Download 1) Add MA soils layer from MA GIS

Tufts University
Urban and Environmental Policy and Planning
GIS for Urban and Environmental Analysis
Luba Zhaurova
12/13/2007
Assignment 9 – Final Project – Short Paper on Description and Analysis
For my final assignment, I have combined two final projects – for the Introduction to GIS class (UEP 232) and for
Water Resources Policy and Watershed Management class (UEP 279). The goal of the project was to prepare a water
resource plan for the Hardy Pond watershed in Waltham and Lexington, MA. The project’s main steps were:
1. Delineating the watershed of the Hardy Pond using the topographic map.
2. Conducting hydrological budget calculation using wetlands, soils and impervious surfaces data layers.
3. Determining phosphorus pollution loading to the lake using the land use data layer.
4. Creating a build-out scenario analysis for the watershed using impervious surfaces, land use, and zoning data
layers.
Throughout the process of doing this semester-long project, I have learned a couple of lessons. Firstly, some of
the steps that I originally wanted to do in ArcGIS involved using advanced ArcGIS tools as well as hydrology that I did
not yet know or was not comfortable using. Also, ArcGIS was very helpful as a spatial tool, but the analysis of the data
derived from the spatial manipulations was much easier to do in Excel. Overall, this project had taught me to think
thoroughly through all the steps necessary to accomplish my goals and to keep a good track of all the steps.
Step 1. Delineating the Watershed

Data Sources
I used paper and scanned topographic maps. I used USGS Topographic Quadrangle Images from Massachusetts
Geographic Information System (MassGIS, http://www.mass.gov/mgis/im_quad.htm). My watershed had fallen at the
intersection of 4 quads, scanned from the 15-minute series, three of which were scanned in 1987 (quads 104, 105, 115),
and one was scanned in 1988 (quad 114). Because topography does not change fast, I consider these maps to be accurate.

Major steps in data preparation and analysis
With the help of my Watershed Management professor (Scott Horsley), I have manually delineated the watershed on the
USGS Topographic Map which I ordered from a map dealer (The Map Shack, www.wendellcenter.com). Following the
paper copy, I then manually delineated the same watershed in ArcGIS using the topographic data layer (see above), a new
shapefile, and the Editor Toolbar. I changed topographic layer symbology and turned on the elevation labels when
creating the watershed boundaries thus double-checking the accuracy of the delineation on paper. The resulting polygon
was the foundation of my future analysis, as I used it to clip all of my other data layers.

Difficulties encountered, etc.
I initially planned on delineating my watershed using ArcGIS. With the help of Barbara I have initially performed
delineation, but it did not match my manual delineation. The computer-generated watershed was approximately only a
third of the manually created one. The program’s delineation matched mine in the mountainous part of the watershed to
the south, but not in the flat wetland-dominated part to the north. There was not enough time (or knowledge about ArcGIS
and hydrology) to understand what exactly went wrong, so I decided to delineate manually in ArcGIS instead. One,
though not the only, problem was the fact that ArcGIS perceived an elevated highway as the barrier to a water flow and
therefore a part of the watershed boundary. One would need to ‘burn in’ a stream network through these perceived
barriers using DEM (digital elevation model) raster and ArcGIS tools.

Concluding thoughts
I think my approach worked well given the circumstances. However, I wish I could verify my delineation with that of
the ArcGIS. Watershed delineation is not as straightforward as one would think; therefore, I might have made some
mistakes in delineation which could affect my further analysis. I would advise whoever is doing it in the future to do their
delineation as soon as they can (if in doubt – draw numerous water flow lines to get a sense of how the water flows
through the watershed) so that you have enough time to get full feedback from the professor.
Also, if you are interested in doing computer-generate watershed delineation, do tutorials and, if possible, get help
from the GIS lab instructors and hydrology experts on the delineation. One resource, as suggested by Barbara, is a “GIS in
Water Resources” course at the University of Texas with a lot of useful tools available online
(http://www.ce.utexas.edu/prof/maidment/giswr2007/giswr2007.htm).
Step 2. Hydrologic Balance

