Watershed Analyst (Page 1)
What is the MassGIS Watershed Analyst?
Uses of the MassGIS Watershed AnalystClick on image to view full-size.
The MassGIS Watershed Analyst is a set of menu choices and tools available in the MassGIS Data Viewer. The menu choices and tools can be displayed or hidden, depending on whether you need them. The menu choices and tools deal with various aspects of watershed analysis. The user can trace upstream in the watershed network, place points or lines on the river network, or delineate a watershed from any chosen point, line or polygon. A "raindrop" tool is also available, allowing the user to follow the path of a drop of rain over land. The raindrop tool and tools for watershed delineation require ArcView’s Spatial Analyst Extension software and also a MassGIS extension called the MassGIS Watershed Delineator (watdelin.avx). These extensions must be loaded through File/Extensions. Regardless of whether or not the MassGIS Watershed Delineator extension is used, a new environment variable is needed, called RIVERNET, equal to /watrshed/tiles (the location of the watershed library) in order to use the trace and event tools and menus. This is set in the environ.txt file on a PC, and at the system level (i.e. .profile) on UNIX.
The Watershed Analyst project is available for ArcView GIS 3.0a/b, ArcView GIS 3.1, and ArcView GIS 3.2 (UNIX or PC).
NOTE: The raster (grid) data for Watershed Analyst was created by the U.S. Geological Survey, which also provided feedback and testing on the overall system. A companion extension is available for obtaining low streamflow statistics for user-selected points on streams: the USGS Lowflow Estimator (lowflow.avx). For questions about the grid processing or about the lowflow estimation, contact Pete Steeves: (email psteeves@usgs.gov or call 508-490-5054)
Click on images to view full-size.
The hydro centerline is a directed network of the flowing of water. The centerline was created from 1:25,000 scale USGS topographic maps and captures the directional flow of water not only through rivers, but also through wetlands, ponds, lakes and canals. The map on page 2 shows an example of this. The centerline is depicted in red on this map with arrows to indicate the direction of flow. The centerline is an abstraction, flowing roughly through the center of features. The map above demonstrates what the centerline for the Ipswich major watershed looks like. The centerline does not know the area of lakes, or the width of rivers. The purpose of the centerline is to prepare for creating the route system. The route system tells us which way the water is flowing. When a route system is made out of the directional arcs, each route "knows" where it is in the major watershed. It knows what route is upstream and what route is downstream. This route system created out of the centerline enables upstream and downstream analysis to be performed.
Click on image to view full-size.
One use of the centerline is to explore upstream of a particular point. This can be useful in water quality analysis. If a poor water quality sample was collected at the black dot on the map above, an upstream trace could be performed from that point in order to determine where water comes from which flows to that point. Many watersheds have complicated and dense networks of hydrography, so an upstream trace can visually aid in determining "what water is upstream".
Click on image to view full-size.
Another use of the centerline is to explore downstream of a particular point. This can also be useful in water quality analysis. If a chemical spill occurs at the black dot on the map above, an downstream trace could be performed from that point in order to determine where the chemicals will flow through the network. Many watersheds have complicated and dense networks of hydrography, so an downstream trace can visually aid in determining "what water is downstream".
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Another feature of the centerline network is that it is a framework for "events" to attach to. There are two types of events: point events and linear events. Point events describe events at one spot on the network such as water quality monitoring sites or storm drains. Linear events describe events that happen along a defined length of a river, such as erosion, bank cleanliness, certain landuse characteristics, or parts of rivers maintained by different organizations. Point events, instead of being latitude/longitude, are stored as the number of meters upstream from the start of a certain route (every route has a unique id number state-wide. Linear events have a starting end ending point, both based on meters upstream from the start of a route. Both point and linear events can be created or modified using the MassGIS Watershed Analyst. Existing point coverages can be converted to point event tables using a variable snap tolerance.
Shown above is the Ipswich watershed centerline with its subbasins and point events on the centerline called "mouth points". Each mouth point defines the mouth of the upstream subbasin.
