Snowmelt in SWMM5
Snowmelt parameters are climatic variables that apply across the entire study area when simulating snowfall and snowmelt. They include:
|·||the air temperature at which precipitation falls as snow|
|·||heat exchange properties of the snow surface|
|·||study area elevation, latitude, and longitude correction.|
The snowmelt routine in SWMM is a part of the runoff modeling process. It updates the state of the snow packs associated with each subcatchment by accounting for snow accumulation, snow redistribution by areal depletion and removal operations, and snow melt via heat budget accounting. Any snowmelt coming off the pack is treated as an additional rainfall input onto the subcatchment.
At each runoff time step the following computations are made:
|1.||Air temperature and melt coefficients are updated according to the calendar date.|
|2.||Any precipitation that falls as snow is added to the snow pack.|
|3.||Any excess snow depth on the plowable area of the pack is redistributed according to the removal parameters established for the pack.|
|4.||Areal coverages of snow on the impervious and pervious areas of the pack are reduced according to the Areal Depletion Curves defined for the study area.|
|5.||The amount of snow in the pack that melts to liquid water is found using:|
|·||a heat budget equation for periods with rainfall, where melt rate increases with increasing air temperature, wind speed, and rainfall intensity|
|·||a degree-day equation for periods with no rainfall, where melt rate equals the product of a melt coefficient and the difference between the air temperature and the pack’s base melt temperature.|
|6.||If no melting occurs, the pack temperature is adjusted up or down based on the product of the difference between current and past air temperatures and an adjusted melt coefficient. If melting occurs, the temperature of the pack is increased by the equivalent heat content of the melted snow, up to the base melt temperature. Any remaining melt liquid beyond this is available to runoff from the pack.|
|7.||The available snow melt is then reduced by the amount of free water holding capacity remaining in the pack. The remaining melt is treated the same as an additional rainfall input onto the subcatchment.|
The Snowmelt page of the Climatology Editor dialog is used to supply values for the following parameters related to snowmelt calculations:
Dividing Temperature Between Snow and Rain
Enter the temperature below which precipitation falls as snow instead of rain. Use degrees F for US units or degrees C for metric units.
ATI (Antecedent Temperature Index) Weight
This parameter reflects to what degree heat transfer within a snow pack during non-melt periods is affected by prior air temperatures. Smaller values reflect a thicker surface layer of snow which result in reduced rates of heat transfer. Values must be between 0 and 1, and the default is 0.5.
Negative Melt Ratio
This is the ratio of the heat transfer coefficient of a snow pack during non-melt conditions to the coefficient during melt conditions. It must be a number between 0 and 1. The default value is 0.6.
Elevation Above MSL
Enter the average elevation above mean sea level for the study area, in feet or meters. This value is used to provide a more accurate estimate of atmospheric pressure. The default is 0.0, which results in a pressure of 29.9 inches Hg. The effect of wind on snow melt rates during rainfall periods is greater at higher pressures, which occur at lower elevations.
Enter the latitude, in degrees North, of the study area. This number is used when computing the hours of sunrise and sunset, which in turn are used to extend min/max daily temperatures into continuous values. It is also used to compute daily evaporation rates from daily temperatures. The default is 50 degrees North.
This is a correction, in minutes of time, between true solar time and the standard clock time. It depends on a location’s longitude (θ) and the standard meridian of its time zone (SM) through the expression 4 (θ-SM). This correction is used to adjust the hours of sunrise and sunset when extending daily min/max temperatures into continuous values. The default value is 0.
The Temperature page of the Climatology Editor dialog is used to specify the source of temperature data used for snow melt computations. It is also used to select a climate file as a possible source for evaporation rates. There are three choices available:
Select this choice if snowmelt is not being simulated and evaporation rates are not computed from daily temperatures.
Select this choice if the variation in temperature over the simulation period will be described by one of the project’s time series. Also enter (or select) the name of the time series. Click the button to make the Time Series Editor appear for the selected time series.
|·||External Climate File|
Select this choice if min/max daily temperatures will be read from an external climate file. Also enter the name of the file ( or click the button to search for the file). If you want to start reading the climate file at a particular date in time that is different than the start date of the simulation (as specified in the Simulation Options), check off the "Start Reading File at" box and enter a starting date (month/day/year) in the date entry field next to it. Use this choice if you want daily evaporation rates to be estimated from daily temperatures or be read directly from the file.
