methodology for identification of suitable sites for ...€¦ · crep wetland gis methodology...
TRANSCRIPT
CREP Wetland GIS Methodology August 31, 2009
1
Methodology for Identification of Suitable Sites for Constructed Wetlands
to Remove Nitrate in Indiana
Documentation of Methods for the Indiana State Department of Agriculture
Margaret McCahon, Graduate Research Assistant, Purdue University
Dr. Jane Frankenberger, Agricultural and Biological Engineering, Purdue University Dr. Indrajeet Chaubey, Agricultural and Biological Engineering, Purdue University
Dr. Eileen Kladivko, Agronomy, Purdue University
CREP Wetland GIS Methodology August 31, 2009
2
Contents Notes ......................................................................................................................................................... 3
Downloading and Preparing Data Layers ...................................................................................................... 4
Roads data ................................................................................................................................................ 4
Cropland data ............................................................................................................................................ 7
Soils data ................................................................................................................................................. 12
Hydrology data ........................................................................................................................................ 18
Elevation data ......................................................................................................................................... 20
Regulated Drains data ............................................................................................................................. 27
Maps of Data Layers................................................................................................................................ 28
Wetland Locating Analysis .......................................................................................................................... 31
1. Target locations where the sufficient contributing area approaches a stream. ............................ 31
2. Define areas on which wetlands may be created. .......................................................................... 32
3. Manually select potential locations using topography. .................................................................. 34
Recommended steps in manual locating analysis .................................................................................. 38
Preliminary Wetland Design ....................................................................................................................... 40
1. Create “dams” at the most suitable locations. ............................................................................... 40
2. Create wetland designs. .................................................................................................................. 41
3. Delineating each site’s Watershed. ................................................................................................ 43
4. Determine suitability of each design. ............................................................................................. 44
CREP Wetland GIS Methodology August 31, 2009
3
Notes An ArcInfo license is required for most steps in this method. ArcHydro is helpful but perhaps note
required, as similar tools exist in the ArcToolbox.
We are bringing together data from many sources, and eventually they will all be used together in the
analysis. Therefore, it is important to create all rasters to have the same cell size and extent, and to
have all data in the same coordinate system. Data preparation usually takes place within an ArcMap
document, so one of the easiest ways to keep data in the same projected coordinate system is to import
it into a document that already had the correct coordinate system. You can do this by pulling data with
the correct coordinate system into an empty ArcMap document first.
Many tools from ArcToolbox are used in this methodology. If you do not know where a tool is, you can
use the Index tab and type in the name of the tool, then select the appropriate tool and click “locate.”
Gathering and preparing data for the analysis is the most difficult and time-consuming task. After this,
the analysis is fairly straight forward and simple. Also, most steps can be performed multiple ways, so
do not hesitate to use tools you are more familiar with.
CREP Wetland GIS Methodology August 31, 2009
4
Downloading and Preparing Data Layers
Roads data Download the Tiger2008 census data from: ftp://ftp2.census.gov/geo/tiger/TIGER2008/
Select the state of Indiana, then each county, one-by-one, and download the file called “edges.” Unzip
each to an appropriate folder.
Preparing roads data
1. In ArcCatalog, right click on a geodatabase and create a new feature class named Roads.
Select line features as type, click next. Import a geographic coordinate system from one
of the county’s “edge” files (GCS_North_American_1983), click next twice. Create a new
field name called “Road_Name”, with data type Text. Finish.
CREP Wetland GIS Methodology August 31, 2009
5
2. Right-click on the new Roads feature class, click load, load data. Browse for each
county’s edge file and click “add.” When all counties are added, click next twice. Match
the Road_Name target field to FULLNAME (string), which contains the name of each
road. This will help you identify locations on the map by address. Click next. Select
“load only the features that satisfy a query”, open Query Builder, and create the
following query: "ROADFLG" = 'Y'. This selects only roads from the county edge classes.
Click OK, next, then finish.
CREP Wetland GIS Methodology August 31, 2009
6
3. Clip the roads to the watershed, either by using the Clip tool or using Select by Location
to select all roads that intersect the watershed.
