Arc User, The Magazine for Esri Software Users Developing a Hydrology Extension for ArcView Spatial Analyst 

by Steve Kopp, Esri

Extensive flooding from the El Niño weather phenomenon has renewed interest in using GIS software to understand and predict flood problems.

Over the past five years, many interfaces have been developed between GIS and hydrologic models. Most have linked rainfall-runoff and hydraulic models, particularly HEC-1 and HEC-2, to ArcInfo. These home-grown converters or data bridges are useful only to their developers or a few people in their office and work with only one GIS package and one hydraulic model. Most of these converters are built for lumped parameter models that use single values of attributes stored for each watershed or stream reach.

Flood Mapping Scenario
Flood prediction mapping involves two aspects -- hydrology and hydraulics. For a given rainfall event, a portion of the rainfall soaks into the ground and the rest becomes runoff which eventually joins a stream. A rainfall-runoff model, such as TR-20 or HEC-1, is used to predict how much rainfall will become runoff. This calculation is based on physical characteristics of the landscape, often stored in a GIS. Using the ArcView Spatial Analyst extension, landscape characteristics can be calculated and used to create an input file for a hydrologic model.

The runoff information from the hydrologic model can then be combined with stream cross-section information in a hydraulic model such as HEC-2 to determine how deep the water would be at given locations. These water surface elevations can then be passed back to ArcView Spatial Analyst and overlaid upon the terrain surface to determine which areas will flood.

Several recent developments have made the process of interfacing GIS and hydrologic models easier. First, the development of ArcView Spatial Analyst provided raster modeling capabilities on the desktop, where most hydrology work is done. ArcView Spatial Analyst also led to the adoption of a simple, open standard vector format -- the Esri shapefile, which facilitates data exchange and conversion.

Finally, new object-oriented models from the Corps of Engineers Hydrologic Engineering Center (COE-HEC) were developed. These models, the Hydrologic Modeling System (HMS) and the River Analysis System (RAS), are HEC's replacements for HEC-1 and HEC-2. However, the information required for all such models is similar.

Project Goals
The hydrologic extension was developed with specific goals in mind.

  • Get the user up and running quickly. The primary focus was on usability and providing the user easy access to key hydrologic functionality in ArcView Spatial Analyst.

  • Develop attribute encoding scheme on a direct-use, open data structure. This provided a common language and format for exchange of spatial hydrologic information.

  • Provide a framework and tools that could be expanded. Depending on the model used, there are dozens of parameters that may need to be calculated. Consequently, the extension has a modular structure that can be readily modified.

With these goals in mind, developers from Esri, Brigham Young University's Engineering Computer Graphics Lab, and University of Texas at Austin's Center for Research in Water Resources designed a software framework, an attribute encoding scheme, and an ArcView GIS extension to work with rainfall-runoff models. By using an open data format, both the GIS and the model can directly read and write in the file without conversion.

The result was the new hydrology extension for ArcView Spatial Analyst Version 1.1. This new tool extends the ArcView Spatial Analyst user interface for creating input data for hydrologic models. With this extension, users can create watersheds and stream networks from a digital elevation model (DEM), a grid data source representing elevation. Users can calculate physical and geometric properties of watersheds, and aggregate these properties into a single attribute table that can be attached to a grid or shapefile.

Depending on your needs there are two ways of approaching this functionality. If you simply want to create watersheds or stream networks, or a particular property of them, work directly with each of the functional choices under the Hydrology pulldown menu. If you want to create watersheds and calculate attributes for them, use the Hydrologic Modeling choice under the Hydrologic pulldown menu.

This chart shows the menu choices and functions available with the hydrology extension.

Working with Models
Using ArcView Spatial Analyst Version 1.1 and a DEM, a scientist can easily delineate drainage basins and calculate basin parameters necessary for runoff modeling. ArcView Spatial Analyst converts this information into shapefiles of watersheds, stream networks, and watershed pour points that can be read directly by the Watershed Modeling System (WMS) to perform runoff calculations using a variety of models such as HEC-1 and TR-20.

Surface water profiles created using HEC-2, RAS, or other modeling software, can be read back into ArcView Spatial Analyst to create flood maps for further map overlay analysis.

Though this project was developed to share information between ArcView Spatial Analyst and WMS, the file format and description for this extension are easy to understand and can be used as a starting point for development of similar GIS-model interfaces.

Work is also under way to combine this extension with the University of Texas PREPRO program which would allow an ArcView GIS user to directly create input files for HEC HMS, Danish Hydraulic Institute's MIKE Basin, and perhaps the SWAT models.

Additional work is being done to further connect HMS and RAS in a single ArcView GIS environment that would allow you to move easily from a DEM to a floodplain map within a single application.

How To Get the Hydrology Extension
The hydrology extension requires ArcView Spatial Analyst Version 1.1 and the Dialog Designer. This extension will be distributed on the ArcView Spatial Analyst Version 1.1 product CD. The Version 1.1 update is no-cost and will be sent automatically to all licensed ArcView Spatial Analyst sites. ) and the University of Texas at Austin's Center for Research in Water Resources Web site (

screen shot as described below
Watersheds in a study area identified by color.

screen shot as described below
The same watershed data with a shaded relief map.

screen shot of pulldown menu as described below
Pulldown menu for the hydrologic extension showing the Hydrologic Modeling interface that allows the user to create watersheds and calculate attributes for them.


Hydrologic Extension Menu -- Choices And Functions 
Menu Choice Function
Hydrologic Modeling Creates watersheds then calculates additional attributes.
Flow Direction Computes the direction of flow for each cell in a DEM.
Identify Sinks Creates a grid showing the location of sinks in a DEM.
Fill Sinks Fills the sinks in a DEM.
Flow Accumulation Calculates the accumulated flow, or number of upslope cells.
Watershed Creates watersheds based on a user-specified flow accumulation threshold.
Area Calculates the area of each watershed.
Perimeter Calculates the perimeter of each watershed.
Length Calculates the straight line distance length of each watershed.
Flow Length Calculates the length of flow path for each cell to the pour point.
Flow Length by Watershed Calculates the maximum distance along the flow path within each watershed.
Shape Factor by Watershed Calculates a shape factor for each watershed.
Stream Network as Line Shape   Creates a vector stream network.  
Centroid as Point Shape Creates a point shapefile of watershed centroids.
Pour Points as Point Shape Creates a point shapefile of watershed pour points.
Mean Elevation Calculates the mean elevation within each watershed.
Mean Slope Calculates the mean slope within each watershed.
Mean Precipitation Calculates the mean precipitation in each watershed.
Mean Curve Number Calculates the mean curve number for each watershed.

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