Rapid runoff via shallow throughflow and deeper preferential flow in a boreal catchment underlain by frozen silt (Alaska, USA)
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- Koch, J.C., Ewing, S.A., Striegl, R. et al. Hydrogeol J (2013) 21: 93. doi:10.1007/s10040-012-0934-3
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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 magnitude, 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 preferential 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.
KeywordsRainfall/runoffPermafrostPipeflowSolute transportAlaska (USA)
Ecoulement rapide par conduit superficiel et flux profond préférentiel dans un bassin versant boréal sous-jacent à un silt gelé (Alaska, USA)
Dans les bassins versants de haute latitude où le permafrost est présent, les dynamiques de lessivage et d’écoulement sont compliquées par le dégel saisonnier des couches productives, qui peut causer une modification des chenaux principaux si l’eau entre progressivement en contact avec des sols minéraux à conductivité hydraulique basse. Une étude de deux ans, menée sur un bassin versant de hautes terres en Alaska (USA) sous-jacent à silt éolien bien calibré gelé, a examiné les changements d’infiltration et d’écoulement avec la dégel. On a pris comme hypothèse qu’un écoulement rapide serait soutenu par un flux à travers des sols peu épais au cours du début de l’été et par un flux préférentiel profond plus tard durant l’été. Les variations saisonnières d’eau du sol, infiltration et intensité du ruissellement, localisation et chimie, suggèrent que le transport est rapide, même au maximum d’extension du dégel. Entre juin et septembre, l’emplacement de l’écoulement change, en rapport avec l’écoulement préférentiel de subsurface, dans les zones en pentes et humides. Des isotopes de l’uranium suggèrent que l’écoulement d’été tardif érode le permafrost, indiquant qu’un écoulement rapide substantiel peut savoir lieu le long de la limite gelée. Simultanément, l’écoulement superficiel et l’écoulement préférentiel profond peuvent limiter le bassin versant du plateau boréal et l’emmagasinement de soluté, et subséquemment le cycle biochimique aux échelles saisonnière à annuelle. L’écoulement profond préférentiel peut être important pour la coupure du flot, le développement du réseau de drainage et la restitution de carbone ancien à l’écosystème.
Escurrimiento rápido vía flujo horizontal somero y flujo preferencial más profundo en una cuenca boreal subyacente a sedimentos congelados (Alaska, EEUU)
En las cuencas de altas latitudes donde el permafrost está presente, la dinámica de lixiviación y escurrimiento se complican por el deshielo estacional de la capa activa, que puede causar un cambio en las trayectorias dominantes del flujo de agua cada vez más en contacto con suelos minerales de baja conductividad hidráulica. Un estudio de dos años de duración, realizado en una cuenca alta de Alaska (EEUU) sustentada por limos eólicos congelados, bien ordenados, examinó los cambios en la infiltración y el escurrimiento con el deshielo. La hipótesis fue que el escurrimiento rápido podría ser mantenido por el flujo a través de suelos someros a principios del verano y por el flujo preferencial más profundo después del verano. Los cambios estacionales en la humedad del suelo, infiltración, la magnitud del escurrimiento, la ubicación y la química sugieren que el transporte es rápido, incluso cuando los suelos están descongelados en su máxima extensión. Entre junio y septiembre se produjo un cambio en la ubicación del escurrimiento, consistente con el flujo subsuperficial preferencial de zonas escarpadas y húmedas. Los isótopos de uranio sugieren que a finales del verano el escurrimiento erosiona el permafrost, lo que indica que un sustancial flujo rápido puede ocurrir a lo largo del límite congelado. Conjuntamente, el flujo horizontal somero y el flujo profundo preferencial pueden limitar el agua de la cuenca boreal alta y el almacenamiento de solutos, y posteriormente el ciclo biogeoquímico en escalas de tiempo estacionales a anuales. El flujo profundo preferencial puede ser importante para la incisión corriente, para el desarrollo de la red de drenaje, y para la liberación de carbono antiguo a los ecosistemas
Escoamento rápido via escoamento subsuperficial e fluxo preferencial profundo numa bacia boreal em siltes congelados (Alasca, EUA)
