In many mountainous headwater catchments snow and soil moisture are key components of the hydrologic cycle, providing valuable information pertaining to the dynamic processes that occur during spring runoff. This has justified large data collection efforts to further understand the distribution of snow and soil moisture across landscapes during the winter and spring seasons (Elder et al., 2009). During spring, much snowmelt will infiltrate into the soil with a noticeable change in soil moisture prior to recharging groundwater storage, producing streamflow, or contributing to evapotranspiration (Bales et al., 2011; Kampf et al., 2015; Webb et al., 2015). The relative saturation in the vadose zone controls the stream connectivity and release of water and nutrients from subsurface storage into stream systems (McNamara et al., 2005; M. W. Williams et al., 2009). Soil moisture during this time is driven by snowmelt that can impact the water availability for plant production (Molotch et al., 2009; Harpold et al., 2015) as well as the ionic signature of soil moisture and stream flow (Harrington and Bales, 1998). For these reasons the connections between snowmelt and soil moisture are critical in understanding the hydrologic cycle in snow-dominated headwater systems (Jencso et al., 2009), particularly in the face of a changing climate that will alter the snowmelt season and resulting hydrological dynamics (Adam et al., 2009; Clow, 2010; Clilverd et al., 2011; Harpold et al., 2012; Rasmussen et al., 2014; Fassnacht et al., 2016).

Processes within headwater catchments such as snow accumulation and persistence are known to vary at multiple scales of interest. From a basin-scale perspective, elevation has been shown to influence the depth and persistence of a snowpack (Richer et al., 2013; Molotch and Meromy, 2014; Sexstone and Fassnacht, 2014), while at finer resolutions the spatial variability of both accumulation and melt may be controlled by aspect (C. J. Williams et al., 2009; López-Moreno et al., 2013; Hinckley et al., 2014), and snow in forested areas is affected by interception during accumulation, shortwave radiation shading, and longwave radiation influences prior to and during melt (Storck et al., 2002; Musselman et al., 2008; Molotch et al., 2009; Adams et al., 2011; Webb, 2017). However, far less is known about the variability that snowmelt has on soil moisture and flow paths during snowmelt at the hillslope scale, in large part due to the difficulty of observing soil moisture beneath a deep snowpack at high spatial resolution. Snowmelt is important to soil moisture storage and resulting streamflow (McNamara et al., 2005; C. J. Williams et al., 2009; Bales et al., 2011; Hunsaker et al., 2012; Kormos et al., 2014). Stream connectivity to the surrounding landscape follows seasonal trends with the highest connectivity during spring snowmelt based on factors such as topography (McNamara et al., 2005; Jencso et al., 2009; Jencso and McGlynn, 2011). The aspect of a hillslope will additionally increase soil water storage and retention on north aspect slopes (Geroy et al., 2011) that can alter runoff processes and result in spatially variable soil moisture beneath a melting snowpack.

There have been recent advancements in the ability to observe soil moisture throughout the water year in capturing high-resolution data at both spatial and temporal scales (e.g., Bales et al., 2011). Similar advances have occurred for observing variables such as the liquid water content of a snowpack (Mitterer et al., 2011; Techel and Pielmeier, 2011; Koch et al., 2014; Heilig et al., 2015). This has allowed for further understanding of hydrological systems and dynamic processes that are vulnerable to climate change (Bales et al., 2006). However, observations of the relative saturation of soil beneath a snowpack has been limited to an array of discrete points with sufficient instrumentation, and few studies have investigated spring snowmelt soil moisture at a scale similar to that used to measure the snow above it. The few studies that have observed these process have shown microtopography to influence infiltration across the snow–soil interface (SSI; French and Binley, 2004) and that wetter areas tend to remain wetter, with slope and aspect being important factors at a low-elevation site (C. J. Williams et al., 2009). In high-elevation alpine environments topography and wind shielding influences soil moisture distribution, though there is less association with these parameters in low snow years (Litaor et al., 2008). These studies, limited to high-alpine and low rain–snow transition zones, suggest that topographic influences on soil moisture are strong but more investigations during varying snow accumulation, melt dynamics, and environments are important to connect the distribution of soil moisture across a landscape to runoff processes, particularly with variable regional and environmental snowpack responses to climate variability (Harpold et al., 2012).

