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Seasonal disconnect between hydrology and retention drives riverine nitrogen export Willamette River Basin, Oregon
Citation:
Compton, J., D. Sobota, K. Goodwin, AND J. Lin. Seasonal disconnect between hydrology and retention drives riverine nitrogen export Willamette River Basin, Oregon. ICRW, Shepherdstown, WV, July 16 - 20, 2018.
Impact/Purpose:
Watershed nitrogen budgets can help inform management at a variety of scales. Using locally-derived information on crops and other sources of nitrogen, EPA researchers along with a team of post-doctoral scientists conducted a seasonal analysis of nitrogen inputs and hydrologic export of nitrogen in the Willamette River Basin. This work was completed using hydrologic and water chemistry data available from USGS and state of Oregon data sets, along with USDA-ARS maps of crop cover validated for the area. Information describing seasonal nitrogen (N) fluxes can provide insights on how N supply, landscape retention mechanisms, and hydrologic processes interact to shape the amount and timing of riverine N export, and can provide guidance for nutrient management. This paper will be presented at the 6th Interagency Conference on Research in Watersheds in July 2018.
Description:
Watershed nutrient balance studies traditionally focus on annual timescales to examine factors controlling landscape level nutrient inputs, processing, and export. In areas with strongly seasonal precipitation, leaching losses may be greater during wet seasons when hydrologic forcing overwhelms retention and removal mechanisms. Information describing seasonal nitrogen (N) fluxes can provide insights on how N supply, landscape retention mechanisms, and hydrologic processes interact to shape the amount and timing of riverine N export, and can provide guidance for nutrient management. In Oregon’s Willamette River Basin (WRB), a large watershed with pronounced dry summers and wet winters, we examined how the spatial distribution of farmland, cities, and forests influence N inputs and interact with hydrology to affect riverine N export. Nitrogen loads affect surface water functions and also groundwater quality in this area. Locally-derived data on N inputs coupled with streamflow and chemistry were compiled to calculate N balances for 25 WRB sub-watersheds for the mid-2000s. For the entire WRB, 80% of the nitrogen inputs came from agricultural activities, largely from synthetic N fertilizer (71%). The second largest input to the WRB was atmospheric N deposition (10%). Fractional riverine N export (annual riverine N export / annual watershed N input) averaged 20% of total N inputs; but ranged widely from 8-66% across the watersheds. Watersheds with the highest fractional export had very high rates of N input, or contained large proportions of urban land. Fall and winter seasons together accounted for 60-90% of the riverine N export across all watersheds. Summer export was generally quite low, but was highest in the watersheds that receive summer snowmelt. Fractional N export in the WRB watersheds is relatively high relative to other areas of the US. The fate of N, whether it is retained in the soils and groundwater or exported downstream or to the coast, is important for considering the net effects of N. Our analysis indicates that the wet winter season drives the high proportion of N inputs exported to rivers during winter in this strongly seasonal climate.