The Runoff Risk Advisory Forecast (RRAF) Map is based on outputs from the National Weather Service's Sacramento Soil Moisture Accounting
(SAC-SMA) Model used for flood forecasting. This model takes into account snow accumulation and melt, soil moisture content,
and forecast precipitation and temperatures to predict the likelihood of runoff at the watershed scale for 242 NWS basins in Wisconsin.
The RRAF team developed and tested a procedure for using the SAC-SMA Model to produce these maps for Wisconsin.
How the SAC-SMA Model is used to make the Runoff Risk Forecast Map:
Why 3 days for rainfall runoff risk assessments?
The team felt that there was a relatively high amount of uncertainty for some precipitation forecasts more than five days out.
In addition, the 590 standard includes a 72-hour incorporation time limit for surface-applied manure. The three-day composite
risk score provides a relatively conservative estimate of runoff risk while maintaining a more realistic degree of accuracy
with regard to the potential uncertainties of precipitation forecasting.
Why 10 days for risk assessments based on snowmelt?
Snowmelt predictions are largely based on a temperature model run in conjunction with the SAC-SMA model. Since there is considerably
less uncertainty in temperature forecasts than in those for precipitation, the team members from the National Weather Service
felt that use of a 10-day forecast window for snowmelt would be reasonably accurate. In addition, the team recognizes the extreme importance
of properly managing manure applications preceding snowmelt runoff. A 10-day window serves to maximize the amount of lead time given to producers,
while at the same time maintaining a realistic degree of accuracy with regard to the potential uncertainties of temperature forecasting. Precipitation
forecasts are also utilized for the longer window; the team balanced the uncertainty of such long-range precipitation forecasts with two factors:
(1) Rainfall (and thus rainfall-derived risk) without temperatures conducive to snowmelt is rare, and (2) The potential runoff from rainfall atop
frozen or snow-covered soils is problematic enough to warrant flagging as high risk, even given the uncertainty in the forecast.