Hydrologic flow path development varies by aspect during spring snowmelt in complex subalpine terrain

Ryan W Webb,Steven R Fassnacht,Michael N Gooseff

doi:10.5194/tc-12-287-2018

Abstract

Abstract. In many mountainous regions around the world, snow and soil moisture are key components of the hydrologic cycle. Preferential flow paths of snowmelt water through snow have been known to occur for years with few studies observing the effect on soil moisture. In this study, statistical analysis of the topographical and hydrological controls on the spatiotemporal variability of snow water equivalent (SWE) and soil moisture during snowmelt was undertaken at a subalpine forested setting with north, south, and flat aspects as a seasonally persistent snowpack melts. We investigated if evidence of preferential flow paths in snow can be observed and the effect on soil moisture through measurements of snow water equivalent and near-surface soil moisture, observing how SWE and near-surface soil moisture vary on hillslopes relative to the toes of hillslopes and flat areas. We then compared snowmelt infiltration beyond the near-surface soil between flat and sloping terrain during the entire snowmelt season using soil moisture sensor profiles. This study was conducted during varying snowmelt seasons representing above-normal, relatively normal, and below-normal snow seasons in northern Colorado. Evidence is presented of preferential meltwater flow paths at the snow–soil interface on the north-facing slope causing increases in SWE downslope and less infiltration into the soil at 20 cm depth; less association is observed in the near-surface soil moisture (top 7 cm). We present a conceptualization of the meltwater flow paths that develop based on slope aspect and soil properties. The resulting flow paths are shown to divert at least 4 % of snowmelt laterally, accumulating along the length of the slope, to increase the snow water equivalent by as much as 170 % at the base of a north-facing hillslope. Results from this study show that snow acts as an extension of the vadose zone during spring snowmelt and future hydrologic investigations will benefit from studying the snow and soil together.

Highlights

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
The precipitation that fell during the melt period in 2015 likely included a number of rain-on-snow events due to the regular warmer than freezing temperatures in late April and May (Fig. 2d), though snow can fall at several degrees warmer than zero (Fassnacht et al, 2013)
Evidence was presented of preferential meltwater flow paths at the snow–soil interface on the north aspect hillslope during early snowmelt

Summary

Introduction

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. 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), 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)

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