Differential Impacts of Baseflow on the Flood Frequency Curve

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Baseflow contributions from groundwater sources have been overlooked with regards to peak flows. Only recently has this component of watershed drainage been shown to exert influence over peak flow behavior. As increased baseflow contributes to increased discharge within a stream, it represents a secondary but potentially key influence on peak flows. One way to quantify the amount of baseflow contributed to a stream is the metric Baseflow Index or BFI. BFI is defined as the average ratio of subsurface flow to surface runoff in a stream which captures the average groundwater a given stream will receives over the selected period of record. As changes to baseflow can be brought about by human disturbances to a hydrologic system including urbanization and groundwater withdrawals, understanding its’ effect on peak flow behavior can provide further insight into how these watershed alterations can impact flood risk assessment. In this analysis, BFI is used to address how the contribution of subsurface flow impacts the flood frequency curve indexed at select return periods. Additional insight is gained by analyzing the effect of BFI at different area thresholds. Small to moderate impacts are observed on average with BFI exerting the most influence over the lower tails of the flood frequency curve. In the extreme lower tails of the flood frequency curve, changes in BFI by 25% can potentially result in up to a 16% change in flood magnitude. Though the effects of BFI were primarily restricted to the lower tails of the flood frequency curve, comparing discharge using the fitted regression equations under a change in BFI that has been observed in natural streams showed how the effects resonate across the flood frequency curve.

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Presented at the AGU Fall Meeting on December 15, 2016 in San Francisco, CA