Figure 1 Compute CN Dialog.
Computing Composite Curve Numbers (CN)
WMS-Hydro can compute composite curve numbers from land use, soil type, and basin boundary shape files. Besides the three shape files used to compute CN you will need to define a mapping table that relates land use and soil ID to curve numbers for soil types A, B, C, and D.
Computing CN's is done through the Compute CN… command found in the WMSHydro menu. The dialog shown in Figure 1 is used to manage the setup and computation procedures.
Figure 1 Compute CN Dialog.
Creating a Land Use Curve Number Mapping Table
The first step in computing a CN is to create a mapping table. This can be done by either specifying a WMS land use text file for import (a sample table file used by WMS accompanies the tutorial data of WMS-Hydro), or creating the appropriate .dbf file in some other manner. To create a table file from the text file you should first select the file by selecting the open file button at the top right of the dialog and after identifying the file selecting the Import SCS Table button.
Alternatively you could create the table file in ArcView either from scratch, or from other existing data source containing similar information. Ultimately what you need is a table file with the item definitions shown in Figure 2.
Figure 2 SCS Mapping
Table Item Definitions.
Preparing a Combined Land Use, Soil, and Basin Theme
In order to compute a composite CN using WMS-Hydro you will first need to create a combined land use - soil - basin theme using the Geoprocessing Wizard available in ArcView 3.1. If you have an earlier version of ArcView you will not be able to compute the composite CN unless you can first combine the themes in ArcInfo. To run the Geoprocessing Wizard in ArcView 3.1, you must first activate the Geoprocessing extension. Then you will see the GeoProcessing Wizard… command in the View menu. The Union option will likely be the best option for overlaying the three themes.
Once you have unioned the three themes into a single theme you will need to identify the fields representing unique basin id, land use id, and soil id in the Compute CN… dialog. This is listed as the first step for computing CN's in the Compute CN… dialog.
Computing CN's
With the id's identified from the combined coverage, and an SCS land use mapping table created, you can compute CN's by selecting the Compute CN button near the bottom of the dialog.
The CN numbers are stored in the CN field created using the Attributes command in the WMSHydro menu. If a CN field is not currently part of the basin shape file one named "CN" will be created and populated when selecting the Compute CN button.
The ability to compute CN's in WMS-Hydro is identical to the capability
already in WMS. The purpose for providing this redundant capability is
because GIS data sets of land use and soil are often large and cumbersome
to transfer between ArcView and WMS. The ability to do this within WMS-Hydro
allows you to transfer a single number, CN, to WMS rather than needing
to export both land use and soil type coverages.
These materials may be used for study, research, and education, but please credit the authors and the Environmental Modeling Research Laboratory, Brigham Young University. All commercial rights reserved. Copyright 1999 Environmental Modeling Research Laboratory.
