Long-term temporal variation of nitrate concentration - stream discharge (C-Q) relationship from an agricultural watershed in Midwest U.S.

Nutrients export from agricultural watersheds have long been recognized as a critical cause of eutrophication in receiving surface water bodies. Since 1970s, many changes have occurred in intensively managed agricultural watersheds in U.S., including enhanced artificial drainage, changing timing and rate of fertilizer applications, and implementation of Best Management Practices. To test the impacts of such changes, we employed the relationship of nitrate concentration - stream discharge (C-Q relationship) as a proxy to reveal the evolution of the inner chemo-physical mechanism of nutrient dynamics. We analyzed a long-term (1976-2017), and relatively high-frequency (up to every six hours) dataset with more than 20,000 nitrate measurements from an agricultural watershed in Ohio, U.S. In addition, we also delineated 700 event-basis nitrate C-Q relationship and evaluated the hysteresis patterns. The results indicated that the chemo-physical relationship for stream water generation and nitrate export remained relative constant in this highly managed watershed, perhaps due to intensive subsurface drainage system and concentrated fertilization. Furthermore, change of fertilization types and timing significantly affected the seasonal patterns of C-Q relationship. Lastly, the seasonality of C-Q relationship indicates that the late spring to early summer is the most active period for nitrate export in the watershed.