Skip to main content

Generate CN Raster

Description​

Calculate curve number (CN) attribute in the input soil and land use polygon based on CN lookup table and create a CN raster. The CN lookup table provides CN values for a combination of land use and hydrologic soil groups. A CN raster is also created based on the polygons and the reference DEM.

The database design for the input datasets is fixed and is based on HEC-GeoHMS design for this function. The following data structures are required for the tool to work (note that these layers can have other attributes, but these are the minimum required ones):

Soil Landuse Polygon:

  • LandUse (integer) - land use code identifier. There are no predetermined values for specific land use, so any classification (e.g. Anderson land cover or similar) can be used as long as it matches the values in the CN Lookup table.

  • PctA (short integer) - percentage of hydrologic soil group A for that land use polygon.

  • PctB (short integer) - percentage of hydrologic soil group B for that land use polygon.

  • PctC (short integer) - percentage of hydrologic soil group C for that land use polygon.

  • PctD (short integer) - percentage of hydrologic soil group D for that land use polygon.

Curve Number Lookup Table:

  • LUValue (integer) - land use code identifier. There are no predetermined values for specific land use, so any classification can be used as long as it matches the values in the soil/land use polygon feature class.
  • A - curve number for hydrologic soil group A for that land use.
  • B - curve number for hydrologic soil group B for that land use.
  • C - curve number for hydrologic soil group C for that land use.
  • D - curve number for hydrologic soil group D for that land use.

If any of the required fields are missing, tool will provide a message and stop execution. If CN field does not exist in the input soil/land use polygon feature class, it will be created. If it exists, its values will be overwritten during tool execution.

The CN computation is based on weighted calculation with Pct fields for each soil/land use polygon, so sum of Pct values does not have to add to 100 (but is recommended). For example, in the figures above, polygon in row 1 will have CN of 68

(= (6150+7550+830+870)/(50+50+0+0))

while the one for row 5 will have CN of 80

(= (540+700+80100+850)/(0+0+100+0)).

The output CN raster uses the same extent and cell size as the input reference DEM and is snapped to it. Default name is cn.tif and is placed in the same folder as the input hydro dem raster.

Usage​

Parameters​

DirectionTypeParameter NameData TypeDialog Reference
InputRequiredInput Hydro DEM RasterGPRasterLayerInput hydro DEM raster. This can be any raster (e.g. dem, fdr, fac), that is foundational for further analysis. It is used to set the cell size, extent, and snapping for the output CN raster.
InputRequiredInput Soil Landuse Polygon Feature ClassGPFeatureLayerInput soil and land use polygon feature class/layer indicating unique combination of land use/land cover type and percent hydrologic soil group. It must have LandUse, PctA, PctB, PctC, and PctD attributes populated. A weighted average CN value for each polygon is calculated based on the CN lookup table and assigned to CN field. If CN field already exists, it will be overwritten.
InputRequiredInput Curve Number Lookup TableGPTableViewInput CN lookup table provides CN values for a combination of land use and hydrologic soil groups. It must have LUValue, A, B, C, and D HydroID attribute populated with CN values for each combination.
OutputRequiredOutput Curve Number RasterDERasterDatasetOutput curve number raster. Cell size and extent are based on the input hydro DEM raster while the value is from the soil/land use polygon CN field.