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Surface Hydrologic Modeling and Watershed Delineation


As population pressures on landscapes increase, land managers are continually looking for new methods of managing and monitoring landscape "health." In order to analyze the properties of a landscape, indeed, in order to monitor any object, it is necessary to break that object into manageable units. In the past, landscapes have been managed on an ownership basis. However, experience has shown that the old methods of land management do not make biological sense. Most biological processes do not stop at an ownership boundary. Animal species migrate across private and public lands (as long as they can get over the fences). Contiguous forested lands may traverse many ownerships. Streams flow across different ownerships and political boundaries.

What has been proposed as a logical unit of land management is a watershed. What is a watershed? The American Heritage Dictionary defines watersheds as: "The region draining into a river, river system, or body of water." Watersheds are always physically delineated by the area upstream from a given outlet point. This generally means that for a stream network, the contributing area upstream to a ridge line. Ridgelines separate watersheds from each other.

Thus, the Columbia River has a watershed extending into a very large region of North America, from the Washington-Oregon border north into British Columbia, South into Oregon, and east to the Continental Divide. The Columbia River watershed is defined by the area upstream from its outlet into the Pacific Ocean. But the Columbia River has many tributaries. The Salmon, Snake, Willamette, are but a few other rivers contributing to the flow of the Columbia. Where these rivers meet the Columbia can be treated as outlet points for each individual tributary, and so on, up the hydrologic network of streams and sub-basins.

The Columbia River watershed

Before landscapes can be managed as watersheds, we need to delineate the boundaries of watersheds, so that we can use a common spatial terminology. Many GIS software applications contain routines to delineate watershed boundaries, and to perform other hydrologic analyses. This section will describe ArcGIS 's hydrologic analysis tools. These include tools as watershed delineation, flow accumulation, and flow length.

All of the hydrologic tools in ArcGIS are available only after enabling the Spatial Analyst Extension. The hydrological tools are accessed through ArcToobox.

Watershed Delineation

Creating a Depressionless DEM

The first step in any of the hydrologic modeling tools in ArcGIS is to fill the elevation grid. You must start with a surface that has no sinks. Sinks are areas of internal drainage, that is, areas that do not drain out anywhere. The reason that sinks need to be filled in is because a drainage network is built that finds the flow path of every cell, eventually off the edge of the grid. If cells do not drain off the edge of the grid, they may attempt to drain into each other, which will lead to an endless processing loop.

Looking at a grid in cross-section, here is a simple image of what FILLing does, either chopping off tall cells or filling in sinks:

Note: this operation is very computer intensive. Only attempt this operation on a large grid if you are using a fast computer, unless you can afford to start the process and return after a long stretch of time.

Flow direction

To calculate a drainage network or watersheds, a grid must exist that is coded for the direction in which each cell in a surface drains. Flow direction is important in hydrologic modeling because in order to determine where a landscape drains, it is necessary to determine the direction of flow for each cell in the landscape. This is accomplished with the Calculate Flow Direction menu choice. For every cell in the surface grid, the ArcGIS grid processor finds the direction of steepest downward descent.

Flow direction is a focal function. For every 3-x-3 cell neighborhood, the grid processor stops at the center cell and determines which neighboring cell is lowest. Depending on the direction of flow, the output grid will have a cell value at the center cell, as determined by this matrix:

If a cell flows northward, then in the output grid, the cell in its location will have a value of 64

input flow direction

output value

If the direction of flow for a cell is due north, then in the output grid, that cell's value will be 64. These numbers do not have any absolute, relative, or ratio meaning, they are just used as numeric place holders for nominal direction data values (since grid values are always numeric).

Flow Direction is a choice on the Hydro menu. It should only be performed on grids that are known to be free of sinks.

In this data frame, cells flowing due north are displayed in yellow.

