In many regions of high annual snowfall, snowmelt modelling can prove to be a vital component of operational
flood forecasting and warning systems. Although Britain as a whole does not experience prolonged periods of
lying snow, with the exception of the Scottish Highlands, the inclusion of snowmelt modelling can still have a
significant impact on the skill of flood forecasts.
Countrywide operational flood forecasts over Britain are produced using the national Grid-to-Grid (G2G)
distributed hydrological model. For Scotland, snowmelt is included in these forecasts through a G2G snow
hydrology module involving temperature-based snowfall/rainfall partitioning and functions for temperature-excess snowmelt, snowpack storage and drainage. Over England and Wales, the contribution of snowmelt is included by pre-processing the precipitation prior to input into G2G. This removes snowfall diagnosed from weather model
outputs and adds snowmelt from an energy budget land surface scheme to form an effective liquid water gridded
input to G2G.
To review the operational options for including snowmelt modelling in G2G over Britain, a project was
commissioned by the Environment Agency through the Flood Forecasting Centre (FFC) for England and Wales
and in partnership with the Scottish Environment Protection Agency (SEPA) and Natural Resources Wales (NRW). Results obtained from this snowmelt review project will be reported on here. The operational methods used by the
FFC and SEPA are compared on past snowmelt floods, alongside new alternative methods of treating snowmelt.
Both case study and longer-term analyses are considered, covering periods selected from the winters 2009-2010,
2012-2013, 2013-2014 and 2014-2015.
Over Scotland, both of the snowmelt methods used operationally by FFC and SEPA provided a clear improvement
to the river flow simulations. Over England and Wales, fewer and less significant snowfall events occurred, leading to less distinction in the results between the methods. It is noted that, for all methods considered,large uncertainties remain in flood forecasts influenced by snowmelt. Understanding and quantifying these
uncertainties should lead to more informed flood forecasts and associated guidance information
