Patterns of DIN export in the Central Valley of California: Increasing the resolution of a global nutrient export model

Background/Question/Methods In California’s Central Valley (CV) high levels of dissolved inorganic nitrogen (DIN) have been associated with hypoxia in the lower San Joaquin River and related adverse impacts on aquatic ecosystems. Modeling of land-based sources and landscape sinks of N provides information that can be used in management of water quality and aquatic ecosystems, and also provides insight into mechanisms controlling N transport downstream through watersheds. We have increased the spatial resolution of an existing global DIN transport model, Global Nutrient Export from Watersheds (Global NEWS) and applied it to the subbasins of the CV in order to gain insight into patterns of N inputs and outputs as well as controls on N transport downstream through watersheds. DIN sources in our model included manure N, inorganic N fertilizers, N-fixation, atmospheric N deposition, and sewage N outflow. DIN sinks included harvest of N in agricultural products, N removed in consumptive water use, and N removal in aquatic systems. 
Results/Conclusions Preliminary results show a good agreement between measured annual DIN export (kg N basin^-1 yr^-1) and modeled DIN export from the subbasins of the San Joaquin and Sacramento Rivers of the Central Valley (r²=0.67, compared with 0.72-0.83 for the Global NEWS-DIN model). For the 23 subbasins modeled, agricultural non-point sources of N (manure and inorganic N fertilizers) contributed 4-90% of DIN export, median 63%. For the subbasins of the Sacramento River, most of which are less impacted by agriculture than those of the San Joaquin River, modeling results suggest that N-fixation was most commonly the dominant source of DIN in river export, accounting for 25-42% of exported DIN. For 7 of the 10 modeled subbasins of the San Joaquin River, inorganic fertilizer was the largest single source of exported DIN. Remaining subbasins were manure-N dominated. Removal of water, and associated N, for consumptive use is an important factor in the N dynamics of the CV. The amount of water removed for agricultural, urban, and industrial uses ranges from 0 to 88% of unimpaired discharge. Future work will include increasing the temporal resolution of the DIN export model in order to investigate seasonal patterns associated with hydrology and agricultural practices.