Related pages: Using MassGIS Impervious Data |
Step 5: Add Impervious Area
A similar exercise was conducted to calculate impervious area on each parcel using MassGIS impervious coverage. The impervious coverage includes roof, road, and unvegetated bare areas (including sand mining area, storage, and berms around cranberry bogs). This is problematic, because each of these areas have different loading coefficients under the MEP loading methodology.
This solution to this problem can be solved in two steps. First MassGIS 2005 Land Use categories likely to have large bare areas such as cranberry bogs cropland, pasture, orchard, and transmission lines, and beaches were used as a mask to the impervious cover for the watershed. Of the remaining impervious cover, that which fell on rights of way (Road layouts) were presumed to be road impervious, that which landed on parcels were presumed to be a combination of roof areas plus driveways (for residential) or parking lots (for commercial properties). These impervious areas were joined in the parcel coverage as IMP_TOT, IMP_BARE, IMP_ROAD, and IMP_PAR for later use in the loading calculations.
Once the impervious “developed areas” were calculated, we needed a mechanism to separate roof area from road and parking areas. Technically this difficult, but a reasonable compromise is available. MassGIS EOT road coverage consists of road centerlines and paved area width (this data is often used to estimate road impervious area. This data set was used to calculate the road area impervious, and non-road impervious (=driveway + parking areas + roof area). If needed, the non-road impervious can have assumptions applied to it relative roof versus driveway or parking area. For this analysis, we will assume the non-road impervious cover consists of 50% roof and 50% parking area.
|Subwatershed||Impervious Coverage Developed (non-agriculture non-bare areas) acres||Road Areas Impervious from EOT roads coverage||implied Roof +parking +driveway area (ac)||road percent of total|
|Broad Marsh River||227.24||44.19||183.06||19.4%|
|Wareham River West||105.07||28.32||76.76||26.9%|
|Wankinco R N LT10||64.40||21.98||42.42||34.1%|
|Wareham River East||47.70||14.37||33.33||30.1%|
|Parker Mills Pond||38.21||17.11||21.09||44.8%|
|White Island Pond LT10||38.13||13.82||24.30||36.3%|
|Wankinco R N GT10||36.90||9.03||27.87||24.5%|
|Agawam Reservoir S LT10||28.91||18.42||10.49||63.7%|
The land use and impervious areas interested for parcels and subwatershed boundaries were joined into a single file we called: shed_parcels_subs_LU.zip
Step 6: Identify wastewater disposal .
The whole parcel file created in step 3 (only parcels along the major subwatershed were clipped) were then defined as to whether they were sewered or on septic systems (this will be based on various sources of information from towns explained below; it is important that municipal officials validate this coverage). There are more than 7000 parcels in the Wareham River watershed. To expedite the relevance of split parcels to the loading analysis, we assumed any tiny parcels along the watershed boundary clipped to less than 0.02 acres did not have their lawn or septic system in the watershed. To confirm this and identify other problems, all parcels along the major watershed boundary were then visually examined to determine if they were to be included or excluded in the wastewater/lawn analysis. (It is worth noting that all parcels, no matter what size, where included in the land cover spatial analysis in Steps 5 and 6.)
Once we validated parcels for sewer/septic service (on this page we have not yet validated the data with Wareham municipal officials), this polygon coverage is converted to point coverage using a label centroid function from an ArcMap tool (“Graphics and Shapes”- centroids are kept within polygon bounds even for odd shaped polygons). This new point coverage coincides with the presumed locations of lawns and septic systems (where are generally coincident). At this step, septic/lawn points can be moved to coincide with orthophotography if the point is on the wrong side of a subwatershed divide. This new point coverage was then spatially joined to the file from step 5. We called this new file shed_parcels_subs_LU_ww.zip. This file now contains all the basic information needed to conduct a nitrogen loading analysis. A summary of the existing wastewater disposal derived from this step is illustrated by the two tables below.
Total Units and breakdown by wastewater disposal
Is this data correct? Open this Google Earth File which shows septic parcels in red, sewered parcels in blue : warehamriver-whole-parcels-ww.kmz (1.4 MB). In Google Earth, click on any parcel for more information. In the popup information box is an email link to report errors.
Wastewater disposal by subwatershed
To use this data for a subwatershed loading evaluation, the parcels were converted to points (label centroids), and these were intersected with the map on the previous page. The table shows a summary by watershed.
|Agawam Reservoir S LT10||9||9|
|Agawam River North||9||9|
|Broad Marsh River||243||976||1219|
|Fawn Pond GT10||15||15|
|Five Mile Pond||10||10|
|Parker Mills Pond||14||14|
|Wankinco R N GT10||202||202|
|Wankinco R N LT10||5||5|
|Wareham River East||307||10||317|
|Wareham River South||26||26|
|Wareham River West||127||906||1033|
|White Island Pond GT10||24||24|
|White Island Pond LT10||231||231|
Step 7: Creating the Loading Spreadsheet
As we noted above, the file we call shed_parcels_subs_LU_ww.zip has all the data necessary to complete a nitrogen loading analysis. Moreover, because it contains town, subwatershed, and various parcel identification centroids, it can be easily manipulated in both a GIS and spreadsheet environment. For the rest of this exercise we will work in the spreadsheet environment. The first step in this process is to open the file shed_parcels_subs_LU_ww.dbf (one of the files in the GIS coverage), and paste into a new worksheet of a loading spreadsheet you create. Your loading calculations will be developed on a separate worksheet tab in the same spreadsheet. In this way it will be easy to paste in updated GIS, without having to rewrite other portions of your spreadsheet.
To save time in creating the spreadsheet and crunching data, we heavily relay on spreadsheet pivot-table functions and array functions. This technique enables the analyst to create tables like those above with only a few key strokes. These tables and data summaries could have also been created in ArcMap, but because it is easy to paste portions of spreadsheets in report documents, and spreadsheets are easier to review by third parties, we will step through the rest of this nitrogen loading exercise using the new spreadsheet. Before constructing the loading spreadsheet, it is important for municipal officials to have accepted the GIS work, especially the location of sewering and locations of developed parcels, because wastewater loading is the most important loading component in most watersheds.
>> Continue to Wareham River Watershed Loading Analysis Demo, PART 3
<< Go Back to Wareham River Watershed Loading Analysis Demo, PART 1