Table of Contents

NAME
MODULE PROCESS (TYPE)
DEFINITION
KEYWORDS
CREATION DATE
PARAMETERS DECLARED
VARIABLES DECLARED
EXTERNAL VARIABLES USED
DESCRIPTION
REFERENCES
DEVELOPER NAME AND ADDRESS

NAME

srunoff_carea_prms.f

MODULE PROCESS (TYPE)

Surface runoff

DEFINITION

Computes surface runoff and infiltration for each HRU using a variable source area method

KEYWORDS

CREATION DATE

July 1992

PARAMETERS DECLARED

Maximum possible area contributing to surface runoff, expressed as a portion of the HRU area, decimal percent.

carea_min

Minimum possible area contributing to surface runoff, expressed as a portion of the HRU area, decimal percent.

imperv_stor_max

Maximum impervious area retention storage for HRU, in inches.

basin_area

Total basin area, in acres [basin]

hru_area

HRU area, in acres [basin]

snowinfil_max

Maximum infiltration rate for snowmelt, in inches per day [snow].

soil_rechr_max

Maximum value for soil recharge zone (upper portion of soil profile where losses occur as both evaporation and transpiration), in inches. [smbal]

VARIABLES DECLARED

Weighted average infiltration for the basin, in inches.

basin_sroff

Weighted average surface runoff for the basin, in inches.

imperv_evap

Evaporation from impervious area, in inches.

imperv_stor

Storage on impervious area, in inches.

infil

Infiltration for each HRU, in inches.

sroff

Surface runoff for each HRU, in inches.

EXTERNAL VARIABLES USED

HRU impervious area in acres. [basin]

hru_perv

HRU pervious area in acres. [basin]

hru_ppt

Precipitation on HRU, rain and snow, in inches. [precip]

hru_rain

Rain on HRU, in inches. [precip]

net_ppt

HRU net precipitation, the sum of net_rain and net_snow, in inches. [intcp]

net_rain

Rain on an HRU (hru_rain) minus interception, in inches. [intcp]

pkwater_equiv

Snowpack water equivalent on an HRU, in inches. [snow]

potet

Potential evapotranspiration for each HRU, in inches. [potet]

pptmix_nopack

Indicator that a rain-snow mix event has occurred with no snowpack present on an HRU. [snow]

snowmelt

Snowmelt from snowpack on an HRU, in inches. [snow]

snow_evap

Evaporation and sublimation from snowpack. on an HRU, in inches. [snow]

soil_rechr

Moisture content for the upper soil zone for each HRU, in inches. [smbal]

DESCRIPTION

carea_min is the minimum possible area contributing to surface runoff,

soil_rechr is the moisture content for the upper soil zone for each HRU,

soil_rechr_max is the maximum value for available water in the upper soil zone, and

(2)
where

carea_max is the maximum possible area contributing to surface runoff.

Surface runoff for the pervious area (srp) is then computed as:

(3)
where

net_rain is the rain on an HRU minus interception,

and infiltration (infil) is:

(4)
Surface runoff from snowmelt on pervious areas is assumed to occur only when the soil zone of an HRU reaches field capacity. At field capacity, a maximum infiltration amount, snowinfil_max, is user defined. Any snowmelt in excess of snowinfil_max becomes surface runoff. Snowmelt generated by rain on a snowpack is treated as all snowmelt if the snowpack is not totally depleted by the rain. If the snowpack is totally depleted by the rain, the resulting rain and snowmelt mix is treated as if it were all rain on a snow-free HRU.
Surface runoff from impervious areas is computed identically for both rainfall and snowmelt in subroutine imperv_sroff. Evaporation from impervious areas is computed in subroutine imperv_evap. Total surface runoff for each HRU is then computed by taking an area-weighted average of the pervious and impervious area surface runoff. This subroutine also computes a basin weighted average for surface runoff and infiltration.
Subroutine imperv_sroff
On impervious areas, rainfall or snowmelt first satisfies available retention storage, and the remainder becomes surface runoff. Available retention storage (avail_stor) is computed by:
(5)
where

imperv_stor is the storage on impervious area, in inches, and

imperv_stor_max is the maximum impervious area retention storage for HRU, in inches.

Subroutine imperv_evap
Impervious area retention storage, imperv_stor, on snow-free HRU's is depleted by evaporation, imperv_evap, at the potential rate, potet. On snow-covered HRU's, the evaporation on impervious areas is set equal to snow_evap.

REFERENCES

Dickenson, W. T., and Whitely, H. Q., 1970, Watershed areas contributing to runoff: International Association of Hydrologic Sciences Publication 96, p. 1.12-1.28.
Hewlett, J. D., and Nutter, W. L., 1970, The varying source area of streamflow from upland basins, in Symposium on Interdisciplinary Aspects of Watershed Management, Montana State University, Bozeman, Montana, 1970, Proceedings, p. 65-83.
Leavesley, G. H., Lichty, R. W., Troutman, B. M., and Saindon, L. G., 1983, Precipitation-runoff modeling system--user's manual: U. S. Geological Survey Water Resources Investigations report 83-4238, 207 p.

DEVELOPER NAME AND ADDRESS

George H. Leavelsey
U.S. Geological Survey, WRD
Box 25046, MS 412, DFC
Denver, CO 80225
Telephone: 303-236-5026
Fax: 303-236-5034
Email: george@usgs.gov

U.S. Department of the Interior | U.S. Geological Survey
URL: http://wwwbrr.cr.usgs.gov/projects/SW_MoWS/software/oui_and_mms_s/prms_files/srunoff_carea_prms.shtml
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