Flow accumulation

Flow accumulation is the next step in hydrologic modeling. Watersheds are defined spatially by the geomorphological property of drainage. In order to generate a drainage network, it is necessary to determine the ultimate flow path of every cell on the landscape grid. Flow accumulation is used to generate a drainage network, based on the direction of flow of each cell. By selecting cells with the greatest accumulated flow, we are able to create a network of high-flow cells. These high-flow cells should lie on stream channels and at valley bottoms.

Once flow accumulation is calculated, it is customary to identify those cells with high flow. This can be done with a Map Query or Map Calculation, or simply by altering the classification of the legend. The display should resemble the vector stream network for the study area.

Higher-flow cells will have a larger value, and in the data frame above, a deeper shade of red.

Here is a display of cells with accumulated flow greater than 5000 cells displayed in red.

Added to the data frame is vector streams. The value of 5000 looks reasonable. Remember that we are eventually going to identify outlet points, so it is more important that the higher-flow downstream cells are identified than all the upland streams. Also, you will always find the vector stream network does not line up perfectly with the DEM-generated flow network, because of the different sources of these data.

Watershed outlet ("pour") points

The next step in delineating watersheds is to select pour points. These are typically points at the edge of the grid, or just downstream of major confluences. Pour points are created by adding a new point layer to the project. Points should be added that are as close to the center of cells as possible. For this reason, it is good to have the high-flow cells displayed and the data frame displayed at very large scale.

Place as many pour points in the data frame as are needed.

Before watersheds can be delineated, the points need to be converted to a grid layer. The points must have an integer attribute that uniquely identifies each point, because the resultant watersheds will have the same value as the grid cells which act as pour points. Use that attribute as the value field in the output grid.

Make sure to use the analysis extent matching the extent of the grid layer representing elevation. Also make sure to set the cell size the same as that of the elevation grid.

Delineating watersheds

The last step in watershed delineation is to perform the function itself. The grid processor needs three grid layers: pour points, flow accumulation, and flow direction.

Here are the contour lines placed atop the watersheds. The watershed boundaries do a fairly good job of following ridge lines.

And if you have a difficult time visualizing contour lines, here is an analytically hillshaded DEM.

You will most likely have to perform watershed delineation on an iterative basis while moving upstream. The first watershed will contain your entire study area. The second round of watershed delineations will create preliminary sub-basins. You will continue to create smaller and smaller sub-basins until the management study objectives are met (e.g., maximum sub-basin size, representation of all pour points, or delineation of entire study area).

Automatically delineating watersheds

Watersheds can be automatically delineated using the Basin command. Pour points are automatically selected from where the grid drains at its edges, and watersheds are delineated.

This method is easy, and only needs limited input from the user. However, this method prevents the user from selecting or viewing pour points, which is one of the most crucial steps in watershed delineation.

Automatic watershed delineation uses a flow accumulation value which you specify. ArcGIS searches for cells at the edge of the grid that have this amount of flow accumulation, and turns these cells into pour points. Here are watersheds generated by using Basin

This method potentially creates a large number of watersheds, none of which match any boundaries that could be used for management planning.

Calculating flow length

One of the tools available in the surface hydrological toolset is flow length. Flow length is the distance travelled from any cell along the surface flow network to an outlet. This can be used to find areas that are closer to headwater locations or closer to stream outlets. In the image below, the cells that are red are farthest from the stream outlet, and those cells that are blue are closest to the outlet. Starting from the groupd of red cells in the center of the forest, the flow pathway goes east, then north into the Little Mashel River, then eastward through the Mashel River, and finally back southward and west into the Nisqually River.

In addition to the overall flow length, it is possible to calculate localized flow length, from every cell to the closest stream location. Here, the cells that are red in color are farther from the stream network along a flow pathway.

Compare this to a simple Euclidean (straight-line) distance result:

The flow length pathway can more closely model downstream effects. For example, the pathway sediments may take in order to enter a stream channel is not same as a simple straight-line distance.


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