Data Sources
1) Watershed Shapefile was created manually, see above.
2) Soils Data Layer from ArcGIS (http://www.mass.gov/mgis/soi.htm) and Related Database Table (specifically –
“muaggatt” table to be opened in Access) from the Soil Data Mart, Natural Resources Conservation Services, U.S.
Department of Agriculture (http://soildatamart.nrcs.usda.gov) for the Middlesex County, Massachusetts.
3) Impervious Surfaces Raster Layer accessed from MassGIS (http://www.mass.gov/mgis/impervious_surface.htm). The
surfaces were extracted from infrared orthoimagery that was acquired in April 2005 as part of the Color Ortho Imagery
project. The pixel size for the impervious surface data is 1-meter. Impervious surfaces were coded 1 for buildings, roads,
parking lots, brick, asphalt, concrete, man-made compacted soil, and more. Zero was assigned to non-pervious surfaces.
4) MA Department of Environmental Protection Wetlands (1:12,000) accessed from MassGIS
(http://www.mass.gov/mgis/wetdep.htm). The wetlands were interpreted from 1:12,000 scale stereo color-infrared
photography in 1993.

Major steps in data preparation and analysis
1. I classified Soils into Hydrologic Soil Groups:
o
Clipped ‘Soils’ Layer to the ‘Watershed’ shape;
o
Joined resulting ‘Soils’ to ‘SOILS_POLY_MAPUNIT2’ .dbf table to be able to see the names of each soil
type;
o
Joined resulting ‘Soil’ Layer to “Muaggatt” table which classifies soils into hydrologic groups A-D. Assigned
value E to the null values.
2. I then calculated total runoff rates in pre-development scenario.
I used “Union” to analyze soil types that do not underlie wetlands or open water (as these are irrelevant for runoff
calculations).
o
Clipped ‘DEP Wetlands’ to the “Watershed” shape;
o
Used “Union” to combine ‘Soils’ and ‘DEP Wetlands’;
o
Selected by attributes all soil types that are not underlying wetlands and water and renamed (Editor Tool and
Attributes icon) them as A, B, C/D, D respectively using the following formula: “Muaggatt17” = A (B,C/D,
D) AND “Soil” <> ‘Open Water’ AND “Soil” <> ‘Wetlands’;
o
Renamed the remaining Wetlands and Water. Reclassified the remaining ‘null’ soil types based on
information available as “E” type;
o
Calculated the total area of each soil class (Field Calculator in the Attribute Table) and exported to Excel;
o
Using runoff rates for each soil type, calculated total runoff in pre-development scenario.
***
o
I used a formula given in Watershed Management class to calculate the water flow in the watershed and the
safe yearly amount of phosphorus that can be safely assimilated by the lake without impacting its health.
3. I calculated total runoff rates in post-development scenario.
o
Converted ‘Impervious Surface’ layer to a shapefile (from raster) and clipped to the ‘Watershed’ shape;
o
Calculated the impervious area within the watershed;
o
Used “Erase” Overlay Analysis tool to erase ‘Impervious’ layer from ‘Soils’ layer;
o
Recalculated the areas of the ‘Soil” attributes (by creating a new field and using ‘calculate geometry’ option);
o
Calculated the total area of each soil group (Filed Calculator in the Attribute Table) and exported to Excel;
o
Calculated total runoff in post-development scenario by adding up runoff for each soil group and runoff
(100% runoff rate) for the impervious surfaces.

Difficulties encountered, etc.
1) The ‘DEP Wetlands’ layer is not very up to date (it is 14 years old) which might cause a certain level of error in
calculations regarding the water flow in the watershed. I had a more current wetlands layer from the city of
Waltham, but not Lexington, and therefore, in order to be consistent with data quality, I did not use it. This issue
underlies the importance of having high quality current GIS data on a regional scale.
2) A small percentages of soil polygons were not assigned a soil group number. I tried to understand the
permeability of these surfaces based on their description, and it seemed to me that most of them are highly
impervious (such as rocky outcrops). I grouped them into a new group E and assigned a 95% runoff rate, higher
than group D. This was not a decision based on previous studies (which I could not find), and therefore there
might be an error attributable to this.
3) Water runoff is a result of complex interactions of soil types, surficial geology, slope, vegetations cover, and
much more. I only used soil types to estimate the runoff rate, which does not account for all the other factors that
play a role in how water moves through the watershed. It is hard to tell if the runoff rate error significant or not.
Also, I could not find a resource that would tell me the runoff rate (as one number, not a range) for each soil type.
I had to estimate based on some literature and discussions in class, but my assigned numbers might contain
considerable error.
4) Since the map of the ground water is not easily available, I completely excluded from considerations a possibly
significant contribution of the ground water to the lake. This information could change the calculations of the
lake’s phosphorus flushing rates.

