Concentration‐Discharge Responses to Storm Events in Coastal California Watersheds

Storm events in montane catchments are the main cause of mobilization of solutes and particulates into and within stream channels in coastal California. Nonlinear behavior of nutrients and suspended sediments during storms is evident in the hysteresis that arises in concentration‐discharge (C‐Q) relationships. We examined patterns in the C‐Q hysteresis of nutrients ( , , DON, and ) and total suspended solids (TSS) during storms across 10 sites and water years 2002–2015 by quantifying the slope of the C‐Q relationship and the rotational pattern of the hysteresis loop. We observed several hysteresis types in the ∼400 storms included in our study. Concentrations of constituents associated with sediment transport ( and TSS) peaked during high flows. Conversely, nitrogen species had hysteretic responses such as dilution with clockwise rotation in urban sites and enrichment with anticlockwise rotation in undeveloped sites. The wide range of C‐Q responses that occurred among sites and seasons reflected the variable hydrological and biogeochemical characteristics of catchments and storms. Responses for nitrate in nested catchments differed in slope and rotation of C‐Q hysteresis. Upland undeveloped and lowland urban sites had anticlockwise rotation at the onset of the rainy season following a dry year, which implied a delay in the transport of this solute to the streams. Slopes by the middle of the rainy season showed that the urban site switched from dilution to enrichment, and then again to dilution with clockwise rotation at the end of the season, which implied high initial concentrations and proximal sources.