A Poor Man's ESP: Statistical Hydrograph Trace Adjustment

Water managers increasingly demand forecasts of hydrograph characteristics beyond seasonal volumes. Currently, most volumetric forecasts of seasonal streamflow are produced using statistical regression techniques. One way to provide additional long-lead information about peak flows, low flows, and number of days to a particular threshold is to apply the Ensemble Prediction System, first developed by the National Weather Service. This Ensemble Prediction System involves the calibration of a hydrologic simulation model, model initialization using current watershed states, and forcing based on a number of observed historical meteorological traces. The output is a series of possible future daily hydrographs, from which the above mentioned characteristics can be derived. As a low-cost alternative, an ensemble hydrograph forecast can be obtained by rescaling historical flow traces by the existing statistical seasonal streamflow volume forecast. The volume of streamflow from past years is mapped into the distribution of the statistical error bound of the official seasonal volumetric forecast to obtain a multiplier factor for each year. This multiplier is then applied to each daily flow value of the historical year. While this simple system has many shortcomings and limitations, it still provides useful information and may serve as a credible naïve forecaster baseline against which to compare the performance of other ensemble forecast systems.