CREP Wetland GIS Methodology August 31, 2009
7
Cropland data Download the NLCD 2001 landcover data from http://www.mrlc.gov/nlcd_multizone_map.php
Determine which zones you need (Indiana is in zones 8 and 11), click the zones on the map and
download “Land Cover zip file.” Unzip files to an appropriate folder.
Preparing cropland data
1. Pull the landcover raster into an ArcMap document
2. Clip the raster to the watershed. You can use Raster Calculator under Spatial Analyst,
setting the 8-digit watershed as a mask under Options and extent and cell size to the same
as the original landcover raster. Input a function like the one below:
Landcover = [landcover11_3k_022007.img]
3. Reclassify the raster to two groups: cropland and non-cropland. The landcover class for
cropland is Cultivated crops, which has a value of 82. Under Spatial Analyst, select
Reclassify. Make sure your options are set so that the extent and cell size will be the same
as the input landcover raster. In the Reclassify tool, set all values except 82 to 0, and set 82
to 1. Now cells with the value 1 are cropland, and those with value 0 are non-cropland.
CREP Wetland GIS Methodology August 31, 2009
8
4. Make sure the cropland raster has the same extent and cell size as the elevation data. First
you need to project the raster into UTM, the same projection as the elevation data, and this
should be the same projection as your map. You can do this by right-clicking on the
cropland raster, go to export data, and make sure to select the spatial reference of the data
frame (current) and the output raster to be square (see image below).
CREP Wetland GIS Methodology August 31, 2009
9
5. Next you match the extent and cell size of the cropland raster to the elevation raster. This is
actually not critical for the following method, but it is a good idea if you want to experiment
with combining the two datasets at some point. This is quite simple. Go to Spatial Analyst,
Options. Set the extent and cell size to the elevation data layer (DEM_watershed). On the
general tab under analysis coordinate system, select “analysis output will be saved in the
same coordinate system as the active data frame”. Now open Raster Calculator, and input:
Cropland_UTM = [DEM_watershed]. Now your cropland raster should have 10 x 10 m cells,
perfectly aligned with the DEM.
6. Finally, I have found that the cropland raster does not always recognize roads as non-
cropland. This is a problem when you want to define a particular field as a suitable site for
wetland placement. To make sure fields break at roads, you must erase roads from the
cropland raster. First, you must create a raster version of the roads data. Under Spatial
Analyst, Options, make sure the Extent and Cell size are set to the Cropland_UTM raster.
Then go to Spatial Analyst, Convert, Features to Raster to convert the roads feature to raster
(see window below).
CREP Wetland GIS Methodology August 31, 2009
10
However, the new roads raster only has values for road cells, and if we were to combine this
raster with the cropland raster, the analysis is limited to only these locations. So we must
reclassify the roads raster to have values over the entire extent of analysis. We can do this
by going to Spatial Analyst, Reclassify. To make the next step simpler, reclassify road cells
(which have a value) to 0, and Nodata cells to 1 (see below). Name the output
Reclass_Roads.
Finally, we must “erase” the roads from the cropland raster. We can again use Raster
Calculator, inputting the function:
Crops_No_Road=[Cropland_UTM2]*[Reclass_Roads]
CREP Wetland GIS Methodology August 31, 2009
11
Now you should have a cropland dataset where fields are broken by roads.
7. The last step is to convert the cropland raster to feature, for ease in later analysis. Go to
Spatial Analyst, Convert, Raster to Features. It’s probably better not to simplify polygons.
Name the raster crops_no_road.shp. The new polygons have gridcode 0 or 1, but you only
want the ones with gridcode 1, which means they are crop fields (gridcode 0 means
anything but croplands). Select by attribute the crop polygons with gridcode 1, and then
export the selected data into a new feature class called feature_cropland.
CREP Wetland GIS Methodology August 31, 2009
12
Soils data This methodology assumes that the user already has access to a SSURGO soils dataset. Soils polygons
should have the following attribute fields: hydclprs and drclasswet.
