Nitrate Storm Flux from Coastal Catchments in Southern California

We have been sampling nutrient concentrations in 19 coastal streams in the vicinity of Santa Barbara, CA., as part of the Santa Barbara Coastal LTER since Oct. 2000. These streams, with land cover varying from coastal sage and chaparral to agricultural and urban development, are representative of the 790 square km of coastal watershed extending 120 km from Point Conception to the Ventura River. The topography of these small coastal catchments (average area of about 10 square km) consists of mountainous headwaters and mild sloping coastal plains separated by transitional foothills. Annual rainfall varied from 27 to 74 cm during the 2001, 2002 and 2003 water years, and year-around sampling, with hourly collections during rainfall, and continuous measurements of stage were used to determine the magnitude and variation of nutrient concentrations and fluxes. Annual volume-weighted mean concentrations of nitrate generally ranged from 5 to 25 μ mol per L in undeveloped areas, increased to about 100 μ mol per L for urban and most agricultural catchments, and were in excess of 1000 in catchments with greenhouse-based agriculture. Different land uses produced different, but relatively characteristic, nitrate fluxes during stormflow. The fluxes, highly correlated with storm discharge, were similar for urban and agricultural catchments, over an order-of-magnitude higher for industrial greenhouse agriculture and, generally, an order-of magnitude lower in undeveloped catchments. A striking feature of the nitrate flux during storms producing large amounts of runoff were high concentrations during the falling hydrograph from upslope undeveloped areas, and the maintenance of these concentrations for several days. An interesting and potentially significant inference based on this pulse of upper catchment nitrate is that the influence of the developed portion of the catchment can become less than that of the undeveloped portion. If nitrate flux per ha is plotted against runoff, it appears that for storm runoff exceeding 2 to 5 cm per unit area the flux from the upper, undeveloped catchment exceeds the flux from lower urbanized or agricultural areas. Interactions among atmospheric deposition, biological nitrogen fixation and mineralization of organic nitrogen, and seasonal runoff from these areas appear to control the supply and export of nitrate.