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| Description of Peakflow Forecasts Formulation |
| INTRODUCTION |
| The seasonal Peakflow forecasts produced at the Northwest River Forecast Center are delivered through two products. The traditional Regression analysis uses the relationship of forecasted seasonal volumes to peakflow. A second method incorporates Ensemble Streamflow Prediction (ESP) methodology using conceptual modeling. |
| REGRESSION MODEL |
| The Northwest River Forecast Center (NWRFC) currently uses a statistical regression model to make water supply forecasts for the Columbia River Basin; the coastal streams of Washington and Oregon; and the Great Basin of Oregon. From January through June there are three forecasts produced every month. Each forecast has a varied amount of observed data available for model input and some diminution of forecast results may occur. There is a strong relationship between seasonal volumes and the peakflows associated with those forecast volumes. The volumetric forecasts are based on precipitation reports from over 400 sites. Included in these procedures are snow water equivalent and observed runoff values from all available sites in Oregon; Washington; Idaho; Western Montana; Western Wyoming; Northern Nevada and British Columbia, Canada. Simple linear and curvilinear relationships between volume and peakflows were then developed. Statistical analysis is performed to account for the most probable forecast and the associated uncertainty forecasts. |
| ESP MODEL |
| The National Weather Service River Forecast System procedures use conceptual hydrologic and hydraulic models to simulate streamflow and reservoir conditions. An added component of the system is the Ensemble Prediction System (ESP). This procedure uses current conditions along with historical meteorological data and forecast meteorological data to make extended probabilistic forecasts. The forecasts are nothing but time series of future streamflow simulations. The simulated streamflow traces can be scanned for different hydrologic parameters, such as max flow, min flow, volumes, etc., for a period in the future. Statistical analysis then produces a probabilistic forecast for each variable and period of interest. For peakflow we look at max flow and the date when the max flow occurs. |
| Available Plots |
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| Plot 1: Exceedance Probability Plot. This plot is a statistical summary of all the ensembles prodcued by the ESP procedure. The line and markers represent stage or flow which exceeds a certain level and the associated probability. The color shades are levels of different critical values. In this example, the colors are levels of minor, moderate, and major flooding. |
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| Plot 2: Days to Peak. The process of summarizing the ensembles also reveals other featurs of the hydrographs. The date of the peak for each period is statistically ordered and a probability fuction is applied. This plot represents the probability of when the peak might occur. The values are days from the beginning of the run. The forecast table at the top of the forecast page makes the conversion to a standard date format. |
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| Plot 3: Historical Peaks. This plot is a simple display of all the historical peaks for a station. The red line when present is the National Weather Srvice established flood stage. The year is the water year in which the peak occured. |
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| Plot 4: Historical Peaks Exceedance. This plot is a simple display of all the historical peaks that have been ordered and a empirical probability distribution has been applied. The peaks in the graph are the highest peaks to occur during the water year for a station. When the flood stage or red line is displayed, this can provide a understanding of the flood frequency at this site. It is not a true flood frequency since some sites expirence multiple floods during a year. |
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| Plot 5: Historical Peaks Exceedance. This plot is a simple display of all the historical peaks that have been ordered and a empirical probability distribution has been applied. The peaks in the graph are the highest peaks to occur during the water year.for a station. This plot only includes peaks which are associated with the snowmelt months. These are then peaks which have occured between April and September. |