Long-Term, Annual, and Seasonal Solute Export Patterns in Two Snow-Covered, Forested Headwater Experimental Watersheds in Northern U.S.

Abstract:

Concentration-discharge relationships (C-Q) or the responses in stream water chemistry to changes in stream discharge (Q) are often described through a power-law relation (C=aQ^b) and can reveal much on biogeochemical and hydrologic processes at a catchment scale. This study used multi-year, annual, and seasonal C-Q patterns coupled with end-member-mixing analysis (EMMA) to understand solute export processes using 20 solutes at W-9 of Sleepers River Research Watershed, VT and 8 solutes at S-2 of Marcell Experimental Forest, MN from 2008-2016. Over a multi-year timescale, some solutes exhibited a positive relationship with Q (W-9: NO_3-, DOC, Fe³⁺; S-2: SO₄²-) while others displayed a negative relationship (W-9: SC, Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄²-, Ba, Li, Mn²⁺, Si, As, Sr²⁺; S-2: Ca²⁺, Mg²⁺, Si, Sr²⁺). Strong chemodynamic behaviors often occur for solutes with larger differences in concentrations in end-members and greater vertical variability of solute concentrations with soil depth. Fewer solutes were chemostatic (W-9: Cl^-, NH₄⁺, DON, TDN, Al³⁺; S-2: Na⁺, Cl^-, K⁺) where concentrations were not controlled by change in Q (-.1 < b <.1). For some solutes, the b-coefficient of the multi-year C-Q relationships switched from chemostatic to chemodynamic behavior (W-9: Cl^-, NH₄⁺, DON; S-2: Na⁺, Cl^-; p <0.05) or switched signs during the annual falling and/or rising limbs of snowmelt (W-9: Mn²⁺; S-2: Na⁺; p <0.05), likely due to seasonal changes in hydrologic pathways and/or shifts in vertical distribution of solute concentrations. Additionally, b-coefficients were often larger during the rising limb of snowmelt than the falling limb (W-9: SC, Ca²⁺, Na⁺, K⁺, Cl^-, SO₄²-, Ba, S, Sr²⁺), indicating solutes were source limited during the falling limb of snowmelt. Other solutes showed the opposite where b-coefficients during the snowmelt rising limb were often smaller than on the falling limb (W-9: NO_3-, DOC, Mn²⁺, Al³⁺, Fe³⁺; S-2: Ca²⁺, Mg²⁺, Na⁺, Cl^-, K⁺, SO₄²-, Si, Sr²⁺). Marcell b-coefficients for the same solutes were frequently smaller on the rising limb than the falling limb of snowmelt, indicating more dilution during the beginning of snowmelt. The variability of C-Q coefficients over sites, years and seasons indicate that the export patterns are controlled by both climate and catchment properties.