30 Sep 96
ADVIS. The ADVIS program (Sweeny 1988), devel-
models "embody a series of functions which are considered to
oped by the NWS for local flood warning, includes an API
describe the basin processes involved" (Linsley, Kohler, and
model as it is primary hydrologic forecast technique. (All
Paulhus 1982). Conceptual models attempt to provide a more
NWS implementations of API are available in ADVIS.)
"physically-based" approach to basin modeling by more
ADVIS is a simplified implementation of hydrologic model-
explicitly accounting for evapotranspiration, interception
ing that produces output appropriate for the local user
storage, retention storage, infiltration, surface runoff,
depending upon what type of information is available. For
percolation, interflow, etc.
example, ADVIS output includes:
(d) Table 4-2 shows the most widely available models
Categorical forecasts for ungaged water-
for local flood warning systems.
sheds. Categorical forecasts are general forecasts
of "minor," "moderate," or "severe" flooding
based on the antecedent precipitation index and
Flood Forecast Models
Sacramento Soil Moisture Accounting
Crest stage forecast. ADVIS will generate a crest
forecast if the unit hydrograph peak is available.
Flood Advisory Tables
Forecast hydrograph. Where the complete unit
hydrograph is available, ADVIS generates a com-
Model (Sittner, Schauss, and Monroe 1969) was developed
plete forecast hydrograph.
by the National Weather Service and has been used in vari-
The ADVIS program is intended to address relatively simple
index reflects the current soil moisture based on recent rain-
hydrologic situations at the local level.
fall. A high index means high soil moisture content, while a
low index indicates dry conditions. The API for a given
Flood Advisory Tables. Flood advisory tables are used
period is used with a rainfall-runoff relationship, the rainfall
to provide a quick estimate of peak stage forecasts using
amount, and the storm duration to estimate runoff. A unit
indices produced by the API or other modeling techniques.
hydrograph is applied to distribute the runoff. At each
The tables are computed in advance for a variety of anteced-
computational period, the index is updated based on the
ent conditions. The current index can be computed onsite or
additional rainfall and by a seasonally dependent factor. The
provided by a local NWS office. Local users apply the
seasonally dependent factor empirically accounts for changes
current index with the latest rainfall estimate to the table to
in the rainfall-runoff relationship due to seasonal changes in
determine the estimated peak stage. An estimated time to
evapotranspiration, infiltration, etc.
peak is usually available based on previous analysis of basin
Complex basins can be modeled by applying the API tech-
nique to individual subbasins that are hydrologically homo-
Sacramento Soil Moisture Accounting Model.
Outflows from subbasins can be routed
The Sacramento Soil Moisture Accounting Model
downstream and combined with other tributary flows and
(Burnash, Ferral, and McGuire 1973) is a conceptual model
inflows calculated by the API model for local areas.
designed as a comprehensive representation of the hydrologic
processes of the upper soil mantel. Figure 4-3 shows how the
Many versions of the API model exist. Most National
various representations of these hydrologic processes are
Weather Service River Forecast Centers that use API have
linked together. Runoff calculated for each period is
added modifications to "customize" the technique for
distributed using a unit hydrograph.
conditions in basins within their area of responsibility. At
least eight different implementations of API are used by the
Each hydrologic process is represented by a function or series
National Weather Service.
of functions with adjustable parameters. The model is
calibrated with historical rainfall and stream flow data by
The API model is simple and relatively easy to understand
adjusting parameters until the model output adequately
and adjust. Forecasters can easily change model parameters
represents basin response. The model is applied to individual
or model runoff based on assessment of the current event to
basins that are hydrologically homogeneous. Complex
improve model performance.