Hydrological and hydrochemical responses of streams in coastal California to land use, fire and climate

Time series measurements in a diverse array of watersheds and sub-catchments reveal that stream discharge and fluxes of dissolved and particulate nutrients and suspended sediments vary greatly as a function of land cover and land use, rainfall amounts and disturbance from fire. For example, annual nitrate fluxes ranged from as low as 0.5 moles/ha to as high as 1100 moles/ha among the watersheds we have studied, while inter-annual variation in nitrate flux varied by almost two orders of magnitude among watersheds. Concentration versus runoff relationships showed consistent nitrate-runoff patterns within three broad land use classes (dilution in agricultural watersheds, invariance in urban watersheds, and enrichment in an undeveloped watershed.   We found that dissolved organic nitrogen (DON) and particulate nitrogen (PN) contributed significantly to the nitrogen exported by streams.

With a recurrence interval of 10 to 40 years in southern California chaparral ecosystems, fires in coastal watersheds result in episodic alternations in hydrology and export of nutrients and suspended sediment to coastal waters. Stream and rainfall gauging and runoff sampling were used to determine changes in hydrology and export of nutrients and suspended sediment from several wildfires in 2004, 2008 and 2009 that burned coastal watersheds in our study area. Burned watersheds had an order of magnitude higher peak discharge compared to unburned watersheds. Suspended sediment exported from a burned watershed was approximately 10 times greater than from unburned watersheds. Highly elevated ammonium export from burned watersheds occurred during the first storms of the rainy season. Nitrate, dissolved organic nitrogen, and phosphate export from burned watersheds increased several fold compared to unburned chaparral watersheds.