Preparing soils data
1. In ArcCatalog, right click on the Soils geodatabase and create a new feature class called
“Soils”. Under “Type” choose “Polygon Features.” Click Next. Import the coordinate
system from one of the county soil layers. Click Next three times. Add two fields, name
them something like “Hydric” and “Drainage,” and give them data type “text.” The “Hydric”
field is the keeper of information about whether or not a particular soil polygon is hydric,
and the “Drainage” field shows whether the polygon is poorly drained, very poorly drained,
etc. Click Finish.
2. In ArcCatalog, look at the table of one of the soils feature classes. Note the field names that
correspond to “Hydric” and “Drainage”. In this case, mine are called “hydclprs” and
“drclasswet”.
3. Right click on the new Soils feature class and go to load, load data. One-by-one, add each
county’s soil feature class created in the previous section (see figure below). Note: if you
have trouble loading all the data at once, you can perform these steps on each soils feature
class separately. Click Next twice.
CREP Wetland GIS Methodology August 31, 2009
13
In the following window, you need to match up the “Hydric” and “Drainage” fields with
“hydclprs” and “drclasswet” (see figure below). Click Next twice, and Finish.
4. Open ArcMap, and drag in your new Soils feature class. Bring in your watershed or area of
interest, and overlay the two to make sure you have successfully brought in all Soils
polygons.
5. At this point you may clip the Soils feature class to the Watershed using the “Clip” function
in ArcToolbox, although it is not necessary if you’re following this method.
CREP Wetland GIS Methodology August 31, 2009
14
Preparing Hydric soils 1. Next, create a new feature class that has only those soil polygons which are categorized as
hydric. You can do this a number of ways, either in ArcMap of ArcCatalog. I chose to use
ArcCatalog.
2. Right-click on the Soils geodatabase and go to New, New feature class. Name this feature
class “HydricSoils”, choose polygon features, click Next. Import the same coordinate system
as the Soils feature class. Click Next three times. Add a field called “Hydric” with “text” data
type. Click Finish.
3. Right-click on Hydric Soils feature class and go to Load, Load data. For input data, choose
the Soils feature class we just created. Click Add, then click Next twice. Make sure the
“Hydric” field matches with the “Hydric” field for the Soils feature class. Click Next. In the
following window, choose “load only features that satisfy a query.” Click Query Builder,
and choose to only include data where the “Hydric” field is “All hydric” or “Partially hydric”
(see figure below). Click Next. Click Finish.
4. Now pull the HydricSoils feature class into the ArcMap document to view the result. Right
click HydricSoils in the Display tab, go to Properties, Symbology, Categories, Unique values.
Choose “Hydric” for the Value Field, click Add Values, click Complete List. Choose to
symbolize “Partially Hydric” and “All Hydric” soils with different colors.
You can see my result in the figure below, where blue is “All Hydric” and green is “Partially
Hydric”. Only two counties, Fountain and Boone, appear to have “Partially Hydric” soils, and
CREP Wetland GIS Methodology August 31, 2009
15
these soils dominate the landscape. We don’t want to have this discrepancy in the soils
layers, so I chose to create a new Hydric Soils feature class that has only “All Hydric” soils.
5. Finally, for later analysis the Hydric soils layer should be a raster dataset. To convert the
feature to raster, first go to Spatial Analyst, Options. Set the Workspace to the Soils
geodatabase, the Mask to the Watershed (or to the DEM), and the Extent and Cell size to
the final DEM (this DEM layer is what we base the extent and cell size on for all raster
datasets). Now click Spatial Analyst, Convert, Feature to Raster. Set the Input Features to
HydricSoils, the Field to 1, NoData to 0, and save the output raster as HydricSoils in the Soils
geodatabase.
Note: if the conversion to raster fails, try setting the Field to SHAPE_length, and after the
conversion, reclassify the raster so that all values are set to 1 and NoData is set to 0.
CREP Wetland GIS Methodology August 31, 2009
16
Preparing tile-drained estimate
1. Follow the same steps as above (1-3), creating a feature class called “PoorlyDrainedSoils.”
Instead of adding the “Hydric” field, add the “Drainage” field. When loading the data,
change the query to include data where “Drainage” field is “very poorly drained”, “poorly
drained”, or “somewhat poorly drained.” See image below.