Nas bacias localizadas a altas latitudes onde ocorre permafrost, a dinâmica de lixiviação e de escoamento é dificultada pelo degelo sazonal da camada ativa, o qual pode causar uma mudança nos sentidos de fluxo dominantes enquanto a água aumenta o contacto com solos minerais de baixa condutividade hidráulica. Um estudo de dois anos realizado na cabeceira de uma bacia hidrográfica no Alasca (EUA) coberta por material congelado, siltes eólicos bem calibrados, observou variações na infiltração e no escoamento durante o degelo. Admitiu-se a hipótese que o escoamento rápido seria mantido pelo fluxo através dos solos superficiais durante o princípio do verão e que o fluxo preferencial mais profundo ocorreria no final do verão. As variações sazonais na grandeza da humidade do solo, da infiltração e do escoamento, na localização e no quimismo sugerem que o transporte é rápido, mesmo quando os solos sofrem descongelação na sua máxima extensão. Entre junho e setembro ocorreu um deslocamento da posição do escoamento, consistente com fluxo subsuperficial preferencial em áreas declivosas e húmidas. Os isótopos de urânio sugerem que o escoamento do fim do verão erode o permafrost, indicando que este fluxo rápido substancial pode ocorrer ao longo da fronteira de congelação. O escoamento subsuperficial e o fluxo preferencial profundo podem em conjunto limitar o armazenamento de água e de solutos nas bacias superiores em zonas boreais e, subsequentemente, o ciclo biogeoquímico às escalas sazonal a anual. O fluxo preferencial profundo pode ser importante para a incisão das linhas de água, o desenvolvimento da rede de drenagem e a libertação do carbono antigo para os ecossistemas.
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 environments (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 greenhouse gas production and subsequently greater inorganic carbon release to surface waters (Striegl et al. 2005). While deep soils may support carbon storage and mineralization, much of the transport and biogeochemistry in upland boreal systems occurs in shallow organic soil horizons because of substantially higher hydraulic conductivity in the overlying peaty material (Hinzman et al. 1991). The hydraulic conductivity of the organic soil horizons decreases with depth, leading to higher runoff potential in the shallowest horizons (Quinton and Marsh 1999). An abrupt change from organic to mineral soil promotes lateral flow at this boundary, and may preclude significant infiltration into the deeper mineral soil (Carey and Woo 2001; Hinzman et al. 1991; Quinton and Marsh 1999).
Preferential flow may provide an important mechanism of water and solute transport through deeper soils where hydraulic conductivity is otherwise limited. Soil pipes are an example of a preferential flow mechanism known to be important in many high-latitude catchments. Soil pipes are large conical voids aligned parallel to the slope in the subsurface that allow flow to bypass the soil matrix (Beven and Germann 1982; Jones 2010). Soil pipes exist in both temperate and polar environments, and may originate in macropores caused by animal burrows and/or root decay following tree death or fire. Pipes are formed when macropores erode due to positive pore pressures that entrain or erode mineral grains (Carey and Woo 2000). In cold regions, pipes occur predominantly on steeper slopes (Quinton and Marsh 1998), when soils are saturated (Carey and Woo 2002; Dingman 1971), upon thaw of ice-rich sediments, and often at the organic/mineral soil boundary (Carey and Woo 2002). Soil pipes may explain rapid hyporheic transport beneath Antarctic streams (Cozzetto 2009; Koch et al. 2011).
An integrated view of catchment runoff processes can be gleaned using multiple physical and chemical methods. Seasonal changes in the ratio of precipitation to discharge and the time scale over which flood hydrographs recess may provide evidence of soil thaw and development of soil pipes upon catchment drainage. Previously, soil thaw has been invoked to explain decreased runoff ratios (Wang et al. 2009) and longer hydrograph recessions (Jones and Rinehart 2010; Lyon et al. 2009). Changes in the magnitude and location of inflows can be observed using stream tracer dilution (Kilpatrick and Cobb 1985), and may provide evidence of shifts in the dominant runoff processes. Tracers can also be used to calculate inflow chemistry, which provides information on the interactions between water and catchment soils.
