Snowmelt Runoff Model

This is the GoldSim implementation of the WinSRM model. It is known to provide support for reservoir operators and water supply managers in anticipating and acting on changes in snowmelt runoff. WinSRM can be used for predicting snowmelt runoff which can then be incorporated into a reservoir model to estimate fill targets for the early summer months andoften provides decision makers with increased knowledge for their annual strategic planning.

The model was developed by U of Berne Switzerland, NM State U, USDA, USBR, USACE. The program has been applied to more than 100 basins and is currently being used for runoff prediction, forecasting, and climate change applications. It has been used successfully in relatively small basins and large ones as big as 350,000 sq. mi.

Each day, the water produced from snowmelt and from rainfall is computed, superimposed on the calculated recession flow and transformed into daily discharge from the basin. The underlying equation relies heavily on a recession constant, which is the main calibration factor. Other factors include direct and snowmelt runoff coefficients, and degree-day factors.

Limitations

Rain infiltration or refreezing within the snowpack is not physically modeled in SRM. Early in the season before the snowpack is ripe, rainfall is assumed to add to the snowpack water equivalent. Precipitation is added to snowmelt when the snowpack is ripe. The user must specify when the snowpack becomes ripe based on judgment.

SRM is most suitable to basins where clear skies allow frequent satellite views of the snow-covered area.

The evaluation of representative areal precipitation is particularly difficult in mountain basins. Also, quantitative precipitation forecasts are seldom available for the forecast mode, although current efforts in this field are improving this situation. Fortunately, snowmelt generally prevails over the rainfall component in the mountain basins. However, sharp runoff peaks from occasional heavy rainfalls must be given particular attention and the program includes a special treatment of such events.
It is assumed that precipitation is basin-wide.

The program accepts a single, basin-wide precipitation input (from one station or from a "synthetic station" combined from several stations. In basins with a great elevation range, the precipitation input may be underestimated if only low altitude precipitation stations are available. It is recommended to extrapolate precipitation data to the mean hypsometric altitudes of the respective zones by an altitude gradient, for example 3 % or 4 % per 100 m. If two stations at different altitudes are available, it is possible to assign the averaged data to the average elevation of both stations and to extrapolate by an altitude gradient from this reference level to the elevation zones. It should be noted that the increase of precipitation amounts with altitude does not continue indefinitely but stops at a certain altitude, especially in very high elevation mountain ranges.

A critical temperature is used to decide whether a precipitation event will be treated as rain (T ≥ TCRIT ) or as new snow ( T < TCRIT). When the precipitation event is determined to be snow, its delayed effect on runoff is treated differently depending on whether it falls over the snow-covered or snow-free portion of the basin. The new snow that falls over the previously snow-covered area is assumed to become part of the seasonal snowpack and its effect is included in the normal depletion curve of the snow coverage. The new snow falling over the snow-free area is considered as precipitation to be added to snowmelt, with this effect delayed until the next day warm enough to produce melting.

It is a typical feature of mountain basins that the areal extent of the seasonal snow cover gradually decreases during the snowmelt season. Depletion curves of the snow coverage can be interpolated from periodical snow cover mapping so that the daily values can be read off as an important input variable to SRM. The snow cover can be mapped by terrestrial observations (in very small basins), by aircraft photography (especially in a flood emergency) and, most efficiently, by satellites. The minimum area which can be mapped with an adequate accuracy depends on the spatial resolution of the remote sensor.

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