Linking GIS Data to Lumped Parameter Hydrologic Models
Introduction
Hydrologic modeling is one of the many natural applications of Geographic Information Systems (GIS) such as Arc/Info. Using GIS software, elevation data in a grid format can be used to delineate drainage basins, create stream networks, and compute drainage basin data. Once computed, several important variables, such as area, slope, and runoff distances, for performing hydrologic analysis can be determined. With grid-based elevation data readily available, it is a simple task to delineate drainage basins and compute sub-basin parameters in a GIS. However, it is very difficult to combine all the drainage data computed in a GIS into a format that is usable by standard hydrologic models.
Lumped
parameter models take all the data for a sub-basin and combine
it into a single number, or set of numbers, that define the response of
the basin to a storm event. On the other hand, Spatial,
or distributed, models take the data for each grid cell inside
the watershed and use the data to compute flow from cell to cell. While
spatial models are ideally suited for use with a spatial GIS database,
lumped parameter models are not.
Lumped parameter watershed modeling programs such as HEC-1
have been used to compute watershed hydrographs for many years. To use
lumped parameter watershed modeling programs, the sub-basin data values
at each grid cell in a GIS must be "lumped" into a few data values
(such as a runoff coefficient and watershed response time) for the entire
sub-basin.
Converting the volumes of data in a GIS to a format that is usable by a
lumped parameter watershed model is a daunting task. Fortunately, a powerful
program called the Watershed Modeling System (WMS)
has been developed that has the capability of importing GIS data, extracting
important watershed information from the GIS database, and running several
hydrologic modeling programs.
This paper focuses on creating a hydrologic model in a GIS and how this
data can be used in conjunction with WMS to run lumped parameter hydrologic
models.
Hydrologic Data Development
Hydrologic data development is the process of delineating a watershed and its sub-basins--then computing geometric sub-basin parameters from the delineated basins. This section will focus on the following steps involved in developing geometric hydrologic data for a watershed:
- Drainage basin delineation using an elevation grid
- Determining geometric parameters such as area, slope, stream length, north/south aspect, and runoff distances
- Developing lumped parameter models in a GIS
Because of its watershed delineation and database capabilities, Arc/Info is used by many hydrologists to perform grid-based watershed delineation. Also, several grid-based watershed delineation tools will soon be available in the Watershed Modeling System (WMS). This section will focus on developing hydrologic data using both of these programs.
Drainage Basin Delineation Using an Elevation Grid
The first step in hydrologic data development is defining drainage basin boundaries. These boundaries normally fall along the ridges in a watershed. On one side of the ridge, water flows into the watershed, while on the other side of the ridge, water flows into a separate watershed. Seven steps are involved in defining these drainage basin boundaries from an elevation grid:
- Fill any pits in the elevation grid.
- Compute the flow direction grid using the elevation grid.
- Compute the flow accumulation grid using the flow direction grid.
- Define the watershed outlet point.
- Create the stream network from the flow direction grid and the flow accumulation grid.
- Delineate the drainage basin and its sub-basins using the flow direction grid and the watershed outlet points.
- Make a polygon coverage of the watershed and its sub-basins.
- Fill any pits in the elevation grid.
A pit forms when a grid cell is lower than all its neighboring grid cells. Before computing a flow direction grid, it is necessary to fill any pits in the elevation grid. Flow directions would be undefined for all unfilled pits (Figure 1). Further, when the watershed is delineated, the grid cells surrounding a pit will not belong to a defined watershed.
Figure 1: Flow paths to a pit.
- Compute the flow direction grid using the elevation grid.
A flow direction grid has one of eight values for each grid cell. The possible values are up, down, right, left, lower right, lower left, upper right, and upper left. These flow directions for each grid cell are computed from the elevation grid (see Figure 2 and Figure 3).
Figure 2: Flow directions computed from an underlying elevation grid.
Figure 3: A flow direction grid with an underlying elevation grid.
- Compute the flow accumulation grid using the flow direction grid.