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Point and linear events can be searched for because they are attached to the route network. Pictured above is the result of an upstream search for point events starting from the black dot. The appropriate point events were selected in ArcView, and became highlighted in the default selection color (yellow).
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Above is an example of some linear events. Linear events can start and stop on different routes. In this example one could use red and green to symbolize "poor" or "good" water quality parameters results.
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The MassGIS Watershed Analyst also includes a tool to calculate the route id number and meters or miles upstream along the route, based on a point entered in by the mouse.
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Shown here is a real-world application of the MassGIS Watershed Analyst. In the MassGIS library is a datalayer of DEP water quality monitoring sites with water quality data. In the Kimball Brook section of the Ipswich Watershed is two monitoring sites. After examining the data for the two points it is noted that the upstream site has significantly better water quality parameters than the downstream site. Using the MassGIS Watershed Delineator extension, a watershed can be created for any point on the centerline network. A watershed polygon, shown in green, is created for the upstream point. In the MassGIS Watershed Analyst software, a watershed is created from a point first using the underlying elevation data. However, the USGS subbasin datalayer is also used to form the final boundary. The MassGIS Watershed Analyst differs in this way from other programs that rely entirely on the elevation data to create the watershed polygon.
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In the MassGIS Watershed Analyst it is also possible to get the watershed area for the second point without considering the area contributing to the first point. This watershed area is shown in gray above.
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Once a watershed polygon is created, it can be used to clip (as if a cookie cutter) other datalayers which might help explain the water quality monitoring site results. Roads or landuse could be clipped. In this example, landuse was clipped to match the watershed polygon. In the watershed flowing to the monitoring site there is a significant amount of forest (dark green).
Click on images to view full-size.Landuse can also be clipped using the intermediate polygon. Note that this polygon has little forest and much more high density residential and industrial landuse. View the coefficients used in the Land Use Analysis (.PDF format, viewable in Adobe Acrobat)
The MassGIS Watershed Analyst can use the clipped landuse information to summarize landuse types by acreage and percent of total area:
Report of Land Use Statistics for upstream "good water quality" point
Cropland: 120.0 00.7% Land use Summary:
Pasture: 33.6 00.2% Agriculture: 153.6 00.9%
Forest: 16093.0 94.8% Forest: 16093.0 94.8%
Wetland: 06.2 00.0% Wetlands: 06.2 00.0%
Mining: 00.0 00.0% Open land: 09.8 00.1%
Open Land: 07.5 00.0% Residential: 680.4 04.0%
Recreation: 00.0 00.0% Commercial: 26.5 00.2%
Spectator: 00.0 00.0% Industrial: 01.5 00.0%
Water Based: 00.0 00.0% Transport: 00.0 00.0%
Multi-Family: 00.0 00.0% Water: 00.0 00.0%
Res. < 1/4: 257.0 01.5% Total Area 16971.1
Res. 1/4-1/2: 321.2 01.9%
Res. > 1/2: 102.2 00.6%
Salt Wetland: 00.0 00.0%
Commercial: 26.5 00.2%
Industrial: 01.5 00.0%
Open Urban: 02.3 00.0%
Transport: 00.0 00.0%
Waste Disposal: 00.0 00.0%
Water: 00.0 00.0%
Woody Perennial: 00.0 00.0%
No Change: 00.0 00.0%Total area for all land uses = 16971.1
Report of Land Use Statistics for downstream "poor water quality" point
Cropland: 114.5 20.4% Land use Summary:
Pasture: 08.6 01.5% Agriculture: 123.1 21.9%
Forest: 02.6 00.5% Forest: 02.6 00.5%
Wetland: 00.0 00.0% Wetlands: 00.0 00.0%
Mining: 00.0 00.0% Open land: 03.5 00.6%
Open Land: 00.0 00.0% Residential: 353.6 62.9%
