Multiscale differences of concentration-discharge relationships: implications for process inference and watershed management

Concentration (c) -discharge (Q) analyses have enabled inference of catchment scale processes that combine aspects of streamflow generation, solute source areas, flowpaths, and biogeochemical processing. Here we examine composite nitrate c- Q patterns at Mahantango Creek and Conewago Creek, Long-Term Agroecosystem Research (LTAR) research sites, to determine if patterns are consistent across watershed size and through time. This data appear to differ from what has been shown in global meta- analyses in other agricultural watersheds. Multi-decadal grab sampling at Mahantango Creek's WE-38 subwatershed, conducted three times per week, reveal a clear enrichment pattern over a 36-year record (1983-2019). The long-term composite data show a decrease in peak concentrations over recent decades and that slopes across all time periods are impacted by both low and high flow conditions in wetter vs. drier years. We also analyze dissolved inorganic nitrogen c-Q patterns at Conewago Creek, a larger mostly agricultural watershed. Conewago Creek has two gaging stations that been less intensively sampled between (2012-2021), and temporal trends in c-Q patterns diverge between upper and lower stations. This implies other controls beyond hydroclimatic variability on the temporal dynamics of c-Q relationships, including watershed management effects. Our results have implications for process level inference and future water quality sampling design.