A flow accumulation grid contains the number of grid cells flowing to each grid cell. In Figure 4, grid cells with flow accumulations above 50 grid cells are displayed for a grid covering the area around Aspen Grove, Utah.
Figure 4: Flow accumulations above 50 cells for the Aspen Grove elevation grid.
- Define the watershed outlet point.
The watershed outlet point is the exit point for all flow in a given watershed. The cells whose flow path eventually passes through this point defines the extent of the watershed (Figure 5).
Figure 5: The watershed outlet point, shown as a triangle at the lower right corner of the picture.
- Create the stream network from the flow direction grid and the flow accumulation grid.
Figure 6 shows a linear stream network in the Aspen Grove watershed. This stream network was created from the flow direction grid and the flow accumulation grid. To create a stream network, define an outlet point at which to begin the stream and a minimum flow accumulation at which to create the stream. The grid stream of Figure 4 is then converted to a vector stream by creating a point at the center of each cell which meets the minimum accumulation threshold value.
Figure 6: A stream network created from background flow direction and flow accumulation grids. Outlet points are shown by triangular symbols.
- Delineate the drainage basin and its sub-basins using the flow direction grid and the watershed outlet points.
After streams are defined, the drainage basin and its sub-basins can be defined from the flow direction grid and the stream network (see Figure 7). Both Arc/Info and the Watershed Modeling System have tools for delineating these sub-basins.
Figure 7: The Aspen Grove drainage basin with its three sub-basins.
To assign a basin ID to each grid cell, a flow path is traced from each grid cell using the flow directions. If that flow path ends up in a stream in the sub-basin, it becomes part of that sub-basin (Figure 8).
Figure 8: Flow directions, streams, and basin boundaries for two sub-basins.
- Make a polygon coverage of the watershed and its sub-basins.
After the drainage basin and sub-basins have been defined, the grid-based watershed model is completely defined. But it is also useful to define the watershed in a vector (line based) format, with polygons created from the watershed boundaries and streams created from stream grid cells (see Figure 9).
Figure 9: A polygon coverage showing three sub-basins in the Aspen grove watershed.
One of the emerging file formats for GIS data is the shapefile format. A shapefile actually consists of two files...a geometry file containing point, vector, or polygon data and a database file containing attribute data for the associated point, vector, or polygon data. Once a vector stream shapefile and sub-basin boundary shapefile is created, the shapefile and attribute data can be linked together and used to define a lumped parameter hydrologic model.
Determining Geometric Parameters
Several useful geometric attributes can be computed using watershed layers created with a GIS. Some of these parameters are:
- The area of each watershed sub-basin.
- The slope of each watershed sub-basin and its associated stream slopes.
- The stream lengths, maximum flow lengths, and other lengths for each watershed sub-basin, and the slopes corresponding to these flow lengths. These parameters are particularly useful for determining the percent of north- and south-facing slopes in each sub-basin.
- The response time of a watershed to a rainfall event.
These watershed geometric parameters are computed from a combination of the grid-based elevation data, the vector stream, and polygon sub-basin boundary data.
Developing Lumped Parameter Models in a GIS
To develop a lumped parameter hydrologic model using a GIS, follow the steps outlined in the previous sections on Hydrologic Data Development. In addition, several other hydrologic parameters need to be defined for each sub-basin in the watershed. For an analysis using the NRCS methods, these parameters would include:
- Sub-basin names
- Watershed response times
- Runoff coefficients
- Storm precipitation totals and temporal distributions
After these values are defined, the lumped parameter watershed model is complete and it is ready to be linked with a program that computes runoff and performs stream reach routing. The next section, Linking GIS Data to Lumped Parameter Hydrologic Models, shows how to link GIS data to any of the lumped parameter hydrologic models available today.
Linking GIS Data to Lumped Parameter Hydrologic Models
After creating a model using a GIS--delineating the basins, computing the basin and sub-basin parameters, and defining the basin hydrologic properties--it may be necessary to use this data in a computer hydrologic model (like HEC-1). To combine all the data in a GIS into a single input file for running a watershed model is a daunting task.
