How do flow peaks and durations change in suburbanizing semi-arid watersheds? A southern California case study

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Forty-three US Geological Survey gauges with records greater than ∼15 yrs located in watersheds less than ∼250 km2 were used to model the effects of suburbanization on streams in semi-arid southern California. The watersheds spanned a gradient of urban development, ranging 0–23% total impervious area in 2001. With little flow control at the subdivision scale, most impervious area in the region is relatively well-connected to surface-drainage networks and hydrologically effective. Consequently, total impervious area was an effective hydrologic surrogate for urbanization, emerging from an expansive array of geospatially-derived hydrologic variables as a statistically-significant (p < 0.05) predictor of instantaneous peak-flow rates at the 1.5- and 2-yr recurrence intervals and the durations of all geomorphically-important flows. To represent the effects of urbanization on flow durations, we developed duration density functions by using power functions (typical R2 > 0.95) to predict occurrence of logarithmically-binned mean daily discharges greater than some nominal value. This approach expands on previous scaling procedures to produce histogram-style cumulative flow durations for ungauged sites using urbanization extent and other watershed descriptors. For a particular watershed size and climatic setting, urbanization resulted in proportionally-longer durations of all geomorphically-effective flows, with a more pronounced effect on the durations of moderate flows. For example, a representative watershed with ∼20% imperviousness could experience five times as many days of mean daily flows on the order of 100 cfs (3 m3/s) and approximately three times as many days on the order of 1000 cfs (30 m3/s) relative to the undeveloped setting. Increased duration of sediment-transporting flows is a primary driver of accelerated changes in channel form that are often concurrent with urbanization throughout southern California, particularly in unconfined, fine-grained geomorphic settings. We did not have comparable studies on flow durations from other regions; however, the peak factors presented herein (e.g., sixfold increase in Q2 at 20% imperviousness) are greater than studies from humid temperate regions suggesting that semi-arid regimes may be more susceptible to urbanization than other climatic settings.