Under favorable circumstances, uranium (U) concentrations and isotope ratios can be a useful tool for constraining the dominant runoff mechanism through mineral soils, because U activity ratios (UARs, 234U/238U) in water may be used to indicate water/sediment contact time over timescales greater than 1,000 years (Kigoshi 1971). Variation in aqueous UARs results from alpha decay of 238U, which may increase 234U/238U activity ratios (UARs) in the surrounding porewater by direct ejection of the daughter product (DePaolo et al. 2006) or increased leaching of the daughter at mineral surfaces due to recoil damage (Andersen et al. 2009; Fleischer 1980). Uranium isotope analysis is particularly well suited for studies in the silt-loess of interior Alaska, because activity ratios are dependent on mineral surface area and hence sediment size and shape, which are predictable and relatively consistent in the silt loess deposits of Alaska (Ewing et al. 2010; Shur et al. 2010). Recent work has used UARs in large arctic rivers to detect hydrologic variation in sub- and supra- permafrost water sources (Kraemer and Brabets 2012). Permafrost U concentrations vary greatly, likely related to dissolution variability that depends on permafrost water content and organic matter concentrations. UARs generally increase with depth in permafrost (Ewing et al. 2010; S. Ewing, Montana State University, personal communication, 2012).
It was hypothesized that in catchments underlain by frozen silt-loess, the dominant mechanism of runoff shifts from shallow throughflow to deeper preferential flow as the active layer deepens seasonally. Soil-moisture data and infiltration modeling results are used to consider mineral-soil-matrix flow and transport. Runoff ratio and stream-hydrograph analysis are used to quantify the seasonal trends in runoff. By quantifying stream inflows and chemistry, temporal and spatial patterns in inflow magnitude and U signatures are identified. The present study improves our understanding of the catchment processes that contribute to the U signature observed at the integrated, river scale. These results provide improved understanding of silt-upland-catchment hydrology relevant to predicting carbon transport and reactivity and landscape evolution as upland catchments in the Arctic and sub-Arctic evolve in response to climate warming.
Vegetation at this site is described by O’Donnell et al. 2011 according to the Alaska Vegetation Classification System (Viereck et al. 1992). In the recently burned black spruce stands, vegetation is dominated by standing dead Picea mariana, and living Vaccinium vitis-idaea, V. uliginosum, Ledum groenlandicum, and Equisetum spp. Burned organic soil surfaces are colonized by Ceratodon purpureus. Mature black spruce stands still exist in some unburned areas immediately adjacent to the stream channel. Feather mosses (Pleurozium schreberi and Hylocomium splendens), sphagnum (Sphagnum fuscum), and reindeer lichens (Cladonia stellaris and C. arbuscula) dominate ground cover.
Throughout the catchment, surface organic soil horizons are approximately 10 cm thick (varying up to 25 cm in swales), below which the soils are homogeneous silt-loess. These soils are seldom saturated, which is common in boreal soils due to high evapotranspiration potential (Bolton et al. 2004), related to low relative humidity and high productivity in the organic soils and mosses. Organics grade from live vegetation at the surface to decaying plant material and thicker peat at the organic-mineral boundary. Soils are commonly frozen to the surface in May, and thaw to approximately 70 cm by early September. Below the active layer is ice-rich wedge ice (yedoma) that is well documented in this area and extends to depths of 26 m or more due to formation with syngenetic permafrost (French and Shur 2010; Kanevskiy et al. 2012). Soil pits were dug in multiple locations on the hilltop, toe slope, and riparian area of the north-facing catchment and in the abandoned floodplain of the south-facing catchment during the summers of 2007 through 2009. During wet periods, north-facing catchment soil pits often filled with water flowing at the organic/mineral soil boundary, or from discrete mineral-pit-wall failures and piping of water into the hole. Soil pipe outlets were observed at the base of the steepest hillslope (just upstream of tributary T4 in Fig. 1), and were characterized by artesian conditions, and upwelling of silty water from discrete holes.
Several tributaries flow into the stream from the north-facing slope. Two inflows (T4 and T6) persisted throughout the summer, and have eroded the organic soils and flow on top of the mineral soil. Three smaller tributaries (T1, T2, and T5) have greater contact with organic soils and were observed flowing only after a large storm period beginning June 27, 2008. Dissolved organic matter concentrations are high in pore and stream water relative to many streams, a common occurrence in boreal systems related to the substantial organic matter cover. Stream and soil waters are generally oxic, consistent with shallow flow above the permafrost. There is evidence of small landslides and slumps along steeper slopes at the toe of the hillside near T3 and T4. Soil pits in these areas display a cohesionless thixotropic silt, that easily collapses upon disturbance. Tracer addition and synoptic sampling results indicate that surface and subsurface inflows are well mixed in the middle of the summer, but that at the end of the season as much as 22 % of the inflowing water may contain a significantly elevated solute load, indicating substantial inflow subject to mineral soil contact (unpublished data).
