NITROGEN AND CARBON DISCHARGE FROM EXTREME PRECIPITATION EVENTS: A CASE STUDY FROM THE SANTEE EXPERIMENTAL FOREST WATERSHED ON THE ATLANTIC COASTAL PLAIN

Watershed-level processes from forests, agricultural, and urban areas, along with multiple management practices are becoming more complicated in recent years due to increasing threats from extreme precipitation, hurricanes, and flooding in the Southeastern U.S. We will present results of nitrogen and carbon transport for 2015-2017 as influenced by large precipitation events, and we put this in context of episodic events and earlier periods. We analyzed long-term data from a relatively less disturbed experimental watershed (WS80; a 160-ha controlled site) consisting of post-Hugo regenerated upland pine and hardwood mixed stands on depressions and bottomland riparian areas drained by a blackwater stream at the USDA Forest Service Santee Experimental Forest in the South Carolina Atlantic Coastal Plain. This low-gradient watershed is characterized by clay-rich soils which affect the surface and subsurface hydrology. Long-term average water balance shows about 20-25% of annual rainfall transfers to streamflow (direct runoff + baseflow), with about 70-75% lost to evapotranspiration, and only a minimal loss to deeper groundwater recharge. However, preliminary results for 2015 with the October 3-4 extreme rainfall event of 500 mm showed that 45% of rainfall became streamflow and 46% of the annual total dissolved nitrogen (TDN; 5.1 kg ha^-1) was exported during this event. The annual load was found to be higher than that for 2016, 2017, and for the post-Hugo (1989-91) and 2006-11 periods. This was primarily volume-driven by event streamflow, with some elevated TDN concentrations; annual mean TDN concentration was 0.56 mg L^-1, similar or less than for 2016-2017 and the earlier periods. Similar observations were noted for dissolved organic carbon (DOC), which showed a strong relationship with TDN . These results are important to understand the biogeochemical processes at work and should be interpreted in terms of episodic events, flow pathways, and downstream ecosystem functions/services to avoid impacts such as eutrophication and reduced DO in receiving waters. We believe these datasets may also serve as references for stormwater quality management and TMDL analysis for similar landscapes in the region, including the rapidly urbanizing greater Charleston area and other coastal cities.