The influence of terracettes on the surface hydrology of steep-sloping and subalpine environments: some preliminary findings

Abstract

Abstract. Alpine and mountain slopes represent important pathways that link high-altitude grazing areas to meadows and rangelands at lower elevations. Given the often acute gradients associated with such environments, they potentially represent highly efficient runoff conveyance routes that facilitate the downslope movement of runoff and associated material during erosion events. Many such slopes host series of small steps, or "terracettes". The juxtaposition of terracettes against the natural downslope flow path of non-complex slopes leads us to hypothesise that they may influence typical hillslope processes by intercepting or capturing surface runoff. Here we report preliminary results and some tentative conclusions from ongoing work to explore this possibility. Google Earth was used to initially identify a ca. 400 m 2 well-developed terracette system situated on a west-facing slope with gradients ranging from 25 to 40 • (46 to 84 %). A digital elevation model (DEM) of the terracettes was constructed using spatial data taken from a relevant section of topographic map. The DEM was then queried using a flow-accumulation algorithm and the results displayed in a geographic information system. The output data provided "proof of concept" that terracettes can capture surface runoff. The generation of empirical data from a series of rainfall/runoff simulations performed on the same section of terracettes supports this finding. Results from both work components indicate that sections of a terracette system may intercept runoff and could act as preferential flow pathways. By contrast, some sections appeared to act as depositional sites. We cautiously predict that these areas could act as retention zones for the temporary storage of runoff-associated substances. Greater understanding of the exact influence of terracettes on surface hydrology in steep-sloping and subalpine environments could benefit the future management of grazing and rangelands in such areas.

Soil Moisture Differences between Terracette Benches and Risers on Semiarid Rangeland Hillslopes

“…We observed less vegetative cover on benches than risers, which contradicts the findings of Greenwood et al (2015) but is supportive of our observed negative correlation between θ v and vegetative cover shown in Fig. 6 and the observations of terracette conditions as described by Rahm (1962) and Radcliffe (1968).…”

“…Locally, soil moisture can be spatially highly variable (Western et al, 2002), play a critical role in the rainfall–runoff response of catchments (Dunne et al, 1991), and serve as an ecological driver controlling plant transpiration and, particularly in semiarid lands, acting as the dominant control affecting the structure, function, and diversity of an ecosystem (Robinson, 2008). It is recognized that conditions such as antecedent moisture play a substantive role in hillslope runoff and erosion (Castillo et al, 2003) alongside slope (Liu and Singh, 2004) and surface morphology (Bergkamp, 1998; Thompson et al, 2010), which often necessitate some parameterization of topography in hydrologic models (Gerke et al, 2010; Harpold et al, 2010; Greenwood et al, 2015). However, because of limitations in the spatial resolution of topographic data and computational resource demands (Tesfa et al, 2011), hydrologic models often do not account for microtopography (soil surface variations of <1m vertical relief as described by Moser et al, 2007), despite a general recognition that influences of microtopography on soil moisture in semiarid ecosystems can increase the precision of the variables used in modeling the processes of surface runoff and sediment transport (Jones et al, 1996; Savabi, 2001; Mapfumo et al, 2004; Easton et al, 2008).…”

“…1) are a combination of repetitious bench, path‐like, and riser, slope‐like, features that exist on hills with >15° slope in semiarid environments of the western United States (Rahm, 1962), as well as throughout the world (Radcliffe, 1968; Gallart et al, 1993; Kück and Lewis, 2002; Wilson et al, 2004; Henck et al, 2010). The origin of terracettes and their respective formational processes is believed to encompass either geomorphic processes such as solifluction (Kück and Lewis, 2002), slumping (Rahm, 1962), soil creep (Bielecki and Mueller, 2002), vegetation control (Bergkamp, 1998), or biologic influences such as hoof impacts from grazing animals (Radcliffe, 1968; Buckhouse and Krueger, 1981; Watanabe, 1994), with attempts at modeling terracette formation (Gallart et al, 1993), relating formation to anthropogenic history (Henck et al, 2010), classification of features (Anderson, 1972), and implications for Hortonian flow (Greenwood et al, 2015) occurring on the periphery. Previous terracette studies have described bench dimensions of 15 to 76 cm and riser heights of 5 to >120 cm on slope gradients between 9 and 60° with individual feature lengths of 3 to >300 cm, variable vegetative cover conditions, and occurrence across a wide range of soil conditions and environments (Ødum, 1922; Rahm, 1962; Radcliffe, 1968; Anderson, 1972; Watanabe, 1994; Bergkamp, 1998; Weihs and Shroder, 2011; Greenwood et al, 2015).…”

“…Studies of these semiarid rangeland environments, terracettes, and the impacts of grazing emphasize the importance of sustainable rangeland management (Alderfer and Robinson, 1947; Dormaar et al, 1989; Holechek et al, 1999) and the need to address water, erosion, and sedimentation challenges in these ecosystems (Laycock and Conrad, 1967; Bilotta et al, 2007; Schmalz et al, 2013). The vital role of vegetation for rangeland use (Holechek, 1988), and in the presence of semiarid terracettes for reducing hillslope runoff (Bergkamp 1998), suggests that the hydrologic interactions of these features may be significant in reducing erosion (Corrao et al, 2015) and increasing the amount of soil water on hillslopes (Greenwood et al, 2015).…”

Introduction to the special issue of Geographica Helvetica: "Mapping, measuring and modeling in geomorphology"

“…The outcome of this work has allowed them to predict spatial and temporal variations of permafrost in sensitive mountain environments under a range of ongoing and future climate change scenarios. Greenwood et al (2015) combine GIS-based and experiment-based approaches to assess the influence of hillslope terracettes on surface hydrology on steep-sloping and subalpine terrain. Their work highlights the fact that the spatial heterogeneity of these relatively common features means that they can influence the surface hydrology in ways that can be both positive and negative.…”