A comparison of algorithms for stream flow recession and baseflow separation

Simple hydraulic models for outflow of groundwater into a stream suggest th at the form of the storage-discharge relationship for groundwater changes f rom linear, for a confined aquifer, to quadratic, for an unconfined flow. T ests of the form of stream flow recessions in 11 streams, during periods of no recharge, show that for most catchments the storage-discharge relations hip is more strongly non-linear than the quadratic form. However, for the c ommonly occurring case of recessions of duration up to about 10 days, the l inear model remains a very good approximation, using a biased value of the groundwater turnover time. In contrast, estimates, from the stream hydrogra ph, of recharge during a storm event are very sensitive to the form of the storage-discharge relationship. The results of this study also show great v ariability in the parameters of the recession algorithm from one recession to another, attributable to spatial variability in groundwater recharge. An extension of the linear model to 'leaky' catchments, where the recession r eaches zero flow, has been tested on two data sets. The second part of the paper deals with algorithms for baseflow during surf ace runoff events - the problem of hydrograph separation. Algorithms with o ne, two and three parameters have been compared, using data for the same 11 streams, and the results show significant differences in the baseflow inde x (BFI) predicted for some catchments. The two-parameter algorithm, which i s fitted subjectively, is more consistent in providing plausible results th an either the one- or three-parameter algorithms, both of which can be fitt ed objectively. Copyright (C) 1999 John Wiley & Sons, Ltd.