The relative saturation of the vadose zone determines runoff processes during spring snowmelt (McNamara et al., 2005). Runoff processes have been shown to change during spring snowmelt compared to summer rain events (Eiriksson et al., 2013; Williams et al., 2015). During snowmelt, soil moisture is influenced mostly in the top 10 cm of soil (Blankinship et al., 2014) with pulses of water that reach further depths varying widely at both the hillslope and catchment scale (Webb et al., 2015). At the catchment scale a south aspect hillslope may display matrix flow during snowmelt as the north aspect displays evidence of preferential flow through the soil (Hinckley et al., 2014). Preferential flow paths have been shown to occur both in the soil beneath a snowpack (French and Binley, 2004) and above the ground surface within the snowpack (Marsh and Woo, 1985; Kattelmann and Dozier, 1999; Williams et al., 2000; Liu et al., 2004; Williams et al., 2010). Preferential flow within a snowpack can form as the result of ice lenses (Colbeck, 1979) or differences in grain size and density (Avanzi et al., 2016; Webb et al., 2018). Each of these can alter the flow of water through snow and resulting infiltration into the soil, from the centimeter scale (Williams et al., 2010) up to tens of meters (Eiriksson et al., 2013; Webb et al., 2018). Preferential flow paths within a snowpack will create spatially variable snowmelt patterns across a landscape depending on the variable metamorphism that occurs within the snowpack (Yamaguchi et al., 2010; Adams et al., 2011; Domine et al., 2013; Katsushima et al., 2013), which increases during melt (Marsh, 1987). These melt patterns have been shown to have correlation lengths of 5 to 7 m in relatively flat alpine terrain (Sommerfeld et al., 1994; Williams et al.,1999) and lesser correlation lengths of 2 to 4 m in subalpine terrain (Webb, 2017). Preferential flow paths within a snowpack will alter soil moisture and resulting runoff processes at the hillslope and catchment scales.

To our knowledge there have been no studies investigating snow and soil moisture interactions specifically to investigate hydrologic flow path development in a sub-alpine environment beneath a deep (2 m) seasonally persistent snowpack. The goal of this study is to gain further understanding through observations of flow path development in a snowmelt-dominated subalpine headwater catchment. Observations of near-surface soil volumetric water content (VWC) were compared to topographical parameters (e.g., slope, aspect) and hydrological variables (e.g., temperature, date of peak snow water equivalent). Statistical analysis of the topographical and hydrological controls on the spatiotemporal variability of snow and soil moisture during snowmelt was undertaken at a subalpine forested setting with north, south, and flat aspects as a seasonally persistent snowpack melts with the following objectives: (1) to investigate if evidence preferential flow paths in snow can be observed and the effect on soil moisture through measurements of snow water equivalent (SWE) and near-surface soil moisture, (2) to observe how SWE and near-surface soil moisture vary on hillslopes relative to the toes of hillslopes and flat areas, and (3) to compare snowmelt infiltration beyond the near-surface soil between flat and sloping terrain during the entire snowmelt season.

To understand flow path development during snowmelt and the resulting distribution of soil moisture, observations of SWE and near-surface soil moisture were correlated to test the influence of topography and snow on soil moisture using Pearson's correlation coefficient, r, and a level of significance determined at p values of 0.05 and 0.01. Near-surface soil volumetric water content (VWC) was compared to SWE at the same location on the date of observations, SWE on the first survey date (representative of peak SWE), the change in SWE between survey dates prior to measurement, near-surface VWC on the first survey date, and topographic slope, elevation, and northness as calculated from a 10 m digital elevation model (DEM; USGS, 2015). Northness is defined as the product of the cosine of aspect and the sine of slope (Molotch et al., 2005; Sexstone and Fassnacht, 2014).

