Tracking water pathways in steep hillslopes by δ¹⁸O depth profiles of soil water

Highlights

• A numerical model successfully simulated observed δ¹⁸O soil water profiles.

• Snowmelt water input into the soils was spatially heterogeneous.

• The method generates a quick overview of soil water paths at the hillslope scale.

Summary

Assessing temporal variations in soil water flow is important, especially at the hillslope scale, to identify mechanisms of runoff and flood generation and pathways for nutrients and pollutants in soils. While surface processes are well considered and parameterized, the assessment of subsurface processes at the hillslope scale is still challenging since measurement of hydrological pathways is connected to high efforts in time, money and personnel work. The latter might not even be possible in alpine environments with harsh winter processes. Soil water stable isotope profiles may offer a time-integrating fingerprint of subsurface water pathways. In this study, we investigated the suitability of soil water stable isotope (δ¹⁸O) depth profiles to identify water flow paths along two transects of steep subalpine hillslopes in the Swiss Alps. We applied a one-dimensional advection–dispersion model using δ¹⁸O values of precipitation (ranging from −24.7 to −2.9‰) as input data to simulate the δ¹⁸O profiles of soil water. The variability of δ¹⁸O values with depth within each soil profile and a comparison of the simulated and measured δ¹⁸O profiles were used to infer information about subsurface hydrological pathways. The temporal pattern of δ¹⁸O in precipitation was found in several profiles, ranging from −14.5 to −4.0‰. This suggests that vertical percolation plays an important role even at slope angles of up to 46°. Lateral subsurface flow and/or mixing of soil water at lower slope angles might occur in deeper soil layers and at sites near a small stream. The difference between several observed and simulated δ¹⁸O profiles revealed spatially highly variable infiltration patterns during the snowmelt periods: The δ¹⁸O value of snow (−17.7 ± 1.9‰) was absent in several measured δ¹⁸O profiles but present in the respective simulated δ¹⁸O profiles. This indicated overland flow and/or preferential flow through the soil profile during the melt period. The applied methods proved to be a fast and promising tool to obtain time-integrated information on soil water flow paths at the hillslope scale in steep subalpine slopes.

Introduction

Knowledge about soil water flow paths is important to assess mechanisms of runoff generation (Stewart and McDonnell, 1991), which include for example overland and subsurface flow (Dunne, 1978). These processes subsequently have important implications for the generation of floods (Beven, 1986), recharge of groundwater (Barnes and Allison, 1988), soil erosion dynamics (e.g. Konz et al., 2010, Lindenmaier et al., 2005, Uchida et al., 2001) and transport of nutrients and pollutants (Schmocker-Fackel, 2004, Weiler and McDonnell, 2006). These processes are of special interest in headwater catchments in mountainous regions, because of their great hydrological importance for the adjacent lowlands (Viviroli et al., 2011, Weingartner et al., 2007).

Hydrological processes at the hillslope scale are influenced by a complex interplay of different factors, including input characteristics, vegetation, geological, morphological and pedological characteristics, all acting on different spatial and temporal scales (Bachmair and Weiler, 2011). Among the topographic controls, slope angle has been identified as a crucial factor influencing hillslope hydrology, i.e. water flows paths (e.g. Hopp and McDonnell, 2009, Lv et al., 2013, Penna et al., 2009). Further, Sayama et al. (2011) found that storage of water was increased with increasing catchment slope. This was due to a greater extent of hydrological active (permeable) bedrock, which is available for water storage in steeper catchments. This underpins the hydrological importance of headwater catchments and the necessity to obtain information on (subsurface) water pathways in these areas. Moreover, Vereecken et al., 2008, UNESCO-IHE, 2011 highlight the importance of knowledge about spatial distribution of hydrological processes and characteristics in the subsurface at different spatial scales.

A great variety of techniques has been applied to study soil hydrological processes. Soil water content can be monitored e.g. by time-domain reflectometry (TDR), electrical resistivity measurements, heat pulse sensors or capacitive sensors (for a review see Vereecken et al., 2008). However, to obtain information on soil water content dynamics at the hillslope scale using these techniques, a high number of sensors has to be deployed during a relatively long time period measuring with high temporal resolution (see review of Dobriyal et al., 2012, Zehe et al., 2010). In addition, the use of hydrometric equipment may be limited in stony soils (Coppola et al., 2013) or due to harsh winter conditions, which often occur in steep hillslopes in alpine headwater catchments. Spatially distributed information on areas where water flow potentially concentrates, can be derived from the topographic wetness index TWI (Beven and Kirkby, 1979), calculated from a digital elevation model. However, Penna et al. (2009) found that flow-related topographic variables (e.g. slope, contributing area and wetness index) could only explain up to 42% of the spatial variation of soil moisture in steep mountainous terrain during two summer seasons. In addition to surface topography, the subsurface topography also plays a crucial role for water flow paths (Freer et al., 2002). Ground penetrating radar (GPR) or electrical resistivity tomography (ERT) can indirectly provide information about possible flow paths in the subsurface, soil water contents, hydraulic properties and soil water dynamics (Jadoon et al., 2012, Lunt et al., 2005, Steelman and Endres, 2012). However, heterogeneities in soils can limit accurate assessment of subsurface characteristics by GPR (Jadoon et al., 2012). All these techniques require a high effort in time, work and economic resources if a monitoring of water fluxes in various compartments over several seasons is investigated (e.g. long-term high-frequency measurements).

