The role of catchment scale for determining hydrological flow paths during spring flood

Abstract: The purpose of this study was to explore the pathways of melt water during the spring flood. Snow melt is a complex process of great importance for boreal ecosystems, and there are major challenges remaining to fully understand the stream flow generation. The study was carried out at Vindeln Experimental Forests, Vindeln in northern Sweden. The hydrological pathways were traced by measuring the δ18O-signals for event water and stream water during the spring flood and the pre-event water δ18O-signal before the snow melt. These δ18O-signals differ because of isotope fractionation, allowing for the separation of different water sources. Event water is defined as melt and rain water entering the catchment during the snow melt episode while pre-event water is water that was present in the catchment prior to snow melt. The separation of the stream water into event water and pre-event water fractions was done using isotopic hydrograph separation. The focus of this study was on comparing the importance of wetlands and scale for the portion of event and pre-event water using 15 streams ranging in catchment size between 3 and 6784 ha with extra focus on small catchments. Small catchments with a substantial wetland part had a bigger event water fraction in the streams and a quicker and greater response to snow melt than small forest dominated catchments. An explanation could be extensive and homogeneous soil frost in wetland areas that caused surface flow paths where less water was lost due to soil infiltration as compared to forest dominated catchments. The stream water during spring flood was dominated by pre-event water. This could be explained by event water entering the soil that increased the ground water flow that consisted of pre-event water.