Concluding thoughts
I think I did all right given the assignment goals, the assumptions I had to make, and my not very deep knowledge
of ArcGIS.
Step 3. Phosphorus Loading

Data Sources
1) Watershed Shapefile, created manually, see above.
2) Land Use Data Layer from ArcGIS (http://www.mass.gov/mgis/lus.htm) based on the 1999 orthophotography. I used
Land Use code definitions 37 to combine land uses into groups for phosphorus loading as following:

o
3 – Forest
o
4 – Wetland
o
6, 17, 24, 31 – Open Land, Urban Open, Power Lines, Urban Public
o
7 – Participation Recreation
o
10 – Multi-family Residential (High Density)
o
11 & 12 – Residential Medium Density (up to ½ acre lots)
o
13 – Residential Low Density (more than ½ acre lots)
o
15 & 18 – Commercial & Transportation
o
20 - Water
Major steps in data preparation and analysis
o
Clipped ‘Land Use’ layer to the watershed size, reclassified the land use symbology (see above) and then
recalculated the areas of each land use class;
o
Exported the results into Excel and performed phosphorus loading calculations in present conditions. Then
compared them to safe loading rates calculated earlier.

Concluding thoughts
o
It is best to use land cover data layer for the phosphorus loading calculations. However, there is no land use
cover available at MassGIS. The National Land Cover Data has poor data quality at the level that I needed.
Therefore, I used the next best data set - the land use data layer (MassGIS, from 1999). Though this method
might have a certain level of error associated with it, I do not think it is significant given that land use and
land cover for residential areas (the majority of my watershed) are similar.
o
I feel confident regarding the phosphorus loading rates that I used for each of the land use because I obtained
them from a very recent scientific report on the Lower Charles River watershed (prepared by the MA DEP).
o
In order to check the accuracy of my calculations it would have been ideal to have Hardy Pondr tested for
phosphorus concentration. Since I did not have that information, I can’t be really sure about how well my
approach worked.
Step 4. Build-Out Analysis

Data Sources
1) Watershed Shapefile, created manually, see above.
2) Land Use Data Layer from Step 3.
3) Zoning Layer from ArcGIS (http://www.mass.gov/mgis/zn.htm), last updated in 2007. The attempt was to standardize
the data across the state, and in a process some of more refined data might have been lost. Also, overlay district data is
only available for some towns. I had more refined data for Waltham, but not Lexington, and therefore decided not to use it
for the sake of consistency.

Major steps in data preparation and analysis
A complex build-out analysis is a major undertaking. Therefore, I performed a visual build-out assessment. I created two
maps: one was a zoning map, and another one contained environmentally constrained lands (wetlands and open water),
current land use, and built areas (I used ‘impervious surface’ layer as a proxy). Based on the second map, no buildable
land was left to build on – everything was either taken up by existing development or by environmentally constrained
land. When comparing the land use of the second map and zoning of the first, it became evident that the two closely
match, which means that no major development will be taking place in Hardy Pond watershed as it is built out to the
maximum allowed by current zoning laws.

Concluding thoughts
o
Because the watershed is so heavily developed, it simplified my build-out estimates and for the purposes of
this project I think I did well. However, if my watershed was not fully developed, the build-out analysis
would involve (as suggested by Scott Horsley):
a) buffering wetlands and waterways;
b) erasing environmentally constrained lands (a) from the watershed;
c) erasing built areas from the watershed;
d) subtracting 15% of the area for future roads;
e) dividing the remaining land by the minimum lot size.
o
Some sources of error could include not using overlay districts information as well as not assessing the
densities of the current developments in comparison to the maximum allowed densities.