About CNRFC - River Flood Forecasting

Traditional flood forecasts are provided for nearly 100 locations in the roughly two-state area.  The diversity of hydrologic regimes, the proximity to the Pacific Ocean, and the complications of a highly-developed water management environment make forecasting extremely difficult.  A great deal of coordination is required as many federal, state, local, and private entities are involved in storing, releasing, and diverting streamflow.  Constant coordination is performed with the U.S. Bureau of Reclamation (USBR), U.S. Army Corps of Engineers (USACE), and the California Department of Water Resources (CADWR).  In addition, numerous municipalities, irrigation districts, and utility companies significantly affect river flows and therefore must be included in the forecast process.  The CNRFC is collocated with National Weather Service (NWS) Weather Forecast Office Sacramento, the USBR, and the CADWR.  This collocation has fostered coordination between the NWS and USBR, who manages some of the largest reservoirs in the area.

Operational flood forecasting is highly dependent upon reliable and timely data.  The CNRFC relies on the data collected by many other agencies and groups to support its flood forecasting function.  It is important to note that the flood forecasting function relies entirely upon automated data as opposed to manual observations.  Automated data are transmitted from field sites using a variety of techniques that include line-of-sight radio, microwave, satellite, telephone, and meteor-burst.  Agencies that collect and provide real-time hydrometeorological data include the CADWR, USBR, USACE, U.S. Geological Survey (USGS), U.S. Forest Service (USFS), U.S. Natural Resources Conservation Service (NRCS), cities and counties with local flood warning systems, numerous irrigation districts, and several utility companies.  In combination, the network consists of over 2500 rain gages, 1400 air temperature sensors, 1300 river gages, and 175 gages that measure reservoir elevation.  Many other parameters such as wind speed, wind direction, relative humidity, and barometric pressure are available.  Data are collected in a local database and screened for quality before being passed along to the river forecasting model.

The winter flood season in the CNRFC area of responsibility typically runs from mid October through mid April.  Additional routine staffing and operational activities take place during this time of the year.   Quantitative precipitation forecasts (QPF) and freezing level forecasts for the next six days in six-hour blocks are provided by the HAS (Hydrometeorological Analysis and Support) function, a staff of meteorologists within the CNRFC.  These forecasters monitor developing meteorological conditions very closely to ensure that the current QPF still reflects expected conditions (Figure 1).  See the Hydrometeorology section for more information.

Figure 1 - Operational Hydrometerological Analysis and Support

The CNRFC uses the Community Hydrologic Prediction System (CHPS) to simulate and project river flows and stages in its area of responsibility.  CHPS is a collection of hydrologic models and infrastructure that handles the required parameters, states, and data.  A typical model configuration for forecasting watershed response at a specific point along a river is shown in Figure 2.

Figure 2 - Typical CHPS Configuration

It is important to note than most of the forecast watersheds in the CNRFC require the use of a snow model.  The snow model mathematically accumulates solid precipitation, calculates the heat content of the snowpack, and computes meltwater for delivery to the soil model when the snowpack begins to melt.  The temporal delay caused by this natural process is important to both flood management and water resources interests.  If all precipitation in the CNRFC forecast area fell as rain, flooding would be significantly worse and more frequent, and agricultural irrigation would be significantly less reliable.

CHPS is operated locally in an interactive fashion through a graphical user interface.  The interface allows the forecaster to run, evaluate, and modify the operation at each forecast point.  Model performance is evaluated over the last several days to several weeks, and the streamflow is projected for the next 5 to 10 days using current model conditions as well as forecasts of temperature, freezing levels, precipitation, and reservoir regulation.  Figure 3 shows the hydrograph portion of the CHPS interface.

Figure 3 - CHPS Interactive Forecast Display (click for larger image)

In practice, the CNRFC breaks up its forecast workload into 11 geographic areas.  This allows multiple forecasters to simultaneously concentrate on different problem areas.  Routine forecasts for the entire region are provided at least once per day year round, and are issued at least twice daily during the winter flood season.  Forecasts are issued more frequently when current or expected stages exceed established criteria.  The art of flood forecasting involves a delicate and skilled balance of experience, analysis of current conditions, and analysis of model guidance (Figure 4).

Figure 4 - Inputs to operational flood forecasting process

California State Law (Section 236) authorizes the CADWR to either act independently or to cooperate with Federal agencies or others in the collection of data for river forecasting, in making forecasts of streamflow, in providing for flood warning, and in providing communications for the collection and dissemination of such information.  As such, it is critical that the CADWR and the NWS cooperate at an unprecedented level.  Over time, the working relationship with the CADWR has resulted in a joint operation whereby CADWR and CNRFC personnel work together to develop flood forecast guidance.  Public dissemination is accomplished through normal NWS channels as well as the State-Federal Flood Center.  This close working relationship benefits NWS customers by taking advantage of the resources and expertise of both agencies.  Duplication of effort is eliminated, and a single message reaches the emergency managers and the public.

In addition to flood forecasts, the CNRFC provides 5-day reservoir inflow forecasts to the appropriate water management agencies.  These forecasts allow reservoir operators to anticipate regulation changes required by very wet or very dry weather.  Reservoir forecasts are produced in the flood forecast modeling process and are updated at least twice per day during the winter.  During heavy runoff events, the inflow forecasts are updated every six hours or more often if required or if conditions change.  Reservoir operators inform the CNRFC of regulation plans and changes so that planned reservoir releases can be incorporated in downstream river forecasts.