Creation and Application of Watershed Nitrogen Budgets
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Watershed nutrient budgets are built by biogeochemists to study sources, loads and retention of nitrogen or phosphorus. Increasingly, in areas where water quality goals are not being met, a watershed nutrient budget is used to inform reduction targets, prioritize source areas and other decisions. Nitrate contamination of private wells in Oregon’s southern Willamette Valley led the state to embark on a multi-year, multi-stakeholder process by declaring the area a Groundwater Management Area (GWMA) in 2004. To inform this process, we are sharing information about watershed nitrogen (N) balances at two scales – the entire Willamette River Basin (WRB) and the tributary Calapooia River Basin (CRB). Locally-derived data on N inputs coupled with streamflow and chemistry were compiled to calculate N balances for 22 WRB sub-watersheds and 58 CRB sub-watersheds. For both areas, 80-90% of total N input comes from agricultural sources. Detailed information about crop types in the CRB allowed us to calculate crop harvest, crop nutrient use efficiency and N surplus, which may be useful metrics for managers. Across CRB tributary sub-watersheds, 19% of annual N inputs were exported by streams, and 40% of N inputs remained as N surplus, which then may be available for N leaching to groundwater, storage in soils and gaseous losses. In contrast to the 68% nutrient use efficiency observed for crops across the US and Canada, crop harvest only removed 41% of N inputs in the CRB. Working with local farmers, crop advisers, fertilizer companies and university extension we will identify and share management practices that can reduce N surplus prior to the fall and winter rains. We will present lessons learned from discussing the results to the farming, regulatory and conservation communities, to better inform efforts that increase crop nutrient use efficiency, reduce N surplus and address water quality goals.