Several features indicate that inflows to the stream are dominated by drainage of the steeper north-facing hillslopes, and that the floodplain to the north does not contribute a significant amount of water. Dry soils, vegetation changes, and thermokarst features on the northern bank suggest that the floodplain was ‘abandoned’, related to decreased drainage and hydrologic connectivity with the stream following the 2003 fire. One small tributary, T3, flows regularly from the north side. This small tributary flows through a large thermokarst feature, which is characterized by reworked silts. Early in the season, the stream remains on the surface, while later in the summer this tributary is a disappearing stream, flowing intermittently in the surface channel and along subsurface pathways. Given that the watershed is dominated by the north-facing hillslopes and because the south-facing floodplain is very dry, it is assumed that the contribution of the south-facing catchment inflows to Richardson Tributary streamflow is minimal. T3 does, however display elevated solute and U concentrations that may be useful to interpreting U signatures from elsewhere in the catchment.
Precipitation was measured at the Hess Creek Gage, operated by the USGS and located several kilometers from the catchment. Stream stage was monitored at two locations in Richardson Tributary and in three persistent inflows (T3, T4, and T6) during the summers of 2008 and 2009 using pressure transducers placed in stilling wells in or above flumes. Pressure was logged every 15 min, corrected for barometric pressure, and field calibrated by independently reading flume staff plates during site visits. Discharge was calculated using flume rating curves and pressure logs. Discharge often exceeded 67.4 L s−1, which is the maximum flume capacity. During these high flows, discharge was modeled using Manning’s equation and verified with wading rod and pygmy measurements
Depth (cm) of soil-moisture sensors at four locations delineated in Fig. 1
Precipitation and discharge
Soil moisture and hydraulic conductivity
Green Ampt infiltration modeling results for major storms
Discharge recession coefficients
Uranium concentrations and activity ratios
Discharge, U concentrations and activity ratios (UAR) from the late-season synoptic sampling of the stream and tributaries. Numbers following stream sample types indicate samples taken at reach endpoints. Diffuse inflow values are calculated based on the stream endpoint and tributary discharge and chemistry. Dashes indicate locations where tracer dilution provided unrealistic discharge values, likely because of incomplete mixing in the stream
Our results support the hypothesis that flowpaths through boreal catchments underlain by frozen silt evolve seasonally, with increasing potential for rapid subsurface flow later in the summer. Despite substantial active layer thawing, water continued to reach the stream quickly (Figs. 5 and 6), implicating preferential flow and not porous media flow through the silt as the primary transport mechanism.
Seasonal variability in runoff
The seasonal shift in the location and magnitude of diffuse inflows indicates a shift in the primary runoff mechanism, from rapid flow through shallow soils in the early season to deeper, preferential flow later in the season. Similar inflow variations in a temperate catchment in Montana have been attributed to a switch from topographic controls during wet periods to geologic controls in drier times (Jencso et al. 2009; Payn et al. 2009). Similarly, inflow magnitude in the Richardson Catchment correlates to drainage area during the wet June sampling and proximity to tributaries in the dry September sampling (Fig. 8). Focused late-season upwelling of deep, sub-permafrost groundwater could explain this trend, but is considered unlikely, given evidence that permafrost acts as an aquitard between supra- and sub-permafrost flow. This lack of deep, sub-permafrost groundwater inflow is supported by (1) the inability of the stream to maintain baseflow without constant precipitation inputs, (2) aerobic conditions and relatively dilute solute chemistry in all catchment waters, and (3) the presence of shallow subsurface ice in every soil pit and frost probe measurement. Supra-permafrost preferential flow is therefore the likely explanation for this late-season flow.
Potential mechanisms of preferential flow
Tracer dilution gaging provides evidence that subsurface preferential flow in the Richardson Catchment occurs in conditions similar to those that promote the shallow pipe flow described by Carey and Woo 2000. Theoretically, preferential flow is likely to occur when positive pore pressures are greater than lithostatic pressures, allowing erosion of silt grains and rapid flow of water through voids (Carey and Woo 2000). This may occur in the high inflow regions of this catchment for several reasons: (1) convergent topography may maintain positive pore pressures, allowing extensive preferential flow networks to develop as suggested by the correlation between diffuse inflow magnitude and proximity to the surface tributaries (Fig. 8); (2) the streams themselves may be important, providing erodible material via ice-rich saturated margins, as well as positive pressures and/or heat from hyporheic exchange to further promote incision and erosion of these margins; (3) steep slopes near T1, T2, and T4 may develop preferential flow networks by promoting high pore pressures and high velocities, similar to observations in previous studies (Quinton and Marsh 1998) and as suggested by the sharp increase in diffuse inflows near the steep, 30° slopes at 800-m stream distance (in Fig. 7b).
