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This model was designed to calculate the area in square miles, pervious curve number and percent impervious for each subbasin to use as input info for a HEC-HMS model using the SCS Method. The model requires four polygon feature classes (subbasins, impervious areas, land cover and soils) and one table (curve numbers).
An ArcGIS Diagrammer schema was developed in conjunction with this model. For optimum results it is suggested that this schema be used to build a new personal geodatabase and then load project data into that format. This will help to ensure that field names and data types are compatible with the model.
The input feature classes for impervious, land cover and soils should cover at least the same extent as the subbasins to be calculated to provide the most accurate results.
Topologies should be run on the land cover and soils feature classes before executing tool to verify that there are no gaps or overlaps within each feature class. Gaps or overlaps may cause incomplete or unexpected results.
The Curve Number table should include a unique CN value for each combination of land cover and soil HSG to be encountered in the model.
When copies of the model are made to run with new dataset, verify that all input and output locations have been updated in the mode. Also verify that the scratch workspace is directed to an appropriate location.
| Expression | Explanation |
|---|---|
| <Input_Subbasins> | |
| <Input_Impervious> | |
| <Input_Land_Cover> | |
| <Input_Soils> | |
| <Input_CN_Table> | |
| <Save_Impervious_Final> | |
| <Save_Pervious_Final> | |
| <Save_Pervious_Statistics> |
| Expression | Explanation |
|---|---|
| Input Subbasins (Required) | |
| Input Impervious (Required) | |
| Input Land Cover (Required) | |
| Input Soils (Required) | |
| Input CN Table (Required) | |
| Save Impervious Final (Required) | |
| Save Pervious Final (Required) | |
| Save Pervious Statistics (Required) |
| Name | Explanation |
|---|---|
| Calculate Field (2) |
This process caluates the area in square feet for each subbasin based on the Shape_Area field. |
| Calculate Field (3) |
This process caluates the area in acres for each subbasin based on the Shape_Area field. |
| Calculate Field (4) |
This process caluates the area in square miles for each subbasin based on the Shape_Area field. |
| Make Feature Layer |
The make feature step is included at this point so that it can be used as an input in the next join. |
| Intersect |
This process intersects the impervious areas with the subbasins to produce an output file showing only the impervious areas within each subbasin. |
| Dissolve |
This process dissolves the impervious intersect by Baisn_ID so that there is a single impervious polygon for each subbasin. |
| Add Field |
This process adds a new field to hold the calculation of acres of impervious area per subbasin. |
| Calculate Field (6) |
This process calculates the acres of impervious area per subbasin. |
| Add Field (3) |
This process adds a new field to hold the calculation of square feet of impervious area per subbasin. |
| Calculate Field (7) |
This process calculates the square feet of impervious area per subbasin. |
| Add Join |
This join combines the subbasin feature class attribute table with the impervious_final table so that the percent impervious for each subbasin can be calculated. |
| Calculate Field |
This process adds the values "Impervious" to each polygon in the feature class. |
| Union (2) |
This process overlays the impervious areas and the landcover to produce an output file showing the impervious and pervious areas within the study extent. |
| Union (3) |
This process overlays the previous union with the soils data so that unique combinations of pervious area can be determined. |
| Intersect (2) |
This process intersects the the impervious/pervious unions with the subbasins so that all of the input information is clipped to the subbasins extent and segmented per subbasin within. |
| Dissolve (2) |
This process dissolves the intersect by Basin_ID, land cover, soils and impervious so that only a single unique combination of impervious, land cover and soils is left for each subbasin. |
| Add Field (2) |
This process adds a new field P_HSG which will be used to calculate the land use and soils combination for each polygon. |
| Calculate Field (5) |
The calculation of P_HSG is a simple joining of two text files. P_HSG = [LandCover]&[HGS] |
| Select |
This process selects all polygons who's impervious attribute is blank (therefore is a pervious area) and exports it to a new shapefile so that the pervious areas are isolated. |
| Delete Field |
This process is there for a bit of house cleaning and now deleting the Impervious field that is no longer needed. |
| Add Field (5) |
This process adds a new field called Wgtd_Area which will hold the future calculation of the polygon area * the curve number. |
| Make Feature Layer (2) |
The make feature step is included at this point so that it can be used as an input in the next join. |
| Add Join (2) |
This join combines the attributes of the pervious shapefile and the curve number table so that additional calculations can be made based on input from both tables. |
| Calculate Field (9) |
Based on the previous join curve numbers are assigned to each polygon for it's unique combination of landcover and soils. This process calculated a weighted area = polygon area * curve number. |
| Summary Statistics |
This process creates a new output table with the sum pervious area of each subbasin and the sum weighted area of pervious areas for each subbasin. |
| Add Join (3) |
This join combines the subbasin feature class attribute table with the pervious statistics table so that the pervious CN for each subbasin can be calculated. |
| Calculate Field (8) |
This process calculates the percent impervious for each subbasin ( % Imp = (Impervious area/subbasin area) * 100 ) and adds the output values to the subbasin feature class attribute table. |
| Calculate Field (10) |
This process calculates the pervious curve number for each subbasin ( Pervious CN = weighted area/pervious area ) and adds the output values to the subbasin feature class attribute table. |