An Elevation-Dependent Snowmelt Model for Upland Britain
Victoria A. Bell1 and Robert J. Moore1
An elevation-dependent snowmelt forecasting model, combining a rainfallrunoff model with a
snowmelt module, is investigated for use in upland Britain. Here, the dynamic nature of snow cover,
and the occurrence of heavy rain along with melt, can exert a considerable influence on major floods.
The model comprises the PACK snowmelt model linked to a lumped conceptual rainfallrunoff
model, the PDM (Probability Distributed Moisture) model. The PACK snowmelt module conceptu-
alizes the lying snow as being made up of dry snow that has yet to melt and wet snow that has melted
but is still held in the snowpack. When the temperature is above the melt threshold the dry snow
melts at a rate proportional to the temperature excess above the threshold and contributes to the wet
snow store. Water is released from the wet snow store at a rate dependent on the proportion of the
pack that is melted snow, and is transformed into flow at the basin outlet by the rainfallrunoff
model.
The variation of temperature in a catchment with elevation and its effect on melt can be incorporated
into the model by partitioning the catchment into a finite number of elevation zones. Model perfor-
mance is generally improved through the use of more zones. This result prompted the development
of a snowmelt model that can use either a near-continuous distribution or a finite number of elevation
zones derived from a digital terrain model (DTM). The new formulation allows the evolution of the
snow line over time to be determined along with the water equivalent of the pack and the discharge
at the basin outlet.
The new snowmelt model is tested on two upland catchments, Monachyle Burn in Scotland (11.4
km2) and Trout Beck in Northern England (7.7 km2). Excellent predictions of flow are obtained for
both catchments with R2 values of circa 0.9. A sensitivity study of the accuracy of flow simulations
to the number of elevation zones employed suggests using a 30-m elevation range for a zone as a
conservative choice. Observations of the position of the snow line in the Monachyle compare very
well with model predictions (R2 values of 0.74 and 0.66 for two snowmelt periods). An assessment
is made of the use of daily snow survey and hourly snow pillow measurements for updating the Trout
Beck model. The results suggest that the pillow data, if used with care, can provide as good if not
better flow predictions. However, there is a tendency for snow to melt preferentially from the pillow
compared to the surrounding vegetation.
1
Institute of Hydrology, Wallingford, Oxfordshire, OX10 8BB, UK
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