While pipes have been previously observed at the organic/mineral boundary, preferential flow in this catchment was found deeper in the mineral soils. Shallow pipes likely form at the organic/mineral soil boundary because of the relative impermeability of the underlying clayey mineral soils (Carey and Woo 2000). It is possible that a similar situation exists at depth, and that preferential flowpaths may be forming at the boundary between the thawed mineral soils and the impermeable ice. Fire is known to increase seasonal active layer depths (O’Donnell et al. 2011), and the recent fire in this catchment may have allowed subsurface water to begin accessing and eroding the transition zone, which is a layer of structurally weak, ice-rich soils at the boundary between the seasonal active layer and underlying permafrost (Bockheim and Hinkel 2005; Shur et al. 2005). Fire may also have created the original macropores (i.e. burned roots) allowing water to move vertically through the mineral soils that would otherwise exhibit low hydraulic conductivities and infiltration potential.
Chemical evolution of runoff and a link to erosion
Trends and variability in U concentrations and activity ratios between the different water sources in Richardson Catchment provide evidence that diffuse runoff and tributary water is in contact with thawing subsurface permafrost before it reaches the stream. Given the lack of connectivity with sub-permafrost groundwaters, the most likely explanation of high and increasing U concentrations and activity ratios is mixing between a low, precipitation/runoff endmember (similar to S3 porewater) and waters with elevated UAR reflecting progressive thaw or cryoturbation along pathways defined by thawing ice-wedge networks. Ice wedges were directly observed in nearby cores (Kanevskiy et al. 2012; O’Donnell et al. 2011) and ongoing ice-wedge thaw is implied by extensive hummocky topography on surrounding hillslopes. While cores indicate contact between water and silt well below the permafrost boundary (5–15 m), processes such as hillslope slumping, cryoturbation, or stream incision may have decreased the overburden, allowing active layer water to contact these soils. Mixing between runoff and permafrost with high UARs could occur due to stream incision and diffuse runoff traveling through preferential pathways at the base of the active layer and along frozen boundaries. Elevated UARs at these boundaries could be maintained by cryoturbation, which inmixes high UAR silts from permafrost zones not previously exposed to thaw. Thus, preferential flowpaths may be modified annually, rather than runoff flowing along the same contact surfaces each year. Constant high UAR runoff may also indicate continual thermo-mechanical erosion by flowing water derived from precipitation. This erosion is likely the explanation for increasing UARs along the tributary flowpaths. Such incision is evident on the landscape and likely related to geomorphologic reorganization as the hillslopes slump and tributaries downcut towards the mainstem stream channel, which may itself be actively incising.
The data suggest variable U concentrations in the high UAR permafrost. High UARs are consistent with increasing UAR (and age) with depth in permafrost cores up to 20 m deep from this area (Ewing et al. 2010; S. Ewing, Montana State University, personal communication, 2011). Uranium variability may be related to surface and subsurface runoff contacting permafrost with depth-variable properties (i.e. water content, organic matter concentrations, etc.). Tributaries likely interact with the top of the permafrost low on the slope and very near the stream, coincident with the greatest downcutting, whereas diffuse inflows are likely contacting the top of the permafrost on the steeper sections of the catchment, which tend to be on the toe slopes and further from the stream. Different mixing lines between precipitation and high UAR diffuse inflows (reaches 2 and 5 in Fig. 10b) may indicate differences in the amount and duration of water/permafrost contact, which may be related to differences in topography in these reaches. For example, the higher UAR and lower U concentrations in reach 5 likely indicate rapid flow related to the steep slopes adjacent to the stream. Some inflows appear to have little contact with high U water, as evidenced by the decrease in U concentrations and UARs associated with large diffuse inflows near the bottom of the stream reach (near T6).
