Long-Term Disconnect Between Nutrient Inputs and Riverine Exports in a Semi-Arid, Agricultural Watershed: Yakima River Basin 1945–2012

Abstract

Nutrient budgets are useful tools in addressing impaired water quality from increasing anthropogenic nitrogen (N) and phosphorus (P) inputs to watersheds. Long term analyses can identify temporal ecological responses to past nutrient management strategies. Multidecadal watershed analyses of nutrient dynamics traditionally have been limited to large humid or mesic river basins focused primarily on a single nutrient (N or P). Here we present a long term, dual N and P input-output budget for the agriculturally rich Yakima River Basin (YRB) located in the semi-arid western U.S. We quantified N and P terrestrial inputs from 1945 to 2012 using past agricultural and water quality data to investigate interactions between water flow and N and P exports in the YRB. Between 1945 and 2012, N and P inputs to agricultural lands increased by over 18 and 10-fold, respectively, with synthetic fertilizer accounting for the majority of input increases. While such increases in nutrient inputs were comparable to other U.S. basins, observed riverine N and P export did not significantly increase over time. Rather, we found that since the mid-1970s both N and P loads and concentrations in the Yakima River have decreased, indicating a temporal disconnect between nutrient inputs and riverine exports in the YRB. This disconnect highlights a need to further investigate nutrient dynamics at local scales in semi-arid watersheds. Our analysis raises questions about long-term N and P retention in the YRB and potential impacts on future water quality across the western US.