The multiple years of observation at a subalpine location with a deep seasonally persistent snowpack offers analysis of SWE and near-surface VWC patterns that have previously been limited to lower elevations near the rain–snow transition zone (C. J. Williams et al., 2009) and higher elevations in an alpine environment (Litaor et al., 2008). In this study, the only topographic parameter that displayed any significance on the near-surface soil moisture at the 0.05 level was northness and this appeared to increase in significance and strength with time, indicating that it is likely more related to the presence or absence of snow and influences from rain (Table 2). However, infiltration of snowmelt beneath the near surface to 20 cm depth was influenced by slope with more infiltration wetting the soils at 20 cm depth on the flat aspect and lesser wetting at this depth on the north aspect in the early melt season (Fig. 6b) where observations of ice veins and saturated layers of snow were made at the SSI (Fig. 5). The soils on the south aspect slope are generally coarser than the north or flat aspects (Table 1), suggesting that soil water retention is higher on north aspects (Geroy et al., 2011). This was reflected with the north aspect soils often having similar and/or higher water contents than south aspect soils, both with and without the presence of a snowpack (Fig. 4, SM vs. NL and NH). As the snowpack melts the shallow subsurface VWC displays clear diurnal fluctuations (Fig. 6b). The locations that were wet relative to other locations during the first survey remained as such for all following surveys when snow persistently covered the study area, comparing well to the study by C. J. Williams et al. (2009) at a smaller scale and beneath a shallow snowpack near the rain–snow transition zone at a lower elevation. However, in contrast to C. J. Williams et al. (2009), near-surface VWC showed a negative correlation to the first-measured SWE (representative of peak) in 2013 and 2014. This negative correlation indicates that locations with lower peak SWE (during the first survey of the season) tend to have greater VWC at the later surveys. This is the result of the shallower snowpacks during the first survey being near the bottom of the slopes and in the flat terrain influenced by canopy interception (Fig. 1a), and the subsequent melt flows downslope at the SSI towards these locations increasing both SWE and near-surface VWC during the following surveys (Fig. 4). However, in 2015, a relatively low snow year, results agreed with C. J. Williams et al. (2009) of higher near-surface VWC at locations that accumulated more snow, indicating the amount of snowfall is also important to these processes. During low snow years, areas where snow persists longer will result in a longer influence on near-surface soil moisture. Near-surface VWC will additionally depend on variability in soil parameters such as soil water retention, with higher moisture retention from finer soil particles, similar to the north aspect slope, affecting the infiltration or lateral flow of meltwater at the SSI when on a slope.

Meltwater flowing downslope near the SSI on the north aspect hillslope is shown by the increases in SWE at locations on and at the toe of the hillslope (Fig. 4), the frozen ice veins observed in 2013 and 2014 early melt seasons (Fig. 5), less infiltration to 20 cm depth on the slope (Fig. 6), similarities in soil moisture between snowmelt and overland flow rain events, and the observations of snow density and wetness increasing with depth downslope in each north aspect snow pit. For the south aspect slope, the increases in SWE at the ST locations were similar to observed precipitation in 2013 and an increase in snow depth for 2014. The south aspect slope may have meltwater flowing downslope near the SSI, though to a lesser extent than the north-facing slope and less apparent. The movement of water across layer interfaces has been shown within a snowpack (Williams et al., 2000, 2010; Liu et al., 2004) and at the SSI (Eiriksson et al., 2013), with evidence of the latter being observed in this study (Fig. 5). This phenomenon will depend on soil parameters, snowpack layer characteristics, slope angle, and the rate that meltwater is percolating through the snowpack. These factors will determine if an interface acts as a permeability barrier, similar to a soil drain, or as a capillary barrier (Avanzi et al., 2016; Webb, 1997; Webb et al., 2018). The primary reasons for lateral flow through the snowpack on the north-facing slope is a result of the slower melt rates and hydraulic conductivity of the soil. When capillary barriers occur, the diversion length will be controlled by the hydraulic properties of the media, slope of the interface (steeper slope increases diversion length), and infiltration rate (slower infiltration rate increases diversion length; Webb, 1997; Webb et al., 2018). It is also possible for lateral flow to be caused by barriers within a layered snowpack well above the SSI, though the large saturated layer of snow was observed only at the SSI in all north aspect snow pits, showing this is where the bulk of the lateral flow occurs. Further testing and field experiments are necessary to quantify the influence of varying slope and soil parameters on these processes in and below a snowpack. Our study shows preferential flow paths during snowmelt on the north-facing slope that are similar to an alpine catchment with water flowing through the snowpack downslope (Liu et al., 2004; Williams et al., 2000), during rain on snow events at lower-elevation sites (Eiriksson et al., 2013), and observations in a coastal climate (Kattelmann and Dozier, 1999). These processes can be combined into a conceptualization of the northern aspect slope having meltwater flow paths near the SSI downslope and the southern aspect slope having more infiltration into the soil (Fig. 7).

