Lake Isabella, CA
Project: Turlock Irrigation District

Determine flood frequencies in both gaged and ungaged watersheds


A common challenge in hydrology is flood frequency analysis. Flood frequency analysis is the determination of flood flows at different recurrence intervals.

The standard procedure to determine probabilities of flood flows consists of fitting the observed streamflow record to specific probability distributions.

However, this method only works for basins:

  • that have streamflow records where streamflow records are long enough to warrant statistical analysis
  • where reservoir regulation does not appreciably alter flood flows, reservoir regulation, channel improvements (levees), or land use change

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Hydrologic Simulation For Gaged Watersheds

Continuous hydrologic simulation is a valuable tool to determine flood frequencies in gaged watersheds that have short streamflow records or are heavily regulated.


Meteorological data for most watersheds in the United States extend 40 to 80 years. Streamflow records, if available at all, are often much shorter. Continuous hydrologic simulation can use the observed meteorological data available to extend the existing streamflow record from a few years to 40 or 80 years.

You may then fit the extended record into a probability distribution.

Hydrologic simulation to determine flood frequencies in ungaged streams


The model is calibrated to a nearby, hydrologically similar, gaged stream.

Measure Changes

The model parameters are then adjusted to reflect the physical changes between the calibrated watershed and the ungaged watershed.


The available observed meteorological data are used to create a long streamflow record for the ungaged stream, which you can then fit into a statistical distribution.

This method produces much better results than alternative simplified approaches such as comparison to similar watersheds or indirect approaches that equate runoff frequency to precipitation frequency, such as unit hydrographs or the rational formula.

Land use changes can have a significant effect on flood flow frequencies. Historical streamflow records may be non-stationary for basins in which widespread urbanization is taking place.

Why Continuous Hydrologic Simulation?

Hydrologic simulation uses historical flow records to calibrate to the historical conditions, and it then incorporates the effects of future urbanization. Similarly, in basins where reservoir regulation significantly affects flood flows, continuous hydrologic simulation can isolate the effect of the reservoir. The model makes it possible to compare flood levels with and without the reservoir and for various reservoir operations.

When you consider the possibility of a dam failure, the results from hydrologic simulation can be used together with a full equations routing model (FEQ). For dam break analysis, Hydrocomp utilizes the output from HFAM as the input for FEQ.

In addition to generating or extending streamflow records, hydrologic simulation can be used to study the validity of an assumed probabilistic distribution for peak flows. The U.S. Water Resources Council recommends the use of the Log-Pearson Type III frequency distribution.

However, based on Hydrocomp’s experience, this distribution may produce unreasonable results in semi-arid areas: The calculated runoff intensity above certain return periods greatly exceeds the precipitation for the same return period.

Since continuous hydrologic simulation maintains a continuous accounting of soil moistures, it provides a unique tool to analyze the complex relationship between frequencies of precipitation, soil moisture and runoff.

Calibration Procedure

The HFAM parameters affecting Snow Accumulation and Heat Exchange and Melt are the only parameters that HFAM typically calibrates. The Snow Aging and Transformation parameters and the Snow and Soil parameters are run using either physically defined or default parameter levels.

There are two parameters that affect Snow Accumulation: TSNOW and SNOWCF. However, they are usually sufficient to fit model results to winter snow course measurements. A change in TSNOW of only 1 degree Fahrenheit can cause a glacier to grow or shrink. HFAM consolidates snowpacks into glacial ice if the supply exceeds melt. SNOWCF (snow correction factor) scales precipitation falling as snow.

For Heat Exchange and Melt, HFAM uses CCFACT (snow condensation and convection melt). CCFACT is effective and used to change the timing of snowmelt as necessary.

Refinements in snowmelt calibration are specific to weather sequences in particular watersheds. There is a need to consider how each process is affecting a particular snowmelt period to find guidance on how process parameters could be affecting that period.

integrateFor example, if melt released from the snowpack needs to be increased, the actions would be:
1) To increase CCFACT
2) Review SHADE level