ELUCIDATING THE IMPACT OF LAND COVER AND CLIMATE CHANGE ON CONCENTRATION-DISCHARGE RELATIONSHIPS IN EPIKARST WATERSHEDS

Accurately predicting how stream water chemistry will vary in response to changes in land cover and climate is needed to insure the future health of ecosystems and quality of drinking water for human consumption. High frequency stream monitoring of solute concentrations (C) over varying discharge (Q) conditions can provide a biogeochemical fingerprint to evaluate the hydrologic connectivity of a watershed and its response to both climate and land cover change. Here, we explore the relationship between land cover change and stream water C-Q behavior at the Konza Prairie (KS), a remnant of the once extensive tallgrass prairie that spanned the central US. The lithology of Konza is composed of repeating couplets of 1‑2 m thick limestone and followed by slightly thicker (2-4 m) mudstone units.

We postulate that the biogeochemical behavior of streams at Konza are responding to two observed phenomena: 1) watersheds with fire frequency return intervals of ~ 4 years or greater have incurred a >20% increase in woody vegetation (1981-2001), and 2) groundwater CO₂ concentrations have increased 20% compared to the 7% increase observed in the atmosphere (1991-2012). To investigate the controls of land cover on C-Q behavior we collected stream water samples three times per week from the main outlet (Kings Creek) and four primary watersheds within the catchment that are managed with 1, 2, 4 and 20 year fire return intervals from Spring 2015 through present. Samples were analyzed for weathering solutes (Ca, Mg, Na, Si, Sr, and K). To evaluate the influence of climatic changes on C-Q we compared current responses to historic datasets (USGS and LTER). Preliminary results indicate Ca, Mg, Na, and Sr mainly exhibited dilution behavior (decrease in C with increased Q) while Si and K demonstrated chemostatic to addition behavior (constant or increased C with increased Q). Cation exchange sites on soils at Konza are dominated primarily by Ca then Mg, K and finally Na near the surface while Na and K change dominance at depth. Inputs of dust and the cycling of nutrients related to phytolith production (both rich in Si and K) likely control the dominance of K on exchange sites at shallow soil depths. We hypothesize that the C-Q behavior of Si and K can result from a predominant input of shallow interflow to the stream, especially during high discharge events.