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

glacier

Identifies if a HRU is covered by a glacier (=1) or is not (=0, default).

glacr_btemp

Base temperature above which glacier melt can occur, in ° F.

glacr_coef

Glacier melt coefficient for each HRU, in inches per ° F.

imperv_stor_max

Maximum impervious area retention storage for HRU, in inches.

smidx_coef

Coefficient in nonlinear contributing area algorithm.

smidx_exp

Exponent in nonlinear contributing area algorithm.

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].

Weighted average surface runoff for the basin, in inches.

glacr_melt

Amount of glacier melt for an HRU, goes to surface runoff, 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.

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_rain

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

net_snow

Snow on an HRU (hru_snow) 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_moist

Soil moisture content for each HRU, in inches. [smbal]

tavgf

Average air temperature for each HRU, in ° F. [temp]

,

where

smidx_coef is the coefficient in the nonlinear contributing area algorithm,

smidx_exp is the exponent in the nonlinear contributing area algorithm, and

,

where

soil_moist is the soil moisture content for each HRU, in inches, and

net_rain is the rain on an HRU minus interception, in inches.

A maximum value is specified for ca_percent using the parameter carea_max. Surface runoff for the pervious area (srp) is then computed as

,

and infiltration (infil) is

.

Estimates of smidx_coef, smidx_exp, carea_max and direct surface runoff can be made from observed runoff and soil-moisture data. Where soil moisture data are not available, estimates of soil-moisture values can be obtained from preliminary model runs. A regression of log ca_percent versus smidx can be done for these data to determine the coefficients. Using the equation

,

then

.

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, 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_et. 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.

Surface runoff from glacier melt is assumed to occur when there is 1) no snow-covered area for an HRU identified as being covered with a glacier and 2) the daily average air temperature is above the user input base temperature. For this case, glacier melt (glacr_melt) is calculated using a a temperature index function as

,

where

glacr_coef is the coefficient or factor used to calculate melt based on the temperature difference between the two following values, in inches per ° F,

tavgf is the daily average air temperature, in ° F, and

glacr_btemp is the base temperature above which glacier melt can occur, in ° F.

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

Subroutine imperv_et

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