Rapid runoff via shallow throughflow and deeper preferential flow in a boreal catchment underlain by frozen silt (Alaska, USA) J. C. Koch & S. A. Ewing & R. Striegl & D. M. McKnight Abstract In high-latitude catchments where permafrost is present, runoff dynamics are complicated by seasonal active-layer thaw, which may cause a change in the dominant flowpaths as water increasingly contacts mineral soils of low hydraulic conductivity. A 2-year study, conducted in an upland catchment in Alaska (USA) underlain by frozen, well-sorted eolian silt, examined changes in infiltration and runoff with thaw. It was hypothesized that rapid runoff would be maintained by flow through shallow soils during the early summer and deeper preferential flow later in the summer. Seasonal changes in soil moisture, infiltration, and runoff magni- tude, location, and chemistry suggest that transport is rapid, even when soils are thawed to their maximum extent. Between June and September, a shift occurred in the location of runoff, consistent with subsurface prefer- ential flow in steep and wet areas. Uranium isotopes suggest that late summer runoff erodes permafrost, indicating that substantial rapid flow may occur along the frozen boundary. Together, throughflow and deep preferential flow may limit upland boreal catchment water and solute storage, and subsequently biogeochemical cycling on seasonal to annual timescales. Deep preferential flow may be important for stream incision, network drainage development, and the release of ancient carbon to ecosystems. Keywords Rainfall/runoff . Permafrost . Pipeflow . Solute transport . Alaska (USA) Introduction Northern hemisphere boreal soils contain as much as one third of the world’ s carbon (Dixon et al. 1994; Schuur et al. 2008). Much of this carbon is stored in frozen soils and is thus unavailable to ecosystems, while the remainder exists in the vegetation and shallow organic soils. Many studies have considered carbon storage and transport in boreal ecosystems, invoking deeper thaw as the cause of trends in stream chemistry (Petrone et al. 2007; Prokushkin et al. 2005; Striegl et al. 2005; Walvoord and Striegl 2007). While these studies seem to implicate a deepening active layer or thawing permafrost as the cause of biogeochemical trends, few have specifically examined the hydrologic flowpaths that move water and solutes from catchments into streams. Physically based investigations are critical in order to predict how the disproportionately large effect of climate warming on high-latitude catchments will affect water and heat flow, and subsequently catchment geomorphology and carbon cycling. High latitude and high elevation catchments commonly contain frozen soils with a shallow, unfrozen “active” layer that expands downward as the summer progresses. Shallow thaw depths in the early summer promote flooding and flashy hydrographs (Roulet and Woo 1986). Subsurface storage capacity and deeper flowpaths may develop during the summer due to increased thawing of soils (Prokushkin et al. 2005) and near-stream environ- ments (Brosten et al. 2006; Zarnetske et al. 2007). This seasonal deepening of the active layer may promote storage and subsequent mineralization of carbon that is leached into the deeper layers from the upper organic horizons (Ågren et al. 2007; Carey 2003; Petrone et al. 2006), thereby leading to a positive feedback of green- house gas production and subsequently greater inorganic carbon release to surface waters (Striegl et al. 2005). While deep soils may support carbon storage and Received: 17 April 2012 /Accepted: 11 November 2012 Published online: 1 December 2012 * Springer-Verlag Berlin Heidelberg (outside the USA) 2012 Published in the theme issue “Hydrogeology of Cold Regions” J. C. Koch ()) US Geological Survey, Alaska Science Center, 4210 University Dr. , Anchorage, AK 99508, USA e-mail: jkoch@usgs.gov Tel.: +1-907-7867119 S. A. Ewing Department of Land Resources and Environmental Sciences, Montana State University, P. O. Box 173120, Bozeman, MT 59717-3120, USA R. Striegl US Geological Survey, Branch of Regional Research, 3215 Marine St, Boulder, CO 80303, USA D. M. McKnight Department of Civil, Environmental, and Architectural Engineering, University of Colorado, P.O. Box 450, Boulder, CO 80303, USA Hydrogeology Journal (2013) 21: 93–106 DOI 10.1007/s10040-012-0934-3