Information, artifacts, and noise in dQ/dt − Q recession analysis

The plotting of the time rate of change in discharge dQ/dt versus discharge Q has become a widely used tool for analyzing recession data since Brutseart and Nieber [Water Resour Res 13 (1977) 637–643] proposed the method. Typically the time increment Δt over which the recession slope dQ/dt is approximated is held constant. It is shown here this that leads to upper and lower envelopes in graphs of log(−dQ/dt) versus log(Q) that have been observed in previous studies but are artifacts. The use of constant time increments also limits accurate representation of the recession relationship to the portion of the hydrograph for which the chosen time increment is appropriate. Where dQ/dt varies by orders of magnitude during recession, this may exclude much of the hydrograph from analysis. In response, a new method is proposed in which Δt for each observation in time is properly scaled to the observed drop in discharge ΔQ. It is shown, with examples, how the new method can succeed in exposing the underlying relationship between dQ/dt and Q where the standard method fails.