As hydraulic barriers form and promote flow paths to develop within the snowpack such as on the north aspect slope, the timing of runoff at the hillslope scale can change dramatically. Snow has been shown to have a hydraulic conductivity orders of magnitude greater than common soils (Yamaguchi et al., 2010; Domine et al., 2013) and will thus be important for hydrologic modeling and flood prediction from snowmelt runoff. From a groundwater recharge perspective, much of the hydraulic gradients driving subsurface flow will be occurring at the base of the north aspect hillslope in this study area due to the lateral flow of water through the snowpack, and soil moisture sensors on the slope will only account for a fraction of the total meltwater as flow paths bypass sensor profiles. Also, at the base of the hillslope (NT) the snowpack can increase in bulk SWE by up to 250 mm (from 146 to 396 mm; Fig. 4aiii), displaying the increased storage capacity of a location by the porosity of the snow. This will result in areas of focused recharge and variable infiltration in the subsurface as observed in other subalpine regions (Webb et al., 2015). However, hillslopes can still display a more classical conceptualization of snowmelt infiltration uniformly and traveling across the soil–bedrock interface to recharge groundwater resources and generate streamflow as on the south aspect slope (Fig. 7).

In order to estimate the ratio of lateral flow to infiltration on the north aspect slope, an energy budget calculation was conducted. The energy budget was calculated assuming an isothermal snowpack on the date of peak SWE observed at the SNOTEL station and utilizing the SNOTEL-measured air temperature, relative humidity, wind speed, precipitation, and shortwave radiation. Fraction of cloud cover was estimated from comparing clear-sky expected shortwave radiation based on latitude and time of year to observed solar radiation at the RAWS location based on Fassnacht et al. (2001). This estimated energy budget was calibrated through adjustments to the longwave radiation component to match average SWE losses at the SNOTEL station. The north aspect energy budget was then estimated through altering the shortwave radiation component by a factor of 0.7 based on average slope and aspect for the month of April. This resulted in the north-facing hillslope having an approximated average melt rate of 0.6 that of the SNOTEL station. Using the 10 m DEM we estimated average contributing areas for the snow pit locations on the north-facing slope. We then used the changes in SWE and observed precipitation to estimate the contribution of lateral flow for the 2-week periods between observations. Given the observed increases in SWE on the north-facing hillslope this results in a minimum of 4 % of melt traveling laterally above the SSI to produce the observed increases in SWE. The 4 % is water that flows downslope above the SSI and remains in the snowpack. Therefore, the percentage may be larger when considering drainage from the snowpack after flowing laterally. Although 4 % of melt flowing downslope within a snowpack is a low number, it accumulates along the 250 m hillslope to increase the SWE at the toe of the slope the most. The 170 % increase in SWE at NT observed in 2014 (Fig. 4) is likely a result of water flowing both above the SSI and below it causing the water table to rise, though the increases in SWE at NL can be attributed to flow above the SSI. Estimating SWE from depth measurements alone in this area will be inaccurate when not considering the effect of preferential flow paths within the snowpack and the resulting effects on snow density.

Preferential flow paths and aspect controls during snowmelt have been observed at lower elevations. At a different site in Colorado, near the rain–snow transition zone, the intermittent snowpack on south aspects displayed matrix flow, whereas north aspects displayed preferential flow paths through the soil (Hinckley et al., 2014). These results are similar to those observed in the present study during the 2015 snowmelt, while the 2013 and 2014 seasons displayed what can be interpreted as lesser melt rates on the north slopes than the south slopes due to temperature and radiation differences (Fig. 2c and d) that result in the preferential flow paths near the SSI. In this study, the north aspect slope displays preferential flow paths early in the snowmelt season similar to alpine regions (Liu et al., 2004; Williams et al., 2015) that can transport a large amount of water relative to the following melt periods (Fig. 4ai and bi). Flow paths then transition to more uniform melting and less preferential flow as the melt season progresses. In 2015, this may have also occurred prior to our observation period, though this is uncertain. The south aspect slope is similar to slopes at lower elevations near the rain–snow transition zone (Eiriksson et al., 2013; Hinckley et al., 2014) that display uniform melt and matrix type of flow with small amounts of water diversion at the SSI.

