Modeling Hydrological Flow Paths During Snowmelt Induced High Flow Event in a Small Agricultural Catchment

Abstract

End-Member Mixing Analysis (EMMA) was used to identify flow paths and source areas controlling river chemistry during a snow melt induced spring high flow event in an agricultural catchment (187 ha) in NE Poland. EMMA using Ca²⁺, Mg²⁺, Cl⁻, and H⁺ showed that stream chemistry could be explained as a three-component mixture of overland flow, shallow groundwater and soil solution from arable soils along the stream margin and deeper groundwater high in base cations and Si. The temporal variability in the flow pathways and solute sources during snowmelt were controlled by soil frost. From the very early beginning of snowmelt overland and rill flows were the main mechanism of runoff generation because of the low permeability of the frozen ground. Solutes transported along with overland flow had the most pronounced impact on river chemistry during peak discharges, when this runoff component contributed up to 70% of stream discharge. High surface runoff contributions produced a pronounced rise and maximum in streamflow PO₄ ³⁻. We found that during investigated snowmelt event only a small percentage of the landscape might be a source of subsurface flow. They were primarily saturated areas well-connected to the drainage network: geomorphic hollows and sideslope benches along the stream margins. Shallow groundwater and soil water discharged from those locations were responsible for pronounced (>60%) export of nitrates. The high concentration of solutes (primarily NO₃ ^–) in the river outflow suggests that during snowmelt, either fluxes of agricultural contaminants bypassed potential buffers, which could constrain their impact on freshwater ecosystems, or that existing buffers were ineffective in removing the contaminants that moved along shallow hydrological pathways. Thus, the short period of snowmelt flood may be perceived as critical from the river water quality perspective.