2. Convert the PoorlyDrainedSoils feature to raster, as in previous step 5, reclassifying so that
all very, somewhat and poorly drained soils have the value 1, and all other soils in the
watershed have the value 0.
3. Take the intersection of the two rasters – cropland and poorlydrainedsoils – to determine an
estimate of land that is tile-drained. You can do this using raster calculator, inputting the
following equation:
TileDrained = [crops_no_road] * [PoorlyDrainedSoil.img]
In your new raster, TileDrained, cells with value 1 are estimated to be tile-drained soils, and
those with value 0 are not. In reality, a field that is mostly poorly drained is most likely all
tiled, and a field that is well drained would not be tiled in one poorly drained spot. But this
is just an estimate that will be used only when many fields are aggregated together.
CREP Wetland GIS Methodology August 31, 2009
17
4. Convert raster TileDrained to feature to use in later analysis. You can use the Raster to
Polygon tool in ArcToolbox. I called this new polygon feature TileDrainedEstimate.
CREP Wetland GIS Methodology August 31, 2009
18
Hydrology data
Download high resolution streams and watershed data from the National Hydrography Dataset for the
watershed (HUA) of interest. You can download the data from this site:
http://nhdgeo.usgs.gov/viewer.htm.
Zoom in, select Subbasins, click “Polygon Extract”, click inside the subbasin of choice, and a window
pops up for you to download the data. Use the High Resolution dataset, which recognizes more open
drains as streams. This layer will be used to narrow the sites for the analysis to those that do not
intercept open drainage.
CREP Wetland GIS Methodology August 31, 2009
19
Preparing Streams and Subbasin data
After downloading and unzipping the data, open the geodatabase and draw the following into ArcMap:
NHDFlowline under Hydrography, and Subbasin under Hydrologic Units.
1. To prepare Subbasin data, select the subbasin of interest and export the selected data into a
new feature class called “Watershed”.
2. To prepare Streams data, “select by attributes” those flowlines that relate to surface flow –
in this case I selected connectors, canals/ditches, streams/rivers, and artificial paths (shown
below; in this case, all of the flowlines are streams, but there may be a situation where
other FTypes must be eliminated). Export the selected data into a new feature class called
“Streams”.
CREP Wetland GIS Methodology August 31, 2009
20
Elevation data
Download National Elevation Data (NED) from http://seamless.usgs.gov/index.php.
You can download by rectangle or coordinates, as shown for the 8-digit watershed below:
CREP Wetland GIS Methodology August 31, 2009
21
Make sure to select the appropriate resolution – in this case, I used 1/3 arc second. You can also choose
to download data in larger chunks if you click “modify data request”, scroll to the bottom of the page,
and under “delivery options” choose the maximum size (250 MB). Download and unzip the data to an
appropriate location, then pull all DEMs into an ArcMap document.
Preparing final DEM
1. If the files exceeded 250 MB, and you have multiple rasters, you will need to merge the DEMs to
one dataset. There are a few ways you can do this. I chose to use Raster Calculator, which is in
the Spatial Analyst toolbar (you need to first select the Spatial Analyst extention under Tools,
Extentions). Select Spatial Analyst, Options, and set Extent to “Union of Inputs.” Then under
Spatial Analyst, select Raster Calculator, and input the following:
DEM = merge(raster1, raster2, raster3...)
In my case this looked like:
DEM = merge([16912071],[48575743],[61257263],[96641403])
2. Now that all of the rasters are in one DEM, you want to clip the DEM to the watershed that is
your study area. One way to do this is to use the “clip” tool, or you can use raster calculator
provided you’ve set the watershed as “mask” under spatial analyst, options. In raster calculator,
evaluate:
DEM_watershed = DEM
3. At this point, the DEM does not have a projected coordinate system. You can pull this DEM into
a new ArcMap document, but first you want to draw another dataset into the document that
has the desired coordinate system. In this case I used UTM Zone 16N.
Now you can use Raster Calculator to create a DEM in the correct coordinate system. Under
Spatial Analyst select options, and set the desired cell size (in this case I used 10 m), and under
the general tab select “analysis output will be saved in the same coordinate system as the active
data frame”.
