Calibration of Snowmelt Parameters


The water balance for the watershed should be calibrated first. The snow accumulation and melt processes should be modeled while doing water balance calibration, but accurate modeling of the snowpack is not necessary for the initial calibration trials. Water balance modeling will establish reasonable soil moisture and infiltration parameters, and will indicate whether or not the meteorological and hydrologic data series that are used are adequate for the watershed.

A summary of the HFAM parameters for snow accumulation and melt processes follows. Each parameter is listed under the snowpack characteristic that it primarily affects.

Snow Accumulation

Snow Aging and Transformation

Heat Exchange and Melt

Snow and Soils

TSNOW

RDCSN

CCFACT

MGMELT

MELEV

MWATER

SHADE

 

SNOWCF

LAT

 

 

SNOEVP

 

 

 

HFAM snowmelt parameter calibration is discussed below for each of these snowpack characteristics. The field data that indicate that parameter changes are needed are also discussed.

Snow Accumulation:

Direct evidence about snow accumulation comes from snow course data. When both snow depth and water content are available, snow density (water content/snow depth) can be compared with model results.

Snow course measurements are a mean or average of several point measurements. Measurement points along a snow course are at the same elevation, and have similar aspect or exposure to solar radiation. When comparing snow course measurements to mean snowpack results over an HFAM segment, the user needs to be aware that;

1) elevation is key to snowpack behavior, affecting both accumulation and melt. A snow course at 4900 ft may not represent an HFAM segment with a mean elevation of 4500 ft.

2) differences in aspect or slope and forest cover will cause different melt rates for snowpacks. Data from any one snow course may not be representative.

Often an HFAM land segment of one acre or one hectare is create at a snow course or Snotel site so a direct comparison can be made between measurements and model results.

To increase or reduce snow accumulation, TSNOW is the most obvious parameter to change. Increasing TSNOW increases snow accumulation, and reducing TSNOW reduces snow accumulation. Changing TSNOW is effective only when significant snowfall occurs at or near 32° F. If winter temperatures are cold, well below 32° F, then TSNOW will have little effect. The allowable range for TSNOW is limited in HFAM to 15°, from 25° to 40° F.

MELEV is a physical characteristic of the land segment. It is important for snow accumulation because it affects air temperature, but it is not changed in calibration.

SNOWCF will affect snow accumulation proportional to its value. Changing SNOWCF from 1.0 to 1.1 will increase snow accumulation by 10 percent. Snow catch in precipitation gages is affected by wind speed during storms. Physically realistic values of SNOWCF are in the range from 1.0 to 1.5. SNOWCF is lower if precipitation gages have shields.

SNOWEVP affects sublimation from the snowpack rather than accumulation. Sublimation is not large in most watersheds, but it can be important where windy, low humidity conditions are common.

In summary there are only limited means to alter snow accumulation in HFAM. Serious apparent difficulties with snow accumulation may indicate unique local lapse rates during storms, or incorrect estimates of mean annual precipitation due to orographic effects.

Snow Aging and Transformation

RDCSN is the initial density of new snow, and can be changed if model snow density is high or low, compared to snow course data. Snow density during the winter and spring is also affected by melt rates, and by MWATER -- the liquid water holding capacity of the snowpack.

LAT is used in the calculation of snow surface albedo. It is a physical characteristic of the watershed that is not calibrated.

The effects of aging on snowpack characteristics are diverse and are of some importance. The albedo of the snow surface is calculated in HFAM, but HFAM does not provide a calibration parameter to alter the snow surface albedo.

Field data for calibration of snowpack characteristics is limited to snow density data from snow courses. Snow density found by HFAM results from RDCSN, modified by melt and freezing of melt water in the snowpack. Snow density does not directly influence melt rates, and has only a limited effect on water leaving the snowpack. Snow density is usually not calibrated.

Heat Exchange and Melt

The processes that melt snow are solar radiation, convection, condensation, ground melt and melt due to rainfall, in the order of their importance for most watersheds. Melt rates for the snowpack require calibration.

SHADE controls the short wave solar radiation that reaches the snowpack. SHADE is increased by forest cover, or by slope in an HFAM land segment. It can be increased or reduced by the segment aspect.

The net radiation exchange on the snow surface is the Short Wave Radiation x (1.0 - albedo) + the net Long Wave Radiation Exchange. Long wave radiation is emitted by the snow, forests and cloud cover in the atmosphere at a temperature dependent, near constant rate. Since snow surface albedo is calculated internal to HFAM, SHADE is the only calibration parameter that alters the net radiation exchange on the snow surface.

CCFACT controls both convection and condensation melt, or more correctly the rate of convective or condensation heat transfer to the surface. The algorithm has the form:

Heat Transfer=CCFACT x Wind Velocity x (Air Temp - Snow Temp) x an elevation adjustment

so wind velocity and air temperature play important roles.

The wind velocity on or at the snowpack is seldom known. Wind velocity measurements may be made many miles from the watershed, and wind is altered by elevation and forest cover. Air temperature can also be uncertain. Thus, CCFACT can vary through a considerable range to account for uncertain meteorologic conditions on the snowpack.

Snow and Soils

MGMELT is the heat transfer, in inches (or mm) of melt per day, from the earth to the bottom of the snowpack. It will be zero if the soil beneath the snowpack is frozen.

Summary of the Calibration Procedure

The HFAM parameters affecting Snow Accumulation and Heat Exchange and Melt are the only parameters that are typically calibrated. The Snow Aging and Transformation parameters and the Snow and Soil parameters are run using either physically defined or default parameter levels.

The two parameters that affect Snow Accumulation, TSNOW and SNOWCF, are limited in their effect, but they are usually sufficient to fit model results to winter snow course measurements.

For Heat Exchange and Melt, CCFACT and SHADE are used. These parameters are effective and can be used change the timing of snowmelt as necessary.

Refinements in snowmelt calibration are specific to weather sequences in particular watersheds. It is necessary to consider how each process is affecting a particular snowmelt period, to find guidance on how process parameters could be affecting that period.

For example, if melt released from the snowpack needs to be increased, the actions would be:

1) to reduce SHADE

2) to increase CCFACT

3) to reduce liquid water storage



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