Introduction to GIS and Hydrology Steve Kopp ESRI Dean Djokic ESRI Al Rea USGS

Outline Hydrology overviewHydrology overview DataData GIS tools for hydrology – the building blocksGIS tools for hydrology – the building blocks GIS data models and specialized toolsGIS data models and specialized tools Hydrologic modelingHydrologic modeling Hydraulic modelingHydraulic modeling Overview of NHD-PlusOverview of NHD-Plus

Water Resources Overview 2 broad categories of water2 broad categories of water –Surface water –Groundwater 2 broad categories of water modeling2 broad categories of water modeling –Quantity –Quality Today’s focus surface water quantityToday’s focus surface water quantity

Surface Water Quantity How much water is there?How much water is there? –Rainfall runoff modeling, a type of hydrologic modeling, determines for a given storm on a landscape, how much water will become runoff. Where will it go?Where will it go? –Hydraulic modeling takes the quantity of water and the shape of the landscape and stream channel and determines how deep the water will be and what area it will cover in the event of a flood.

Hydrologic Modeling Goal: Find stream discharge, Q, at a location for a given precipitation.Goal: Find stream discharge, Q, at a location for a given precipitation. GIS is used to summarize terrain and hydrologic characteristics of the watershed for input to a model.GIS is used to summarize terrain and hydrologic characteristics of the watershed for input to a model. Many ways to calculate Q.Many ways to calculate Q. –Statistical methods USGS regression equations (NFF, StreamStats)USGS regression equations (NFF, StreamStats) –“Physical” modeling (rainfall-runoff models) HEC-HMS (successor to HEC-1), TR-20, etc.HEC-HMS (successor to HEC-1), TR-20, etc.

Hydrologic Modeling Map natural processes onto software tasksMap natural processes onto software tasks Aggregate landscape characteristics to simplifyAggregate landscape characteristics to simplify –“Lumped parameter model”

Hydraulic Modeling Goal: to predict water surface elevations for the creation of flood inundation maps.Goal: to predict water surface elevations for the creation of flood inundation maps. –Also velocity, sedimentation, quality Input: channel and floodplain geometry with hydraulic characteristics, plus discharge and initial water surface level.Input: channel and floodplain geometry with hydraulic characteristics, plus discharge and initial water surface level. Output: water surface elevation at each cross section and other characteristics.Output: water surface elevation at each cross section and other characteristics.

GIS Data for Hydrologic and Hydraulic Modeling Digital Elevation ModelDigital Elevation Model Watershed boundariesWatershed boundaries HydrographyHydrography SoilsSoils LandcoverLandcover Current and historic water recordsCurrent and historic water records Climate, weather, rainfallClimate, weather, rainfall Channel geometry (cross sections)Channel geometry (cross sections)

Elevation Data TypesTypes –DEM : Digital Elevation Model –DSM : Digital Surface Model Data StructureData Structure –Raster –TIN

Where do you get DEM data? SourcesSources –USGS DEM, NED, DTED, ETOPO30, SRTM –Interpolated from points and lines –Generated photogrammetrically –LiDAR Created with interpolation toolsCreated with interpolation tools –especially TOPOGRID, TopoToRaster What cellsize and accuracy?What cellsize and accuracy? –Horizontal and Vertical resolution must be appropriate for the landscape and scale being modeled.

DEM construction issuesDEM construction issues –Resolution and extent –Projection (for hydrology - equal area) –Source elevation data –Interpolation techniques (IDW, spline, via TIN) Problems with contour inputProblems with contour input –Specialized DEM construction software/components (ANUDEM, TOPOGRID, TopoToRaster) DEM Construction

Hydrologically correct DEMHydrologically correct DEM –Sinks Do not fill in the Great Salt LakeDo not fill in the Great Salt Lake –Streams in the correct place? To burn or not to burnTo burn or not to burn –Watershed boundaries in the correct place? To fence or not to fenceTo fence or not to fence DEM Construction continued

Drainage System Watershed (Basin, Catchment, Contributing area) Watershed Boundaries (Drainage Divides) Pour Points (Outlets)