Preparing contributing (watershed) area calculation
1. Add the ArcHydro toolbar. It may also be possible to do these calculations in ArcToolbox, under
Spatial Analyst Tools, Hydrology, though I have not tested this.
2. Under Terrain Preprocessing on the ArcHydro toolbar, select DEM Reconditioning (Agree). Use
the final DEM and Streams data.
CREP Wetland GIS Methodology August 31, 2009
22
3. Under Terrain Preprocessing on the ArcHydro toolbar, select Fill Sinks. Use the Agree DEM from
the previous step.
4. Under Terrain Preprocessing on the ArcHydro toolbar, select Flow Direction. Use the DEM
created in the previous step. You may set the 8-digit Watershed as the Outer Wall Polygon.
CREP Wetland GIS Methodology August 31, 2009
23
5. Under Terrain Preprocessing on the ArcHydro toolbar, select Flow Accumulation. Use the DEM
created in the previous step.
6. Next determine which locations have sufficient contributing area, which we define as 500 to
2000 acres. Each cell is 10x10 m, or 100 square meters, so this comes to 20,234-80,937 cells.
Right click on the Flow Accumulation grid (Fac_8dig), select Properties, Symbology, Classified.
Symbolize the grid manually in the following 3 classes: 0-20234 cells, 20234-80937 cells, and
above 80937 cells (see figure below). Click OK and OK.
CREP Wetland GIS Methodology August 31, 2009
24
7. Go to Spatial Analyst, Options and make sure the mask is set to Watershed, and the extent and
cell size are set to the layer Fac_8dig. Go to Spatial Analyst, Reclassify, and reclassify the raster
so that cells with sufficient contributing area have the value 1, and all other cells have the value
0. I saved the raster as FlowAcc_8dig.
CREP Wetland GIS Methodology August 31, 2009
25
8. These next steps are to limit the sufficient contributing area to those locations that also drain at
least 500 acres of tile-drained land. Make sure you have the TileDrained raster in the map.
Then go to Weighted Flow Accumulation under Terrain Preprocessing in the ArcHydro toolbar.
Using the same flow direction raster from above, and using the tile drained estimate as the
weight grid (0 = not tile drained, 1=tile drained), create a new flow accumulation grid (see image
below). The result is that only grid cells that are tile drained “count” in the flow accumulation
calculation. Now, perform steps 6 and 7 for the weighted flow accumulation grid, using the
threshold of 20234 cells (or 500 acres) of tile drained land.
9. Now we need to take the intersection of the two rasters created – the original, reclassified flow
accumulation raster, and the reclassified tile drained flow accumulation raster. One way to do
this is using Raster Calculator, by the following equation:
CREP Wetland GIS Methodology August 31, 2009
26
SuffCA_8dig=[rTileFac_8dig] * [FlowAcc_8dig]
10. Under Spatial Analyst, select Convert, Raster to Feature. Input the SuffCA_8dig raster, select
geometry type Polyline. It’s best NOT to generalize lines for later parts of the analysis.
11. Locations that are in the stream are not suitable for wetland placement, so you need to
eliminate streams from the analysis. One way to do this is by buffering the streams layer and
“erasing” the buffered streams from the contributing area feature class. In the ArcToolbox, find
the buffer tool, and fill it out according to the image below, with a buffer of 50 m that is flat at
the ends of stream segments. Now under ArcToolbox find the erase tool, and erase the
buffered streams from the contributing area feature class (SffCA_8dig).
CREP Wetland GIS Methodology August 31, 2009
27
Preparing contours calculation
1. Topographic contours of the land are needed to see where a wetland might be placed, and will
be used in creating the preliminary wetland shapes/designs. It is helpful to have contours in
feet rather than meters, so the first step is to convert the z units from meters to feet. You can
do this in raster calculator, by inputting the following function:
dem_z_in_ft=[dem_utm] * 3.2808399
2. Now find the Contour tool in ArcToolbox, and create contours from dem_z_in_ft with 1 foot
intervals. Alternatively, you can use the Contour tool in Spatial Analyst, Surface. If you are
working with a very large watershed, such as the 8-digit watershed I am using, you may run into
errors as you try to create contours. In this case, you may want to create contours in a small
portion of the watershed. I only created contours on the crops/fields that I found to be suitable
for wetland placement later in the analysis.
