Working with Freezing and Melting - User Guide

Snow melt is an important phenomenon that can dramatically affect the spring runoff timing and volume. Therefore, a realistic description of the snow melt process is important.

The snowmelt module in MIKE SHE is a modified degree-day method, whereby the rate of melting increases as the air temperature increases. The main input parameters required for the snowmelt process includes melting threshold temperature, degree day coefficient, minimum snow storage for full coverage, and maximum wet snow fraction as well as functions to account for incoming solar radiation, heat content of rainfall, and sublimation.

Degree-day based methods are commonly used in hydrology because the models require limited additional data and are relatively simple to calibrate. In general, degree-day methods are most applicable in open and lightly forested areas. A well-parameterized degree-day method, such as the one available in MIKE SHE, can usually be calibrated for all climatic conditions.

1. Elevation lapse rates

Snow typically accumulates at higher elevations, but the temperature measuring network is rarely dense enough to represent the spatial variation of temperature that is known to exist in the catchment. Thus, MIKE SHE allows you to correct the temperature and precipitation with elevation. The change in value with elevation is called the lapse rate.

1.1 Air temperature lapse rate

Air temperature decreases with elevation. MIKE SHE assumes that this decrease is linear with the elevation. However, the lapse rate depends on the relative humidity. So, MIKE SHE includes two different lapse rates - one for dry conditions and the other for wet conditions (i.e. when it is raining or snowing). The exact formulae are shown in Equations 23.1 and 23.2. Wet conditions are defined as any time step were the precipitation rate is greater than zero.

Note

Air temperatures are specified as instantaneous values. Thus, for air temperature time series, an average air temperature is used in each time step based on a linear interpolation between two instantaneous values.

1.2 Precipitation lapse rate

Precipitation tends to increase with elevation. MIKE SHE assumes that this increase is linear. Similarly, precipitation varies spatially across the catchment, but the amount of local precipitaiton is also a function of the elevation. However, the different areas will have precipitation-elevation relationships.

So, for precipitation, it may be important to have a distribution of lapse rates that depends on the slope orientation, etc. Thus, it is likely necessary to have locally varying lapse rates that depend on the orientation of valleys.

2. Evapotranspiration

ET is also very sensitive to elevation. However, the function is rather complex. The lower temperatures will reduce ET with elevation. However, the wind speed may be higher which will increase ET. If there is more rainfall at higher elevations, or if the humidity is higher, then this will also act to reduce ET. And, solar radiation may be more intense at higher elevations.

There is currently no means to automatically adjust ET with elevation. So, if you need to consider ET versus elevation, then you will have to generate either a spatially varying ET grid, or local time series of ET and apply these in different elevation zones.

Overall, if your winter evapotranspiration is too high, you will underestimate your snow pack accumulation.

3. Time varying infiltration

A common characteristic in cold climates is that infiltration is reduced during the winter months. When the air temperature is cold enough to maintain precipitation as snow, then infiltration will be limited in any case. However, in the spring, when snow storage is melting, then infiltration may still be limited for some period of time.

To achieve this a time varying surface-subsurface leakage coefficient can be specifyed in the Surface-Subsurface Leakage Coefficient dialog.

By default, the Time Series Types is Instantaneous, but there is an option that allows you to use Mean Step Accumulated values. The use of Mean Step Accumulated does not change the meaning of the item, but changes the way the values are interpolated.

Note

The code does not check for the time series type.

Parameter Name	Type	Value
mean step accumulated leakage coefficient	Boolean	On

4 Snow water equivalent

MIKE SHE does not distinguish between rainfall and snowfall. If the rainfall is in a period where the air temperature is below the Melting threshold, then the rainfall accumulates as snow as a Snow Water Equivalent (SWE). In other words, you cannot compare the MIKE SHE results against snow pillow depths. These must be converted to an equivalent depth of water - the Snow Water Equivalent.