In 2014, the large increase in SWE at the toe of the north aspect slope (NT) is from the lateral flow of water in snow and the rising of the water table above the soil surface (Fig. 4bi) as evidenced through observations of a deep saturated layer at the bottom of the snowpack and saturated soils. This is a result of snowmelt primarily influencing the top 10 cm of soil on the slope (Blankinship et al., 2014) and water flowing downslope near the SSI decreasing the travel time of water on the hillslope and increasing connectivity at the toe of the hillslope and water table similar to observations in the northern Rocky Mountains (Jencso et el., 2009). Some locations on the north aspect slope in 2014 remained consistent in the amount of bulk SWE while other locations on the hillslope decreased in SWE due to preferential flow paths causing non-uniform flow across the hillslope (Fig. 5). The 2015 observations show the result of rain-on-snow events occurring that are known to produce lateral flow within snowpacks (Eiriksson et al., 2013). The major difference of 2015 is earlier peak SWE and melt season along with increased solar radiation and warmer temperatures (Fig. 2). It is possible that preferential flow paths caused more lateral diversions earlier in the season; however no evidence of this was observed in our study. Future hillslope-scale investigations of these phenomena may benefit from larger-scale runoff lysimeter studies similar to Eiriksson et al. (2013) and observing the entire melt season to capture peak SWE processes in low years.

In flat terrain, snowmelt patterns are known to have correlation lengths of 5 to 7 m in alpine environments (Sommerfeld et al., 1994; Williams et al., 1999) and 2 to 4 m in a subalpine environments (Webb, 2017). These correlation lengths are less than the distances between measurement locations in this study. However, these correlation lengths are explained by flow across snow layer interfaces and snow topography in flat terrain (Sommerfeld et al., 1994; Williams et al., 1999, 2010). Increasing the topographic slope will thus increase the correlation lengths as the snow layer interfaces tilt with the slope of the ground (Webb, 1997; Webb et al., 2018). This study shows that the resulting correlation lengths in complex terrain with steep slopes can increase towards the scale of the terrain variability and result in increases in SWE at the toes of hillslopes. Further investigations are necessary to determine the scale that water may flow through snow or at the SSI on steep slopes. Future studies will benefit from the use of numerous soil moisture sensors to obtain time series data of soil VWC at multiple locations within a watershed to observe the variable infiltration characteristics during snowmelt that are difficult to detect from the near-surface soil moisture alone.

When considering dynamic hydrologic processes that occur during spring snowmelt in subalpine headwater catchments, it is important to consider the variable flow paths that develop based on factors such as slope, aspect, soil parameters, and snowpack characteristics to move beyond single-point measurements and one-dimensional assumptions. The toe of a hillslope is an important location to observe and estimate the amount of hillslope runoff occurring near or above the SSI relative to flow through the soil in future investigations. Future studies will benefit from considering the snowpack as an extension of the vadose zone during spring snowmelt due to the variable saturated flow that occurs.

The observations of this study occurred during above-normal, relatively normal, and below-normal snow seasons capturing bulk SWE and soil VWC variability in space and time during spring snowmelt with varying meteorological forcing conditions, including rain-on-snow events in 2015. Evidence was presented of preferential meltwater flow paths at the snow–soil interface on the north aspect hillslope during early snowmelt. The effect of these preferential flow paths were observed in changes in SWE and infiltration in the shallow subsurface at 20 cm depth, but not observed in the near-surface soil moisture. Near-surface soil moisture is correlated the strongest with soil moisture measured during the first survey relative to other topographic parameters or hydrologic variables. Infiltration beyond the near surface occurred more on flat terrain when compared to sloped conditions during the entire snowmelt season, resulting in greater relative saturation in the shallow subsurface in the flat area.

The snowpack is a porous medium that is an extension of the vadose zone and increases the water storage capacity of a region within a watershed. Water flowing downslope near the snow–soil interface increased SWE at the toe of the north aspect hillslope by as much as 250 mm (170 %), which additionally affects the soil moisture at the toe of the slope. This is a result of a minimum of 4 % meltwater being directed downslope through the snowpack rather than infiltrating. The south aspect hillslope did not display evidence of this phenomenon. The differences in flow path development on the two opposite-facing hillslopes are due to differences in soil, snowpack characteristics, slope and aspect, and snowmelt rates as a result of meteorological forcing variability. The formation of hydraulic barriers at the snow–soil interface will be dependent upon the snow characteristics, soil parameters, and meteorological conditions during melt. During 2015, when a relatively low peak SWE occurred early and rain-on-snow events were observed, the variability of snow and soil moisture increased, displaying the connection and interactions between snow and soil moisture. Results from this study show that the snow acts as an extension of the vadose zone during spring snowmelt, and future investigations will benefit from studying both the snow and soil together.