Regulated Drains data For this analysis, it is ideal to have a layer that shows were main tiles exist and which drains are open vs.
closed. Yet this layer is not easy to find, and only a few counties appear to have digitized regulated
drainage maps.
CREP Wetland GIS Methodology August 31, 2009
28
Maps of Data Layers Below are some maps to show you what my data layers looked like after this preparation.
CREP Wetland GIS Methodology August 31, 2009
29
CREP Wetland GIS Methodology August 31, 2009
30
CREP Wetland GIS Methodology August 31, 2009
31
Wetland Locating Analysis
1. Target locations where the sufficient contributing area approaches a
stream. These places are very likely near the intersection of closed and open drains, and are also a useful
upper limit for later analysis.
a. Create points at the endpoints of the segments of sufficient contributing area. Open the
tool Feature Vertices to Points, and choose the Point Type BOTH_ENDS.
b. For each set of points, select only the point of higher contributing area, which is near
the stream. Under Selection, choose Select by Location, and select from the new layer
points which are within 101 m from the stream. 101 m is used because all sufficient
contributing area segments were erased within 100 m of the stream (because the
contributing area did not always line up closely with the stream segments), and the
stream is not considered a suitable location for wetland placement.
CREP Wetland GIS Methodology August 31, 2009
32
c. Right-click the suff_ca_endpoints feature class, go to data, export data, and export the
selected features to a new feature class called suff_ca_outlets.
2. Define areas on which wetlands may be created. Wetlands can be placed on crop fields. If the suff_ca_outlets are the most “downstream”
wetland outlet possible, then the wetlands will be placed somewhere within this point’s
watershed. Wetlands also cannot cross roads, and must be of a suitable size.
a. Find the Watershed tool in ArcToolbox, and input the flow direction raster (created on
the way to the contributing area raster), and the sufficient contributing area outlet
points from above. This tool will create watersheds above each of the outlet points.
After completed, browse through the map to make sure every outlet point has a
watershed. If you don’t have the right cell size or extent, or if you’re using a different
flow direction raster than the one used to create the sufficient contributing area, it’s
possible you will not have watersheds for each point. If this is a persistent problem, you
can manually create points in the correct locations to capture the entire watershed.
CREP Wetland GIS Methodology August 31, 2009
33
b. Convert the watersheds from raster to feature, using Spatial Analyst, convert, raster to
feature. I didn’t simplify polygons, but it shouldn’t really matter.
c. Now we want to intersect these watersheds with the cropland fields. You can use the
Clip tool, with feature cropland as the input and feature watersheds as the clip features.
d. But not all fields in the watershed are suitable for wetland placement – only those fields
that also intersect a segment of sufficient contributing area. To identify these, select by
location those WatershedsOnCropland that intersect the sufficient contributing area
segments, and export the selected features into a new feature class called
SuitableCropFields.
CREP Wetland GIS Methodology August 31, 2009
34
e. At this point you may decide to impose a size constraint, eliminating all fields/locations
that are not large enough to place a wetland. You can do this by using Select by
Attribute. You may decide on which lower limit to use. I might suggest at least 10-30
acres, because the wetland and buffer are placed based on topography and a very large
piece of land is required to find the appropriate topography. I did not impose such a
constraint, but went on to the manual search for suitable locations using topography.
3. Manually select potential locations using topography. The ideal wetland location has a particular topography. You can think of it as a sort of bowl-
shape, with an opening on one side of the bowl. The wetland itself should be relatively shallow
(no more than 25% of the wetland more than 3 feet deep; this is called “deep wetland”), to
allow for efficient nitrate removal. The buffer land around the wetland must be relatively steep
(no more than 4:1 ratio of buffer to wetland area), and must rise at least 4 feet above the
wetland elevation. Finally, you want to place a relatively small dam, so the contours will ideally
wrap around the wetland to form a small outlet.
