Evapotranspiration
ET rates are relevant only if the device network contains an Aquifer for
computation of baseflow. ET is computed from Air Temperature, Vegetative
Cover, & Daylight hours (Haith & Shoemaker,1987).
Vegetative cover factors & daylight hours are entered on a monthly basis.
Air temperatures are also used to simulate snowfall/snowmelt & frozen soil conditions. The specified mean monthly air temperatures are used if daily air temperatures are not provided.
The ET calibration factor (normally= 1) can be used to adjust computed ET values (e.g., when calibrating against observed streamflow, for example)
The last column shows the mean monthly ET rates computed from the previous model run, based upon the specified time interval and evapotranspiration parameters.
Snowfall/Snowmelt
Both rainfall and snowmelt contribute to runoff. The snow
budget considers snowfall, snowmelt, and snowpack. Snowfall is assumed
to be uniformly distributed over each watershed. Precipitation occurs as
snowfall if daily-mean air temp is <= snowfall temp. Snowmelt occurs if
air temp >=
snowmelt temp and if a snowpack exists.
The melt rate is expressed in water-equivalents (not actual snow
depth.< /FONT>
To avoid simulating snowfall/snowmelt, set the critical snowfall temperature to a value below the minimum value in the air temperature file, say -999 deg F.
The rate of snowmelt is governed by the SCS degree-day equation:
SnowMelt (inches/day) = MeltCoef x ( AirTemp - MeltTemp )
MeltTemp = 32 deg F
(typically)<
BR >
Typical Values for the
snowmelt coefficient, MeltCoef (in/day/deg-F)
GLWF
0.098
Haith et al, 1992 various
studies
0.03-.16
Water Encyclo,Leeden et
al.,1995
heavily forested
/ north slopes .04-.06
SCS
National Eng Handbook 1964
open country /
south slopes
.06-.08
The maximum
abstraction factor ranges from 0 to 1 and is interpreted as follows:
0 - all snowmelt becomes runoff (independent of watershed curve number)
1 - snowmelt is combined with rainfall before computing runoff based upon curve number
A linear average is taken for values between 0 and 1.
The appropriate melt coefficient depends on the specified maximum
abstraction scale factor SCS values for MeltCoef (e.g., .04-.08) assume that the
maximum abstraction factor = 0, (all snowmelt contributes to runoff).
Higher values for MeltCoef (e.g. GWLF =
.098) assume that the maximum abstraction factor =
1 (melt driven by curve
number).
Runoff from Frozen Soils
The frozen soil temperature
(Tfreeze) can be adjusted to control the rate of runoff from pervious areas
when the soil is likely to be frozen.
At the start of each event, P8 computes the 5-day-average antecedent air temperature (TAir). If TAir < TFreeze, the following adjustments are made to the runoff simulation for the duration of the event:
1. Antecedent Moisture Condition= III
2. The maximum abstraction computed from Curve Number equation is multiplied by the specified scale factor for maximum abstraction. The scale factor would range from 0-1. If scale factor = 0, the soil will be treated as completely impervious. If the scale factor= 1
This capability has been included to permit simulation of conditions in northern climates (e.g., long cold spell followed by rainfall).
To turn this option off, set the frozen soil temperature to a very low number.
Pervious-Area Runoff
Parameters
The
growing season boundaries and the AMC coefficient values control simulation of
runoff from pervious areas using GWLF algorithm (Haith et al, 1992).
Default (GWLF) values of 5-day antecedent rainfall+snowmelt to select AMC2 are 1.1 during the growing season and 0.5 inches during the non-growing season. Corresponding values for AMC3 are 2.1 and 1.4 inches, respectively.
Antecedent Moisture Condition 3 (highest runoff potential) is forced if the 5-day avg air temp <= soil freeze temp (e.g., frozen ground condition).
See Simulation
Methods
and model
documentation reports for more
details..