To properly forecast river flows from melting snow, with or without the addition of rainfall during the ablation period, requires a good estimate of the water equivalent and thermal condition of the snow on the ground. To this end, various authorities carry out snow surveys, but the work of performing snow surveys is very labour-intensive, requiring personnel to make field trips and record observations. The data obtained are generally very sparse, both in terms of space and time (generally many kilometers between snow course locations, and two weeks between observations). However, the availability of digital RADAR data offers a unique opportunity to “fill-in” the data, and provides more detailed snow water equivalent estimates across a large area. The capability of RADAR to estimate precipitation has been well documented throughout the years. When coupled with a hydrological model such as WATFLOOD tracking the hydrological variables such as snow water equivalent, soil moisture and the thermal quality of the snow, the prediction of stream flow becomes possible. Using RADAR data and a hydrological model, the information on snow water equivalent is always current.

WATFLOOD is a distributed hydrological model that subdivides the watershed into grids. Therefore, it is ideally suited for use with gridded data sets, such as RADAR data. WATFLOOD uses the Grouped Response Unit methodology to account for land cover in-homogeneity. All areas of similar land cover within a grid (not necessarily contiguous) form a GRU, and hydrological processes are calculated for each GRU separately. The model has been used for a variety of basins within Canada.

The authors have applied the RADAR precipitation from the J.S. Marshall RADAR Observatory in Montreal, Quebec to estimate the snow water equivalent for the 2002-2003 season for drainage basins in eastern Ontario. A grid size of 1.5 minutes by 1.5 minutes was used, and the snow water equivalent was calculated in each grid. These were compared to the bi-weekly snow survey results provided by the Ontario Ministry of Natural Resources (MNR). It was found that the RADAR-WATFLOOD estimates of snow water equivalent agreed with the observed measurements. The WATFLOOD model was also able to show areas and times of greater (or lesser) snow depth, and these would be helpful to the flow forecasters to pinpoint areas/times where/when extra snow surveys should be performed to validate the RADAR based estimates.