Therefore, it would be nice to have a way of automatically linking GIS data layers to a lumped parameter model. This section will discuss how this automatic link between GIS point, line, and polygon data and hydrologic model data is created in the Watershed Modeling System (WMS). The following ideas will be discussed:
- How is the GIS watershed data organized?
- How is data organized for a watershed model and how can GIS data be converted to the "tree" representation for input into a hydrologic modeling program?
How is GIS Watershed Data Organized?
GIS Watershed data can be arranged into the following categories:
- Watershed and sub-watershed outlet points (Figure 10).
Figure 10: Watershed outlet points are shown as triangles.
- Stream lines (or arcs-see Figure 11).
Figure 11: Stream lines in the Aspen grove watershed.
- Watershed boundary polygons (Figure 12).
Figure 12: Boundary polygons representing sub-basin boundaries for the Aspen Grove watershed.
How is Data Organized for a Lumped Parameter Watershed Model?
A lumped parameter model is more easily "conceptualized" using a tree diagram as shown in Figure 13.
Figure 13: The "Tree" representation of a watershed with its sub-basins for input into a lumped parameter model.
This tree format represents how all sub-basin and outlet data is "lumped" to a single node. A watershed modeler's task is to convert spatial GIS data to this tree format.
Figure 14 shows how the watershed sub-basins, stream reaches, and outlet points are converted into this tree format using WMS. Each of the sub-basins and the points at which the sub-basins combine are represented on the tree.
Figure 14: The relationship between GIS watershed data and tree-based data.
WMS: The Common Gateway Between GIS and Hydrologic Models
Creating a tree and hydrologic model input file by hand is a long and time-consuming task. In WMS, a method has been devised to read in GIS shapefile data with the associated database attribute files and automatically convert this data to a tree format. In addition, sub-basin properties such as area, curve number, time of concentration, and a host of other attributes can be imported to WMS (see Figure 15).
Figure 15: The map attributes dialog in the Watershed Modeling System.
Since WMS contains a complete, stand alone interface to hydrologic models, any remaining data is easily defined using dialogs similar to the one shown in Figure 16.
Figure 16: The Watershed Modeling System makes it possible to go from a GIS representation of a watershed to a tree representation of a watershed to a hydrologic model.
Finally, WMS will create input files that can be used with several hydrologic modeling programs that perform the following tasks:
- Create sub-basin hydrographs
- Perform stream reach routing
- Perform reservoir and detention basin routing
- Perform channel, weir, and curb/gutter water depth calculations.
As a part of this project, a lumped parameter TR-20 model was created for the Aspen Grove Watershed in the mountains of Utah. Figures 17 and 18 show the runoff results at sub-basin outlet points in this watershed for a 5-inch 5-hour storm.
Figure 17: A TR-20 solution for runoff at sub-basins in the Aspen Grove Watershed.
Figure 18: A hydrograph of Flow vs. Time for two points in the Aspen Grove Watershed.
Conclusions
Hydrologic modeling is one of the many natural applications of a GIS. Grid-based elevation data can be used to delineate drainage basins, create stream networks, and compute drainage basin data.
GIS grid-based databases are ideal for use in spatial hydrologic modeling.
However, spatial hydrologic models are not yet accepted by many government
agencies. Also, spatial models need to be calibrated using existing, tested,
lumped parameter models. There is a need to unlock this grid-based basin
data and use it for lumped-parameter hydrologic modeling.
Converting these volumes of data in a GIS to a format that is usable by
a lumped parameter watershed model can be accomplished using the Watershed
Modeling System (WMS).
WMS has the capability of importing GIS data, extracting important watershed
information from the GIS database, and computing watershed runoff information.
After these hydrologic modeling programs are run, runoff and stream routing
results can be read in and viewed in WMS.
References
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Other References
GIS Modules and Distributed Models of the Watershed Task Committee Home Page
Fully Automated Geographic Information System Watershed Stormwater Modeling
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HECPREPRO - GIS Preprocessor for HMS
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