CREP Wetland GIS Methodology August 31, 2009
35
a. At this point you may want to print off a map of the outlets, contributing area segments,
and crop fields where a wetland may be placed. Then as you browse through the map
you can make note of any promising site on paper, and also keep track of where you
have already worked. See image below.
b. In the Arcmap document, turn on the following layers: SuitableCropFields, contours,
orthophotos, sufficient contributing areas, and suff_ca_outlets. Note: You may not
want to turn on orthophotos or contours until you’ve zoomed in on a particular field.
Zoom in to a suff_ca_outlet, and look at the one field where a wetland may be placed.
Search the image for a location with sufficient contributing area and topography that
appears to be promising. The image below shows a SuitableCropField (green) near the
CREP Wetland GIS Methodology August 31, 2009
36
potential wetland outlet (purple dot), and you can see how the contours give a sense of
a potential wetland shape.
c. Create a new field in the SuitableCropFields feature class, and call this field Suitability,
leaving it as a short integer (see first image below). If you’d rather have this field be
text, that is OK too, as long as you use consistent text for the categories. In this field
you’ll make the suitability of various crop/fields for wetland placement, perhaps like
this: 5 (very promising), 4 (promising), 3 (possible), 2 (unlikely), 1(no way). To change
the value of this field for a particular cropland polygon, you can select the polygon, open
the SuitableCropFields attribute table, right click on the Suitability field, go to Field
Calculator, and type in the number that you’ve decided describes this field’s suitability
(see second image below). Make sure the field is only changed for the selected polygon.
CREP Wetland GIS Methodology August 31, 2009
37
d. Complete these steps for all SuitableCropFields, browsing throughout the map, making
sure to record in some way those locations that you think are most promising.
CREP Wetland GIS Methodology August 31, 2009
38
Recommended steps in manual locating analysis
1. Zoom in to the outlet, and look at orthophotos to make sure the sufficient contributing area is
not in an open ditch. It’s best to see that the outlet occurs at the interface between closed and
open drains.
Above: you can clearly see from the orthophotos that the ditch is open approximately below the
outlet point, and closed above. This is perfect, and now you can consider the sufficient contributing
area segment (red) just above the outlet point to see if this location has suitable topography.
2. Zoom out so that you can see the local topography. Visualize a dam near the outlet point, like
the black dotted line drawn below. Select a contour at least 5 feet above the lowest elevation
beneath the dam. This is an example of where a buffer may be placed. If this contour appears
reasonable, and does not cross roads, then select the contour four feet beneath this one,
creating your wetland. How do the sizes compare? Is the buffer huge, more than 4 times the
size of the wetland? The wetland should most likely have some portion greater than 3 feet
deep, if it is really large enough for its contributing area. Once you’ve done this a few times,
many sites will stand out more quickly as impossible or improbable, and you may not need to
actually select contours to see the shapes of these features.
CREP Wetland GIS Methodology August 31, 2009
39
Above: the contours representing a hypothetical wetland and buffer are selected. You can see the
buffer is much too large, greater than 4 times the wetland size, and the wetland is much too small.
The dam (black dotted line) is a relatively short compared to the size of the wetland. There aren’t
really any other scenarios, because the road to the south intercepts higher contours. I might rate
this wetland a 2 in Suitability.
3. Rate the suitability by selecting the cropland polygon, right-clicking the Suitability field in the
SuitableCropFields attribute table, selecting Field Calculator, and inputting the appropriate
rating with 5 as most suitable and 1 as entirely unsuitable for wetland placement. Only one
critical failure is required to earn the rate of 1 – for instance, topography that makes wetland
placement impossible.
CREP Wetland GIS Methodology August 31, 2009
40
Preliminary Wetland Design
1. Create “dams” at the most suitable locations. You will be using the contours to create polygon features for wetland, deep wetland, and buffer.
