## NAME

r.cn - Generates a SCS curve number map layer
(GRASS Raster Program)

## SYNOPSIS

r.cn
r.cn sg=hy_soil_group lu=landuse_map pr=practice_map hc=hy_condition_map cn=cn_map amc=AMC_number

## DESCRIPTION

r.cn is a command-line interface for generating an SCS CN (Curve Number) map. The command format is:

  r.cn sg=hysoilgroup lu=landusemap pr=practicemap hc=hyconditionmap cn=cnmap amc=AMCnumber

  ### Parameters:

  sg=map
    Hydrologic soil group map name
  lu=map
    Landuse map name
  pr=map
    Practice or management map name
  hc=map
    Hydrologic condition map name
  cn=map
    Output CN map name
  amc=number
    Equivalent AMC condition number for the CN output
  The command-line ordering can be in any form but all key words must be there to run the program.

  ## EXAMPLE

  r.cn sg=hy.soil.group lu=landuse hc=hy.cond pr=veg.cover cn=cnname amc=2
  will create a CN map cnname for equivalent AMC condition II

  ## ATTENTION

  r.cn uses information from the four map layers by reading the cell layer and its category files. So it is important to update the category files (text attributes) for all four layers and it expects the categories of the map layer exactly as shown below:

  For the hydrologic soil group map the text categories may be either "A", "B", "C" or "D". You will reclassify an existing soil type map using r.reclass or v.reclass.

  For the landuse map layer the text categories may be either "fallow", "row crops", "small grain", "close-seeded legumes", "rotation meadow", "pasture", "range", "meadow", "woods", "farmsteads", "roads (dirt)", "hard surface" or "water".

  For the practice or management map layer the categories may be either "straight row", "contoured" or "contoured and terraced".

  For the hydrological condition map the categories may be "poor", "fair" or "good". The rating (good, fair, poor) is based on a combination of factors that affect infiltration and runoff, which includes:

  - density and canopy of vegetated areas
  - amount of year-round ground cover
  - amount of grass or close-seeded legumes in rotations
  - percent of residue cover on the land surface (good is more than 20%)
  - degree of surface roughness

  AMC is the antecedent moisture condition. AMC I: dry, AMC II: normal, AMC III (wet). Specify this number during module run.

  The category text labels can be applied to above maps using r.support ("Edit category file").

  If the combination of the four layers categories does not exist in the SCS CN table, an error message is printed and the program quits.

  ## NOTE

  The r.cn program is sensitive to the current window setting. Thus the program can be used to generate a CN map of any sub-area within the full map layer. Also, r.cn is sensitive to any mask in effect.

  ## REFERENCE

  SCS Curve number method: TR-55, Urban Hydrology for Small Watersheds

  ## SEE ALSO

  r.weighted.cn r.support r.reclass v.reclass

  ## AUTHORS

  Raghavan Srinivasan and Dr. Bernie A. Engel, Agricultural Engineering Department, Purdue University
  Extensions by Markus Neteler, University of Hannover, Germany (fixes for AGNPS 5)

  Last changed: $Date: 2002/03/01 00:08:31 $