### Interpretive Notes

**PEAK FLOW DEFINED**

The peak flow forecast represents the maximum mean daily flow at a point during
the April through July period (the highest average flow for an entire day
during the runoff season). It does not represent the instantaneous peak (the
maximum flow at a single moment). In the case of smooth snowmelt regimes
(hydrographs), it may be acceptable to approximate one with the other.
In Arizona, the normal snowmelt period is from March to May. Occasionally,
heavy rainfall events can produce higher peak flows than the snowmelt peak
flows. For verification and calibration purposes, the maximum mean daily flow
during the March through May period was used regardless of the runoff source.

The Average Peak, Historic Peak, and Normal Time of Peak are all derived from
the entire period of record for a given gauge.

**FORECAST PROBABILITIES**

Peak flow forecasts are presented in terms of probabilities or, more
specifically, exceedance probabilities. The so-called "most probable" forecast
actually corresponds to a flow that is equally likely to be too high or too
low, the 50% exceedance level (i.e., 50 chances out of 100 of being exceeded).
The other exceedance probabilities associate the likelihood of exceeding other
levels. In general, a close bunching of the exceedance forecasts indicates low
variability and that the user can have a high degree of confidence in the
forecast information. Conversely, a large spread in the exceedance forecasts
indicates high variability.

**MODELLING TECHNIQUES**

The peak flow forecasts that follow have been derived using a combination of
(1) physically-based conceptual models and (2) statistical regression models.
The conceptual model is the National Weather Service River Forecasting System
(RFS) in the Extended Streamflow Prediction (ESP) mode. Since the conceptual
model requires reservoir operation plans for up to five months into the future,
ESP application is limited to basins where regulation is minimal (mostly in the
headwater areas).

The farther downstream a forecast point is, the more likely it is that a
statistical regression was used between natural snowmelt runoff volume and the
observed maximum mean daily flow to generate the forecast. Such an approach
assumes that the correlation between regulated and unregulated flow is strong
and is constant from year to year.

**Snowmelt Peak Flow Forecasts**
- April, 1996
Colorado Basin River Forecast Center
(http://www.cbrfc.gov)

Last Modified: *Wednesday, 10-Apr-1996 16:18:14 MDT*