30 Sep 96
Figure 4-3. Sacramento soil moisture accounting model
basins are modeled by combining outflows from individual
organizations throughout the United States have used HEC1
basins using a variety of available routing techniques.
to generate flood hydrographs for a variety of purposes from
bridge design to flood plain mapping. As a result, many local
HEC1-F. The Hydrologic Engineering Center (HEC)
engineers understand the model and the transition to HEC1-F
has developed a forecasting system (Pabst 1986; USACE
is relatively easy.
1989) for COE offices that is also available for local flood
warning systems. The forecast technique uses an initial and
(2) Quantitative Precipitation Forecasts. Flood forecasts
uniform loss rate to compute runoff, which is applied to a unit
based on rainfall observations will likely underestimate future
hydrograph to produce a basin forecast. Results from each
flood flows if the forecast is released while it is still raining.
basin can be combined and routed to develop forecasts for
Sequentially forecasting floods based on observed rainfall only
complex systems. HEC1-F uses observed stream flow to set
results in "stair-step" forecasts. Each forecast is higher than
proper loss rate parameters.
the previous ones due to the incremental rainfall observations
at each timestep. To avoid "stair stepping," forecasts of
HEC1-F can be calibrated relatively easily. Most of the
future rainfall (quantitative precipitation forecasts or QPF) are
necessary parameters can be easily obtained from maps.
Infiltration parameters and certain characteristics of the unit
hydrograph can be estimated initially. During a flood event,
(a) Using QPF's along with the observed rainfall
HEC1-F evaluates model performance against observed
provides a more realistic look at possible flood elevations.
stream flow and automatically adjusts the appropriate
Not only is the effect of what has already occurred included in
the forecast, but what the effect of additional expected rainfall
for the remainder of the storm is also included. See
HEC1-F is the forecast version of HEC-1, a widely used
hydrologic design tool. Many different public and private