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Snow melt base temperature
The snow pack will not melt until the snow pack temperature exceeds melt_tmp.
Name of the snow record
The name of the snow record is a primary key referenced by snow in hru-data.hru.
Snowfall temperature
Mean air temperature at which precipitation is equally likely to be rain as snow/freezing rain.
Melt factor for snow on June 21
The variables melt_max and melt_min allow the rate of snow melt to vary through the year and account for the impact of snowpack density on snow melt.
If the watershed is in the Northern Hemisphere, melt_max will be the maximum melt factor. If the watershed is in the Southern Hemisphere, melt_max will be the minimum melt factor.
In rural areas, the melt factor will vary from 1.4 to 6.9 mm H2O/day-°C (Huber and Dickinson, 1988). In urban areas, values will fall in the higher end of the range due to compression of the snowpack by vehicles, pedestrians, etc. Urban snow melt studies in Sweden (Bengston, 1981; Westerstrom, 1981) reported melt factors ranging from 3.0 to 8.0 mm H2O/day-°C. Studies of snow melt on asphalt (Westerstrom, 1984) gave melt factors of 1.7 to 6.5 mm H2O/day-°C.
Initial snow water content at start of simulation
Fraction of snow volume corresponding to 50% snow cover
SWAT+ assumes a non-linear relationship between snow water and snow cover. This parameter defines the fraction of the snow volume represented by that corresponds to 50% snow cover.
Minimum snow water content that corresponds to 100% snow cover
Due to factors such as drifting, shading, and topography, the snow pack in a HRU will rarely be uniformly distributed over the total area. A fraction of the HRU area will be bare of snow. This fraction must be quantified to accurately compute snow melt in the HRU.
The factors that contribute to variable snow coverage are usually similar from year to year, making it possible to correlate the areal coverage of snow with the amount of snow present in the HRU at a given time. This correlation is expressed as an areal depletion curve, which is used to describe the seasonal growth and recession of the snow pack as a function of the amount of snow present in the HRU.
The areal depletion curve requires a threshold depth of snow to be defined, above which there will always be 100% cover. The threshold depth will depend on factors such as vegetation distribution, wind loading of snow, wind scouring of snow, interception, and aspect and will be unique to the watershed of interest.
If the snow water content is less than snow_h2o, a certain percentage of ground cover will be bare. It is important to remember that once the volume of water held in the snow pack exceeds snow_h2o, the depth of snow over the HRU is assumed to be uniform. The areal depletion curve affects snow melt only when the snow pack water content is between 0 and snow_h2o. Consequently, if snow_h2o is set to a very small value, the impact of the areal depletion curve on snow melt will be minimal. As the value for sno_h2o increases, the influence of the areal depletion curve will assume more importance in snow melt processes.
Melt factor for snow on December 21
The variables melt_max and melt_min allow the rate of snow melt to vary through the year and account for the impact of snowpack density on snow melt.
If the watershed is in the Northern Hemisphere, melt_min will be the maximum melt factor. If the watershed is in the Southern Hemisphere, melt_min will be the minimum melt factor.
In rural areas, the melt factor will vary from 1.4 to 6.9 mm H2O/day-°C (Huber and Dickinson, 1988). In urban areas, values will fall in the higher end of the range due to compression of the snowpack by vehicles, pedestrians, etc. Urban snow melt studies in Sweden (Bengston, 1981; Westerstrom, 1981) reported melt factors ranging from 3.0 to 8.0 mm H2O/day-°C. Studies of snow melt on asphalt (Westerstrom, 1984) gave melt factors of 1.7 to 6.5 mm H2O/day-°C.
Snowpack temperature lag factor
The influence of the previous day’s snowpack temperature on the current day’s snow pack temperature is controlled by a lagging factor, which inherently accounts for snow pack density, snowpack depth, exposure and other factors affecting snowpack temperature.
As tmp_lag approaches 1.0, the mean air temperature on the current day exerts an increasingly greater influence on the snow pack temperature and the snow pack temperature from the previous day exerts less and less influence. As it approaches zero, the snowpack temperature will be less influenced by the current day's air temperature.
This file controls the simulation of snowfall and snowmelt processes.
| Field | Description | Type | Unit | Default | Range |
|---|---|---|---|---|---|
Name of the snow record
string
n/a
n/a
n/a
Snowfall temperature
real
ºC
1
-5 - 5
Snow melt base temperature
real
ºC
0.5
-5 - 5
Melt factor for snow on June 21
real
mm H2O/day-ºC
0.0
0.0-10.0
Melt factor for snow on December 21
real
mm H2O/day-ºC
0.0
0.0-10.0
Snowpack temperature lag factor
real
none
1
0.01-1
Minimum snow water content
real
mm
0.0
0.0-500.0
Fraction of snow
real
fraction
0.50
0.0-1.0
Initial snow water content at start of simulation
real
mm
0.0
0.0-0.50