Estimation of Baseflow Recession Constants

Hydrograph recession constants are required in rainfall-runoff models, base flow augmentation studies, geohydrologic investigations and in regional low-flow studies. The recession portion of a streamflow hydrograph is shown to be either an autoregressive process or an integrated moving average process, depending upon the structure of the assumed model errors. Six different estimators of the base flow recession constant are derived and tested using thousands of hydrograph recessions available at twenty-three sites in Massachusetts, U.S. When hydrograph recessions are treated as an autoregressive process, unconditional least squares or maximum likelihood estimators of the baseflow recession constant are shown to exhibit significant downward bias due to the short lengths of hydrograph recessions. The precision of estimates of hydrograph recession constants is shown to depend heavily upon assumptions regarding the structure of the model errors. In general, regression procedures for estimating hydrograph recession parameters are generally preferred to the time-series alternatives. An evaluation of the physical significance of estimates of the baseflow recession constant is provided by comparing regional regression models which relate low-flow statistics to three independent variables: drainage area, basin slope and the baseflow recession constant. As anticipated, approximately unbiased estimators of the baseflow recession constant provide significant information regarding the geohydrologic response of watersheds. !