However, you need to decide where to cut off these features at the wetland outlet, so you will
create lines at hypothetical dam locations.
a. In ArcCatalog, you may want to create a new geodatabase called
“PreliminaryWetlandDesign.gdb”.
b. Right click this new geodatabase and add a new feature class. Call this feature class
“Dams”, select Line Features, click Next. Import a coordinate system from one of the
layers you’ve been using, such as the DEM in UTM. Click Next three times and Finish.
c. Add this new “Dams” feature class to your ArcMap document.
d. At each site, the first step of wetland design is to place the dam. You do this by using
the Editor toolbar. Select Editor, Start Editing. Choose to edit from the
“PreliminaryWetlandDesign.gdb” location. Choose “Task: Create New Feature” and
“Target: Dams”. With the sketch tool (pencil), click on one side of the dam and then
double click on the other. Save edits. You may move the dam around and change its
end points as you further decide where it should go.
e. A good place for a dam is a location where the contours narrow or come together, so
that the dam may be smaller. You don’t always have much choice, however, as the dam
location is determined by the location of wetland components. You would also prefer
the dam to be close to the interface between closed and open drains. Make sure your
dam is located on cropland.
CREP Wetland GIS Methodology August 31, 2009
41
2. Create wetland designs. You will be using the contours to create polygon features for wetland, deep wetland, and buffer
at each site. If there are multiple wetland designs, create them all, and narrow the possibilities
later. Most likely there will be only one to three feasible designs at most sites. You want the
buffer elevation to be four feet above the wetland elevation, which is four feet above the deep
wetland (deep wetland is not required, so don’t worry if you don’t have any deep wetland in
many designs). The dam should cross all of these contours, and the dam and the contours
should create complete polygons.
a. Note: this process is made easier if your “Contours” shapefile is broken at the roads. I
chose to create contours only on crop fields that that intersected a sufficient
contributing area. The reason you want to do something like this is that you will be
selecting contours, and you would prefer the contours you select to only intersect the
particular dam you are working with, not the other dams in the map.
b. Use the Select tool in the Tools toolbar to select contours that represent wetland,
buffer, and deep wetland. Use Selection method: Add to current selection so that
multiple contours may be selected simultaneously. Make sure the buffer is the contour
4 feet above the wetland, and the deep wetland is the contour 3 feet below the wetland
surface.
c. Once all contours are selected, find the Feature to Polygon tool in ArcToolbox. Input
“Dams” and Contours, and click OK. If you are creating very many wetland designs, you
may use the default names and later pull all of these designs into a new feature class.
“Dam” (used to create the polygons)
“Deep wetland” (>3 feet deep)
“Wetland” (usual water level)
“Buffer” (4 feet above wetland)
1-foot contours
Orthophotos
Sufficient Contributing Area
CREP Wetland GIS Methodology August 31, 2009
42
Result of Selecting Contours Result of Feature to Polygon tool
(colors added by symbology)
CREP Wetland GIS Methodology August 31, 2009
43
3. Delineating each site’s Watershed. To know more about each site that we are considering for wetland placement, we really want to
be able to see its watershed. We already know that this site drains at least 500 acres of
cropland with poorly drained soils, so this is not necessary. But it is helpful for visual purposes
and to check your contributing area calculation to make sure it makes sense.
a. Use the Intersect tool in ArcToolbox to create intersection points where the “Dams”
cross the sufficient contributing area segments. These points are hypothetical wetland
outlets, so you can call this new point feature class WetlandOutlets. Make sure you
select Output Type: POINT. If you did NOT generalize lines when creating the
contributing area feature, then these points should be located exactly above the correct
cells in the Flow Direction raster.
b. Use the Watershed tool in ArcToolbox to delineate watersheds for this set of
WetlandOutlets points.
CREP Wetland GIS Methodology August 31, 2009
44
c. Convert the raster data to polygon (feature) data. You can do this by using the Raster to
Polygon tool in ArcToolbox.
4. Determine suitability of each design. You want to be able to compare these designs based on the original specifications of the
comparative size of each wetland component.
a. For each wetland design, calculate and put in a table the following:
i. Watershed size in acres
ii. Wetland size in acres; Percent of watershed that is wetland (remember to add
the “deep wetland” to the “wetland” polygons for entire wetland)
iii. Deep wetland size in acres; percent of entire wetland is deep wetland
iv. Buffer size in acres; ratio of buffer to wetland
CREP Wetland GIS Methodology August 31, 2009
45
b. These are the ranges you are aiming for:
i. Watershed size 500-2000 acres
ii. Wetland size0.5-2% of watershed size
iii. Deep wetland no more than 25% of wetland
iv. Buffer less than 4:1 ratio buffer to wetland