Areal depletion refers to the tendency of accumulated snow to melt non-uniformly over the surface of a subcatchment. As the melting process proceeds, the area covered by snow gets reduced. This behavior is described by an Areal Depletion Curve that plots the fraction of total area that remains snow covered against the ratio of the actual snow depth to the depth at which there is 100% snow cover. A typical ADC for a natural area is shown below.
Two such curves can be supplied to SWMM, one for impervious areas and another for pervious areas.
Wind speed is an optional climatic variable that is only used for snowmelt calculations. SWMM can use either a set of monthly average speeds or wind speed data contained in the same climate file used for daily minimum/maximum temperatures.
|Wind Speed Page|
The Wind Speed page of the Climatology Editor dialog is used to provide average monthly wind speeds. These are used when computing snowmelt rates under rainfall conditions. Melt rates increase with increasing wind speed. Units of wind speed are miles/hour for US units and km/hour for metric units. There are two choices for specifying wind speeds:
|·||From Climate File|
Wind speeds will be read from the same climate file that was specified for temperature.
Wind speed is specified as an average value that remains constant in each month of the year. Enter a value for each month in the data grid provided. The default values are all zero.
|Snow Pack Editor|
The Snow Pack Editor is invoked whenever a new Snow Pack object is created or an existing snow pack is selected for editing. The editor contains a data entry field for the snow pack’s name and two tabbed pages, one for Snow Pack Parameters and one for Snow Removal Parameters.
|Snow Pack Editor – Parameters Page|
The Parameters page of the Snow Pack Editor dialog provides snow melt parameters and initial conditions for snow that accumulates over three different types of areas: the impervious area that is plowable (i.e., subject to snow removal), the remaining impervious area, and the entire pervious area. The page contains a data entry grid which has a column for each type of area and a row for each of the following parameters:
Minimum Melt Coefficient
The degree-day snow melt coefficient that occurs on December 21. Units are either in/hr-deg F or mm/hr-deg C.
Maximum Melt Coefficient
The degree-day snow melt coefficient that occurs on June 21. Units are either in/hr-deg F or mm/hr-deg C. For a short term simulation of less than a week or so it is acceptable to use the same value for both the minimum and maximum melt coefficients.
The minimum and maximum snow melt coefficients are used to estimate a melt coefficient that varies by day of the year. The latter is used in the following degree-day equation to compute the melt rate for any particular day: Melt Rate = (Melt Coefficient) * (Air Temperature – Base Temperature).
Temperature at which snow begins to melt (degrees F or C).
Fraction Free Water Capacity
The volume of a snow pack’s pore space which must fill with melted snow before liquid runoff from the pack begins, expressed as a fraction of snow pack depth.
Initial Snow Depth
Depth of snow at the start of the simulation (water equivalent depth in inches or millimeters).
Initial Free Water
Depth of melted water held within the pack at the start of the simulation (inches or mm). This number should be at or below the product of the initial snow depth and the fraction free water capacity.
Depth at 100% Cover
The depth of snow beyond which the entire area remains completely covered and is not subject to any areal depletion effect (inches or mm).
Fraction of Impervious Area That is Plowable
The fraction of impervious area that is plowable and therefore is not subject to areal depletion.
|Snow Pack Editor – Removal Page|
The Snow Removal page of the Snow Pack Editor dialog describes how snow removal occurs within the plowable area of a snow pack. The following parameters govern this process:
Depth at which snow removal begins (in or mm)
Depth which must be reached before any snow removal begins.
Fraction transferred out of the watershed
The fraction of snow depth that is removed from the system (and does not become runoff).
Fraction transferred to the impervious area
The fraction of snow depth that is added to snow accumulation on the pack’s impervious area.
Fraction transferred to the pervious area
The fraction of snow depth that is added to snow accumulation on the pack’s pervious area.
Fraction converted to immediate melt
The fraction of snow depth that becomes liquid water which runs onto any subcatchment associated with the snow pack.
Fraction moved to another subcatchment
The fraction of snow depth which is added to the snow accumulation on some other subcatchment. The name of the subcatchment must also be provided.
The various removal fractions must add up to 1.0 or less. If less than 1.0, then some remaining fraction of snow depth will be left on the surface after all of the redistribution options are satisfied.
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