Alternatively, soil water stable isotopes can be a valuable tool to track movement of soil water and to gain integrative information about subsurface flow processes like mixing, preferential flow and hydrodynamic dispersion (Asano et al., 2002, Barnes and Allison, 1988, Dusek et al., 2012, Klaus et al., 2013, McDonnell et al., 1991, Stewart and McDonnell, 1991, Stumpp and Maloszewski, 2010, Stumpp et al., 2009). The seasonally varying stable isotope signals of precipitation and the subsequent potential attenuation or propagation of distinct peaks in the soil water can be used to determine recharge rates (Adomako et al., 2010, McConville et al., 2001, Saxena and Dressie, 1983), soil water movement (Gehrels et al., 1998) and to calculate soil water transit times (Stewart and McDonnell, 1991, Stumpp et al., 2012). As such, pore water stable isotope signals have the potential to give a fingerprint integrating over time (one season to several years) and a certain space.

Soil water for stable isotope analysis can be extracted by suction lysimeters (Stewart and McDonnell, 1991), centrifugation of soil samples (Gehrels et al., 1998) or distillation techniques (Ingraham and Shadel, 1992), which are time-consuming and prone to isotopic fractionation (Wassenaar et al., 2008). Wassenaar et al. (2008) developed a fast and effective method for soil water stable isotope analysis, which is based on H₂O_liquid–H₂O_vapor equilibration laser spectroscopy. Garvelmann et al. (2012) applied this approach and used a combination of soil water stable isotope profiles along two relatively smooth hillslope transects and digital terrain analysis to investigate subsurface hydrological processes. With these methods they were able to deduce the relative importance of dominant subsurface flow paths (vertical percolation and lateral subsurface flow) at the hillslope scale. Their approach offers a way to generate a time-integrated overview of soil water flow paths in the subsurface without the need of extensive hydrometric equipment. However, a more physically based description of soil water flow and transport processes to support their conceptual model was not realized in their study. The Richards equation for variably-saturated flow combined with advection–dispersion equations can be used to quantitatively describe water flow and solute transport in soils (e.g. van Genuchten and Simunek, 2004). More complex models to account for non-equilibrium and preferential flow include for example dual-porosity and dual-permeability approaches (for reviews see Beven and Germann, 2013, Simunek et al., 2003). Stable isotopes of soil water in combination with modeling tools were successfully used to describe soil hydrological processes at the plot scale (e.g. Shurbaji and Phillips, 1995, Stumpp et al., 2012) and the hillslope scale (e.g. Dusek et al., 2012). Studies on the plot scale using lysimeters give detailed information on transport parameters on the one hand (e.g. Stumpp et al., 2012). Studies on hillslope hydrographs provide integrated information at the hillslope scale (e.g. Dusek et al., 2012), missing the spatial variability of e.g. transport parameters on the other hand. A method linking this gap could provide important additional information that can be included in detailed spatial hydrological models at the hillslope scale. The aims of the presented study were therefore: (i) to use depth profiles of soil pore water stable isotopes as an indicator of water flow paths and its heterogeneity at the hillslope scale, (ii) to combine measurements of stable water isotopes of soil profiles with a numerical model of the Richards equation coupled with the advection–dispersion equation including fractionation processes to identify water pathways and transport processes in the shallow subsurface and (iii) to apply this method in steep hillslopes in a remote alpine headwater catchment, where installation of more conventional equipment to investigate water flow and solute transport would not only be extremely time consuming but also very difficult (e.g. due to harsh winter conditions, which can impede continuous measurements or due to high stone contents, which might hamper proper installation of TDR probes). The method was tested with 28 depth profiles of soil water δ¹⁸O values at a transect of a north- and a south-facing subalpine hillslope in the Swiss Alps. The suggested method is designed as a diagnostic tool to obtain a time-integrated overview of hillslope hydrology, without the necessity to collect long time series data of hillslope hydrology.