Relationships Among Nutrient and Sediment Fluxes, Hydrological Variability, Fire, and Land Cover in Coastal California Catchments

Catchments along California's coast, and those with mediterranean climates elsewhere, experience large seasonal and interannual variations in rainfall, and occasional fires; land cover varies from natural vegetation to agricultural and urban development. Multiyear sampling of streams, with intensive collections during rainfall, and continuous measurements of stage, was used to determine the magnitude and variation of concentrations and fluxes of nutrients and suspended sediments during rainfall events and per year from catchments bordering the Santa Barbara Channel. Annual rainfall varied from 18 to 122 cm in water years 2002 through 2015. Hydrological variability, catchment topography, and land cover have strong statistical relationships to nutrient and suspended sediment fluxes. The link between nutrient fluxes and runoff during storms was strongest in sites with greater portions of impervious surfaces. The occurrence of fires followed by above average rainfall resulted in highest ammonium and suspended sediment fluxes. Nitrate fluxes during large storms in undeveloped catchments following wildfire were similar in magnitude to storm fluxes generated in catchments with intensive agriculture. Decreasing storm fluxes were observed with increasing cumulative discharge within a water year, which is consistent with the reduction of flux from each subsequent storm as the rainy season progressed. Plain Language Summary Coastal California's picturesque landscape bordering the Santa Barbara Channel is a mosaic of natural chaparral, orchards, and towns with appealing temperatures year-round. Most of the year is rainless, and winters can experience large storms as well as droughts. The many streams that flow from the mountains carry nutrients and sediments into the coastal ocean and reflect the landscape and climate. Wildfires are common and cause abrupt changes in stream flow and transport of nutrients and sediments. To understand how land uses, climate, and fire interact to determine the responses of streams in this region, we conducted intensive sampling throughout storms over many years in numerous watersheds. After fires, concentrations of nutrients and sediments increased dramatically. Nitrate fluxes during large storms in undeveloped catchments following wildfire were similar in magnitude to fluxes generated in catchments with intensive agriculture. During the course of rainy seasons the transport of nutrients per storm declined. Climate change projections of increasing incidence of droughts and floods will intensify these findings.