r.cn

NAME

r.cn - Generates a 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 for all four layers and it expects the categories of the map layer exactly as shown below:

For the hydrologic soil group map the categories may be either A, B, C or D.

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

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).

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

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)