Catchment and subcatchment scale linkages between land use and nutrient concentrations and fluxes in coastal California streams

Streams draining three coastal watersheds near Santa Barbara, California were studied for three years. Intense winter rains and short steep catchments produced flashy runoff from the dominant land uses, i.e., chaparral, avocado orchards, greenhouse agriculture, open-field nurseries, and residential and commercial development. Samples were analyzed for nitrate, ammonium, dissolved organic nitrogen, and phosphate; and volume-weighted mean annual concentrations ranged from 0.4-130, 6-1500, 8-180, and 2-130 mumol L-1, respectively, between the chaparral and intensive agricultural sites. Concentration and discharged data were converted to nutrient fluxes at a high temporal resolution and aggregated by storm, month, season, and year during water years 2002, 2003, and 2004. Nitrate and phosphate nutrient export coefficient (NEC) models were developed in one watershed and tested in adjacent catchments. Stepwise linear regressions were performed on potential surrogate variables for antecedent soil moisture conditions to create watershed response functions that simulated runoff conditions for low, medium, and high rainfall events; variables included rainfall, runoff, and position in the water year. Nutrient input rates by land use and atmospheric depositions were also evaluated in a parallel modeling approach using fertilizer input data and atmospheric deposition. The nitrate and phosphate models were loosely coupled with an urban growth model (SLEUTH) to predict stream nutrient loading under different urban growth scenarios 50 years into the future. The results showed direct application of the model for agricultural and municipal tailwater and stormwater management.