Application of alternative hydrograph separation models to detect changes in flow paths in a watershed undergoing urban development

Runoff characteristics in a low‐gradient central‐Florida watershed were analysed using environmental δ18O and a series of conservative‐tracer mass‐balance models applied to a storm event (109 mm over approximately 30 h) monitored in May and June 1993 on the Econlockhatchee River, Florida. The assumption of steady‐state conditions implicit in the widely used two‐component mixing model was tested by applying steady‐state and non‐steady‐state models for a subcatchment (215 km2) of the river. Both models indicated that about 76% of the resulting river flow was composed of pre‐storm water. A third mass‐balance model (steady‐state) was developed to separate pre‐storm from storm‐event runoff over a discrete reach of the river, which had a contributing area of 135 km2. This model indicated that approximately 47% of the water entering the reach could be attributed to pre‐storm water. The greater proportion of event water entering the reach was attributed to suburban development in the watershed and indicates that urbanization in watersheds not only affects the timing, peak and total runoff, but also may change flow‐paths for runoff, and may significantly affect downstream water quality. Copyright © 2000 John Wiley & Sons, Ltd.