Evidence of preferential flow through silts
Discharge recession coefficients provide further evidence for preferential flow in the Richardson Catchment. Discharge recession coefficients are inversely correlated to date (Fig. 6), indicating slower drainage of the catchment later in the season, similar to what has been witnessed in other catchments with increasing thaw depth (Jones and Rinehart 2010; Lyon et al. 2009; McNamara et al. 1997; Wang et al. 2009). However, because calculated runoff responses are more rapid than can be reasonably expected from matrix flow through a silt loess, preferential flow is a likely mechanism explaining trends in runoff. A trend of seasonally increasing specific conductance in the surface waters of this catchment provides further evidence that late-summer runoff is in contact with the mineral soils.
Evidence for shallow storage and minimal matrix flow through the silt-loess soils
Soil-moisture trends (Fig. 3) and Green-Ampt modeling (Table 2) indicate that matrix flow through silt soils is not a significant pathway of hillslope runoff due to low hydraulic conductivity inhibiting both infiltration and subsurface matrix flow. Soil moisture varies minimally at depth and does not respond to storms (Fig. 3), indicating that a substantial portion of the storm water does not infiltrate to greater than approximately 25 cm. Minimal infiltration precludes significant flow in deeper soils due to the constantly low hydraulic conductivities (0.04–0.4 cm hr−1). Shallow mineral soils approach saturation in the early summer and following storms, but even at saturation, these soils exhibit low hydraulic conductivities (approximately 1.65 cm hr−1). These data indicate that regardless of thaw depth in the silt soil, the hydraulic conductivity is too low to explain observed short runoff lag times (Fig. 5a) and recession curves (Fig. 6). In fact, given that shallower soils are often drier than the deep soils, calculations using silt soil properties from Carsel and Parrish 1988 suggest that evapotranspiration creates an upward hydraulic head gradient that indicates movement of water towards the surface rather than down the hillslope.
Infiltration into the mineral soil matrix may significantly decrease and retard catchment runoff. Green-Ampt modeling indicates that the greatest infiltration occurs in dry soils (Fig. 4a), and runoff coefficients confirm that less water runs off dry soils (Fig. 4b). In the early and late summer, wet soils preclude significant subsurface storage resulting in high runoff ratios, while in the dry mid-season, soil storage potential increases (Fig. 5b).
These results present a view of boreal catchment runoff that focuses on the seasonal shift from shallow, topographically controlled runoff to deeper flow through preferential flowpaths that form in moist areas and on steep slopes. Given that late-season preferential flow is accessing and thawing the top of the permafrost, and that permafrost contains high concentration of ancient carbon, runoff may be delivering ancient carbon to ecosystems. Catchments underlain by frozen silt may be particularly important to climate change in the coming decades, given that silts are highly erodible, display significant thaw settlement because of their ice rich nature (Kanevskiy et al. 2012), contain vast stores of frozen carbon, and in interior Alaska are undergoing geomorphic reorganization related to an accelerating fire cycle and permafrost thaw.
The data from this study provide compelling evidence that drainage of silt-dominated permafrost-bound catchments of interior Alaska shifts from near-surface topographically controlled throughflow to deeper pipeflow later in the season. Moisture content and infiltration modeling indicate that silts are typically unsaturated, and in the absence of preferential flow, are incapable of rapidly conducting water. Seasonal trends in runoff coefficients and flood recessions indicate greater potential for storage, and decreased runoff velocities later in the summer. The change in location of runoff indicates that late summer flow is associated with steeper and wetter near-stream slopes, and is likely related to soil pipes that have been observed in these locations. Uranium isotopes indicate that preferential flowpaths are renewed seasonally, achieving rapid transport of water and solutes and thawing the top of the permafrost. The data and observations highlight steep slopes, the near-stream environment, and recently exposed silts along thawing margins as likely locations of preferential flow and erosion. The near-stream environment has been recognized as an important location of biogeochemical activity, and this work provides evidence that in high latitude regions, such areas may be of significance for catchment geomorphic evolution via erosion of permafrost as well.
This work was supported by a National Science Foundation grant, OCE-BE 0628348 to Q. Zhuang. We thank P. Schuster and C. Hart for laboratory analysis of the bromide tracer, and J. Paces for assisting with the U isotope analysis. B. Rajagopalan and K. Bencala provided invaluable discussions on the data and results. Field support was provided by K. Wickland, R. Runkel, K. Kelsey, M. Bourret, D. Halm, M. Dornblaser, G. Aiken, and R. Spencer. The manuscript was significantly improved through reviews by B. Ebel and two anonymous reviewers. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily represent the view of the National Science Foundation or US Geological Survey. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.