GIS Tools for Describing Surface Water Movement DEM FLOW DIRECTION SINK FILL FLOWLENGTH FLOW ACCUMULATION WATERSHED SNAP POUR Depressionless DEM Are there any sinks? No Yes STREAM LINE STREAM ORDER STREAM LINK Apply Threshold

Flow Direction Elevation Direction Coding Flow Direction

Flow Accumulation Direction Coding

Function Processing DEM FLOW DIRECTION SINK FILL FLOWLENGTH FLOW ACCUMULATION WATERSHED SNAP POUR Depressionless DEM Are there any sinks? No Yes STREAM LINE STREAM ORDER STREAM LINK Apply Threshold

Creating Vector Streams Value = No Data NET_GRID StreamToFeature RasterToFeature

Stream Link Assign a unique value to each stream segment.Assign a unique value to each stream segment. –Can be used as input to Watershed

Stream Ordering Strahler Shreve

Watershed Delineate the contributing area to a cell or group of cells.Delineate the contributing area to a cell or group of cells.

SnapPour Snap the “pour point” of a watershed to the cell of highest flow accumulation within a neighborhood.Snap the “pour point” of a watershed to the cell of highest flow accumulation within a neighborhood. –Prevents accidental creation of tiny watersheds on channel side slopes. Snap distance Cell you clicked on The cell that will be selected (cell with highest flow accumulation) Stream

Flow Length Calculate the length of the upstream or downstream flow path from each cell.Calculate the length of the upstream or downstream flow path from each cell.

DEM Errors – Sinks and Spikes Sinks: when sinks are (or are not) sinks – lakes, depressions, …Sinks: when sinks are (or are not) sinks – lakes, depressions, … –Global fill –Dealing with internal basins –Selective fill DepthDepth AreaArea Sink Filled sink

DEM Editing Streams: when streams are not where they “should” beStreams: when streams are not where they “should” be –Flat areas – difficulty in determining the flow pattern –Barriers (roads) diverting the flow paths How to “model” bridges and culverts in DEMHow to “model” bridges and culverts in DEM How to model damsHow to model dams –Imposing the flow pattern - to burn or not to burn (beware of the scale issues and artifacts – Saunders, 2000) Simple burnSimple burn AGREEAGREE OMNROMNR

DEM Editing (cont.) Boundaries - when watershed boundaries are not where they “should” beBoundaries - when watershed boundaries are not where they “should” be –To fence or not to fence –Ineffective flow areas

DEM Editing (cont.) ToolsTools –Global operators FillFill Agree (burn/fence)Agree (burn/fence) OMNR (when it becomes available)OMNR (when it becomes available) SWFWMDSWFWMD Custom – model builderCustom – model builder –Micromanagement DEM Editing toolsDEM Editing tools Custom – model builderCustom – model builder

Enhanced Flow Direction (OMNR – Kenny & Matthews) Main stepsMain steps –Standard flow directions (D8) –A hybridized raster/vector topological analysis to assign D8 flow directions to cells that intersect the network. –An iterative raster single cell dilation and D8 assignment from within water bodies to focus flow towards the virtual segments. –A merging of results from steps 1, 2 and 3 to produce a final “enhanced flow direction grid”.

Summarizing Watershed Characteristics (Zonal Statistics) A zone is all the areas/cells with the same value.A zone is all the areas/cells with the same value. Calculate a statistic within the zones for each cell in a raster.Calculate a statistic within the zones for each cell in a raster. Input zones can be feature or raster.Input zones can be feature or raster. Output as a raster, summary table, or chart.Output as a raster, summary table, or chart. –Max flow length per watershed –Average slope per watershed –Average curve number per watershed

Zonal Overlay (cont.) Slope Watersheds Mean Slope per Watershed

Where is this functionality? ArcInfo GRID FunctionsArcInfo GRID Functions ArcView 3.x Spatial AnalystArcView 3.x Spatial Analyst –Avenue requests –Sample extension ArcGIS Spatial Analyst 8.xArcGIS Spatial Analyst 8.x –HydrologyOp containing VB methods –Sample Toolbar on ArcObjectOnline –Arc Hydro data model tools ArcGIS Spatial Analyst 9.xArcGIS Spatial Analyst 9.x –Tools in the Spatial Analyst Toolbox