Introduction to Geographic Information Systems in Forest Resources
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Introduction to Geographic Information Systems in Forest Resources |
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Discussion:
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.
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 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 ArcView's hydrologic analysis tools. These tools include watershed delineation, flow accumulation, flow length, and mean elevation per watershed, to name a few.
All of the hydrologic tools in ArcView are available only after enabling the Hydrologic Modeling Extension (part of the sample extensions). This extension is available only if the Spatial Analyst is present on the system. The Hydrologic Modeling Extension will add a new menu to the View GUI called Hydro, through which all the hydrological tools are accessed.
The first step in any of the hydrologic modeling tools in ArcView 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 sideways, here is a simple image of what FILLing does:
To fill an elevation grid, select Fill Grid from the Hydro menu.
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.
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 ArcView 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:
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 view, cells flowing due north are displayed in yellow.
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 at stream channels and at valley bottoms.
Generate a flow accumulation grid by selecting Flow Accumulation from the Hydro menu.
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 view above, a deeper shade of red.
Here is a display of cells with accumulated flow of > 5000 cells displayed in red.
Added to the view 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 downstream cells are identified.
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 theme 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 view displayed at very large scale.
Before watersheds can be delineated, the points need to be converted to a grid theme. 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 theme representing elevation. Also make sure to set the cell size the same as that of the elevation grid.
The last step in watershed delineation is to perform the function itself. The grid processor needs three grid themes: pour points, flow accumulation, and flow direction. The actual task of delineating watersheds is performed with an Avenue script that may need to be edited to suit the current project.
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, delineation of entire study area).
Automatically delineating watersheds
Watersheds can be automatically delineated, and metrics calculated for existing watershed grids by selecting Hydro > Hydrologic Modeling from the menu. The grid processor will automatically generate a filled grid, flow direction grid, flow accumulation grid, and drainage network line shapefile. Pour points are automatically selected, and watersheds are delineated. Several metrics are calculated.
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 contains several options that are specified with check boxes. The choice Delineate from DEM does all steps. The choice Use Existing Watershed simply calculates selected watershed metrics for an existing watershed grid.
Automatic watershed delineation uses a flow accumulation value which you specify. ArcView searches for cells at the edge of the grid that have this amount of flow accumulation, and turns these cells into pour points.
Here is a view displaying automatically derived watersheds. Note that there are certain areas within watersheds that are null. This is the result of not using a filled grid (I intentionally used an un-filled grid, which results in null areas where sinks exist).
And the same watershed boundaries on the hillshaded grid:
Note how the sinks are directly above road cuts.
And the theme table for the vectorized grid, including the watershed metrics:
Calculating watershed metrics
Apart from the watershed metrics that are added to the vectorized watershed polygon theme with Hydrologic Modeling, it is possible to create new themes that represent other watershed metrics.
These operations, available in the Hydro menu, create output grid themes whose cell values represent the quantity in question.
Conversion of hydrologic themes is often performed for further analysis or mapping.
Three operations are available from the Hydro menu.
Converting a watershed grid theme to a polygon theme
To create a watershed polygon theme from a grid, either use the Hydrologic Modeling dialog in the Hydro menu, or use Theme > Convert to Grid. The former choice will result in a polygon theme with several watershed metrics. In the latter case, only simple polygons will be generated, without watershed metrics.
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