Modeling nitrogen transport in the Newport Bay/San Diego Creek watershed of Southern California

The Newport Bay/San Diego Creek watershed has had a total maximum daily load (TMDL) established for the allowable loading of nitrogen into the bay. Baseline allocations for the TMDL indicate that 30% of the total nitrogen load is from agriculture; however, observations from a monitoring study and estimates from a conceptual model show that nitrogen from agriculture contributions is more likely between 2 and 8% of the total. An analysis using a nitrogen transport model indicates that agriculture contributes 2% of total N loading directly to surface waters, and an additional 6% from leaching to shallow groundwater and subsequent mixing with surface water. The time lag involved in groundwater mixing suggests that if all agricultural production in the watershed were ceased today, nitrate loading from fertilization would continue for 10–30 years. The greatest input of nitrogen to the surface-water system was found to be from urban development with 63% rather than the 21% estimated in the TMDL baseline allocations.