Recreation: 00.0 00.0% Commercial: 30.4 05.4%
Spectator: 00.0 00.0% Industrial: 41.4 07.4%
Water Based: 00.0 00.0% Transport: 07.3 01.3%
Multi-Family: 00.0 00.0% Water: 00.0 00.0%
Res. < 1/4: 275.3 49.0% Total Area 562.0
Res. 1/4-1/2: 78.4 13.9%
Res. > 1/2: 00.0 00.0%
Salt Wetland: 00.0 00.0%
Commercial: 30.4 05.4%
Industrial: 41.4 07.4%
Open Urban: 03.5 00.6%
Transport: 07.3 01.3%
Waste Disposal: 00.0 00.0%
Water: 00.0 00.0%
Woody Perennial: 00.0 00.0%
No Change: 00.0 00.0%Total area for all land uses = 562.0
Estimate of annual Nonpoint Source Pollution Loads for upstream point
Land use Summary (Acres):
Agriculture: 153.6
Forest: 16093.0
Wetlands: 6.2
Open land: 9.8
Residential: 680.4
Commercial: 26.5
Industrial: 1.5
Transport: 0.0
Water: 0.0
Total Area 16971.1Estimated Nonpoint Source Pollution loads based on Landuse (and area)
Average Annual Nitrogen Load = 49567.6 pounds
Average Annual Phosphorus Load = 3059.8 pounds
Average Annual Suspended Solids Load = 747930.3 poundsEstimate of annual Nonpoint Source Pollution Loads for downstream point
Land use Summary (Acres):
Agriculture: 123.1
Forest: 2.6
Wetlands: 0.0
Open land: 3.5
Residential: 353.6
Commercial: 30.4
Industrial: 41.4
Transport: 7.3
Water: 0.0
Total Area 562.0Estimated Nonpoint Source Pollution loads based on Landuse (and area)
Average Annual Nitrogen Load = 5350.3 pounds
Average Annual Phosphorus Load = 1041.0 pounds
Average Annual Suspended Solids Load = 298689.3 poundsThese Estimates are based on the Watershed Management Model.
They are estimates of the amount of contamination generated by the
distribution of land use in an average year. Real loads vary greatly
from year to year and season to season.Observed loads tend to be much lower then these estimates for two
reasons. NO mitigating factors are considered. No in stream
assimilation, No BMPs. The larger the area, the more natural
assimilation should take place. Also few observations are made during
the most dramatic storm events. Thus observed loadings tend to be
biased away from nonpoint source events.THESE RESULTS SHOULD BE USED TO COMPARE AREAS OF SIMILAR SIZE AND
PHYSIOGRAPHY.
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A watershed can also be created based on a polygon. In this case only the elevation data is used – the centerline and USGS subbasins datalayers are not used. Suppose that the yellow polygon above is a plot of land to which fertilizers were applied. A "downland" watershed (purple polygon) could be created to determine where any fertilizer washed off by rain might go. This watershed polygon is created up until the point or points that it hits a stream or streams. Another tool called the "raindrop" tool can be used to check on the accuracy of the created polygon. The black dots above are where two "raindrops" were placed on the parcel of land. The red lines emanating from these dots represent the downland path of the two raindrops.
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Suppose that the yellow polygon above is a farm and the farmer is concerned about where surface water comes from which washes onto his land. A "upland" watershed (purple polygon) could be created to determine where water is coming from. In this case water is coming from a hill adjacent to the parcel. The raindrop tool shows that water placed on the side of the hill near the parcel does indeed wash down onto the parcel. Two other raindrops placed on the hill outside the polygon correctly indicate by their red flow paths that they do not flow to the yellow parcel.
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The MassGIS Data Viewer also has a system to linking text documents and images to points, lines, polygons, or point or linear events. This can be useful in watershed analysis, as water quality monitoring reports and photographs can be linked to specific geographic features and brought up on the screen with a click of the mouse.
Next section: Tools
not requiring ArcView's Spatial Analyst
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Analyst Index
Last Updated 11/1/2000
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