Buttons

The MassGIS Watershed Analyst has 3 buttons available: 

The "Landuse Sum" button

Application: Runs the "Table.LanduseSum" script. 
Prerequisites: A feature attribute table of a landuse theme must be active. 
User Action: User clicks the button with the mouse. 
End Result: A dialog box appears with landuse summary information. This information can 
be printed or saved as a text file. 
Defaults:
Notes: Contributed by Brian Brodeur, MA DEP GIS 
Important: After clipping Landuse, be sure to run the Theme/Calc. Geometry menu choice (aka the calcapl.ave sample script) to recalculate area and perimeter.

The "Landuse Non Point Source Runoff" button

Application: Runs the "Table.NPS" script. 
Prerequisites: A feature attribute table of a landuse theme must be active. 
User Action: User clicks the button with the mouse. 
End Result: A dialog box appears with landuse summary information and nonpoint source runoff estimates for different categories of land use. This information can be printed or saved as a text file. 
Defaults:
Notes: Contributed by Brian Brodeur, MA DEP GIS 
Important: After clipping Landuse, be sure to run the Theme/Calc. Geometry menu choice (aka the calcapl.ave sample script) to recalculate area and perimeter.
Background:
The model is based on the Watershed Management Model developed by Camp Dresser & McKee, who in turn draw very heavily on the EPA's Urban Runoff Program. The calculations are based on a "event mean concentration" for each contaminant multiplied by a volume of water to get yearly load. The EMCs come from the EPA and CDM.  The volume of water is the depth of rain {assumed to be 42 inches}times the area of each land use, times a percentage of runoff calculated as 0.8 * the impervious fraction of a land use + 0.2 * the pervious fraction of the land use. So basically runoff ranges from 20% for completely pervious areas to 80% for completely impervious. Of course there is a constant thrown in to make the units work out. That is the theoretical basis anyway.

If you look at the script however, rather than do all those calculations over and over the results were reduced to a loading in terms of pounds per acre per year which were more readily comparable with other studies. These loading rates are stored in data dictionaries with the land use code the dictionary key and the loading rate the corresponding value. So the calculation in the script is the loading rate times the area for that land use which is also stored in a dictionary keyed on land use code.

The "Percent Impervious Surface" button

Application: Runs the "Table.Imperviousness" script. 
Prerequisites: A feature attribute table of a landuse theme must be active. 
User Action: User clicks the button with the mouse. 
End Result: A dialog box appears with landuse summary information and nonpoint source runoff estimates for different categories of land use. This information can be printed or saved as a text file. 
Defaults:
Notes: Contributed by Brian Brodeur, MA DEP GIS 
Important: After clipping Landuse, be sure to run the Theme/Calc. Geometry menu choice (aka the calcapl.ave sample script) to recalculate area and perimeter.

A % 5 imperviosness is assigned to each of the 21 land use codes used by MassGIS. Values were originally based on literature and revised by B. Brodeur based oninterpretation of half meter ortho photos for the imperviousness of the area under landuse polygons. Further work with a largeer number of polygons from diverse areas of the state would be a good means of refining these estimates.

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Last Updated 3/2/2004
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