Flood Forecasting

Traditional flood forecasts are provided for approximately 80 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 (COE), 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. A recent office move resulted in the collocation of the CNRFC, WFO Sacramento, the USBR, and the CADWR. This collocation has dramatically improved the coordination between the NWS and USBR who manage 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, COE, 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 1200 rain gages, 600 air temperature sensors, 500 river gages, and 120 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 runs from late October through mid April. Additional routine staffing and operational activities take place during this time of the year. Quantitative precipitation forecasts (QPF) for the next three days in six hour blocks are provided by the Hydrometeorological Prediction Center during the winter. Forecasts of the snow level are also provided. These weather forecasts are critical to the NWS’s ability to provide adequate public warning time in the event of a winter flood. Within the CNRFC, a staff of meteorologists operate the HAS (Hydrometeorological Analysis and Support) function to integrate observed and forecast weather into the river forecast models. These forecasters quality control incoming temperature and precipitation data and monitor developing meteorological conditions very closely to ensure that the current QPF still reflects expected conditions (Figure 5).

Figure 5 - Operational Hydrometerological Analysis and Support

The CNRFC uses the NWSRFS (National Weather Service River Forecasting System) to simulate and project river flows and stages in its area of responsibility. NWSRFS 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 6.

Figure 6 - Typical NWSRFS 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 fell as rain, flooding would be significantly worse and more frequent and agricultural irrigation would be significantly less reliable.

NWSRFS is operated locally in a quasi-interactive fashion through a graphical user interface. The interface, called the Interactive Forecast Program (IFP), allows the forecaster to run, evaluate, and modulate 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 3 to 5 days using current model conditions as well as forecasts of temperature, precipitation, and reservoir regulation. Figure 7 shows the hydrograph portion of the IFP interface.

Figure 7 - NWSRFS Interactive Forecast Program (IFP) Display

In practice, the CNRFC breaks up its forecast workload into eight geographic pieces. This allows forecasters to simultaneously concentrate on specific problem areas. Routine forecasts for the mainstem of the Sacramento River are provided daily. Other river points are forecast when the current or expected stage exceeds 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 8).

Figure 8 - 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 3 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 once a day during the winter. During heavy runoff events, the inflow forecasts are updated every six hours or more often if required or conditions change. Reservoir operators inform the CNRFC of regulation plans and changes so that planned reservoir releases can be incorporated in downstream river forecasts.