This procedure assumes you have downloaded your watershed from EPA’s BASINS website, and have obtained digital aerial photography (e.g., DOQQs) of your study area. It is recommended that the photos be compressed with software such as MrSID and indexed to facilitate rapid retrieval.
DIVIDE AND CONQUER
Subdivide your watershed into subwatersheds using BASINS’ Manual or Automatic Watershed Delineation Tool (BASINS User’s Manual, Section 8.1 or 8.2).
In AcrView 3x., add the 2 files that are needed (Landuse and Watershed) to the project. Select ŕFileŕ, then check the box next to Geoprocessing. After that, navigate to View, then select the GeoProcessing Wizard. From there, check the button next to Union two themes. There are 3 steps in the wizard that you should see: 1.) Select the input theme to union. For this theme, select the Landuse theme. For step 2.) Select the polygon overlay theme to union, add the watershed them. In step 3.) Specify the output file. In other words, give your new theme a name, and put it in the directory of your choosing.
Make the new theme you just created active by clicking on it. Click Theme and select Query. Make sure your cursor is between the parentheses, and then double-click on Subwaterid in the Fields column. After that, double-click on the = (equals) symbol. Then, click on the Value (the actual subwaterid) of the subwatershed. Finally, click on New Set. The subwatershed you just selected should be highlighted in yellow. If this is the case, click on Theme and select Convert to shapefile. Name the file appropriately and put it in a folder or your choosing. Repeat this paragraph for every subwatershed.
FINE TUNE THE LAND COVER
Creating a personal geodatabase
A personal geodatabase will allow you to set rules and create special features (ex. Drop down menus), that you cannot do with shapefiles alone. You can find more information on how to do this by going to Chapter 14 in your, GETTING TO KNOW ARCGIS desktop Manual.
To create a personal geodatabase for a subwatershed, open up ArcCatolog. Navigate to the folder you want to add your geodatabase to. Click FileŕNewŕPersonal Geodatabase. Name the geodatabase appropriately. Right-click on your new geodatabase and select ImportŕFeature class (single). In the Input Features box, select the subwatershed shapefile of your choice. Leave the Output Location as is. Name your subwatershed in the Output Feature Class Name(it would be best to give it the same name as the shapefile you are importing). Finally, click on Ok. Your personal geodatabase is now created.
Adding a domain
Adding a domain will allow you to create drop-down menus for specific attributes. Right-click on your geodatabase and select Properties. Click on the Domains tab. In the first line of the Domain Name column, type in the name of the field in your subwatershed that you want to edit (ex. LEVEL2). In the Description column, type in a description of the Domain Name. Now, in the Domain Properties section, make sure the Field Type is correct, and change the Domain Type to Coded Values. In the Code field of the Code Values tab, type in the name of the each attribute in your field. (Ex. the LEVEL2 field of your subwatershed contains some of the following attributes: Commercial, Industrial, Residential, Transportation, etc.) Make sure to include all of the attributes in that field, and be sure the spelling and punctuation is identical to that in the field. Do the same thing in the Description column, making sure that the attributes match up to those next to them in the Code column.
Right click on your feature class within your geodatabase, and select Properties. Go to the Fields tab and click on the field that contains the attributes which you are creating a drop-down menu for, in the Field Name column. Then go down to the Field Properties tab and click in the blank box next to the Domain box. You should see a drop-down menu containing the name of the domain that you created in the previous paragraph. Make sure this is selected, and click, Ok. You have now created a drop down menu for a specific field in your attribute table.
Import the needed files
In ArcGIS, import the subwatershed feature class from the geodatabase. Make sure it is properly projected to your aerial photo, so it will overlay correctly. Then, import the aerial photo and display it as a backdrop to the subwatershed. You will also want to change your symbology to display all of your land use types correctly. To do this, go to the properties of your watershed shapefile, and navigate to the Symbology tab. In the Show column, select CategoriesŕUnique values. In the Value drop down menu, select LEVEL2. You will then need to check select the Add All Values box. If the colors (symbols) need to be changed, do this, if not, select, Ok.
Differentiate a given land cover (e.g., Agriculture) into more specific types of land use (e.g., row crops, livestock operations, pasture, etc.) depending on the project objectives and resolution of the photos by splitting and reclassifying the Agriculture polygons (GTKAG[1] Ex. 16b). Make sure to change the attribute table to reflect the changes in splitting the polygons. To do this, first select the new polygon that was created by the split. Then, click on the Value of the LUCODE. There should be a 2-digit number there. Change this number to the new Land Use code. After this, click on the Value of the LEVEL2 field. Again, change the value according to the new land use type (you should be able to use your drop-down menu here, if you activated the domain).
TURN THE TABLES
Create Event Mean Concentration (EMC), Impervious Factor, and Export Coefficient data tables using values corresponding to the various land uses compiled from the PLOAD User’s Manual Appendix, scientific literature, or the Internet.
NOTE: If you want to run the model using BMPs, you have to make sure that your BMP table includes the same Fields (in this case pollutant type) as the EMC table. If the fields are not identical, the model will not run properly.
RUN THE MODEL