculation only; it does not address groundwater hydrology. Therefore all references in
