Spatio-temporal variability of piezometric response on two steep alpine hillslopes D. Penna, 1,2 * ,† N. Mantese, 1 L. Hopp, 3 G. Dalla Fontana 1 and M. Borga 1 1 Department of Land, Environment, Agriculture and Forest, University of Padova, Legnaro, Italy 2 Faculty of Science and Technology, Free University of Bozen-Bolzano, Bolzano, Italy 3 Department of Hydrology, University of Bayreuth, Bayreuth, Germany Abstract: Information on the main drivers of subsurface flow generation on hillslopes of alpine headwater catchments is still missing. Therefore, the dominant factors controlling the water table response to precipitation at the hillslope scale in the alpine Bridge Creek Catchment, Northern Italy, were investigated. Two steep hillslopes of similar size, soil properties and vegetation cover but contrasting topography were instrumented with 24 piezometric wells. Sixty-three (63) rainfall-runoff events were selected over three years in the snow-free months to analyse the influence of rainfall depth, antecedent moisture conditions, hillslope topographic characteristics and soil depth on shallow water table dynamics. Piezometric response, expressed as percentage of well activation and water peak magnitude, was strongly correlated with soil moisture status, as described by an index combining antecedent soil moisture and rainfall depth. Hillslope topography was found to be a dominant control only for the convex-divergent hillslope and during wet conditions. Timing of water table response depended primarily on soil depth and topographic position, with piezometric peak response occurring later and showing a greater temporal variability at the hillslope bottom, characterized by thicker soil. The relationship between mean hillslope water table level and standard deviation for all wells reflected the timing of the water table response at the different locations along the hillslopes. The outcomes of this research contribute to a better understanding of the controls on piezometric response at the hillslope scale in steep terrain and its role on the hydrological functioning of the study catchment and of other sites with similar physiographic characteristics. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS water table dynamics; hillslope topography; antecedent conditions; time lag; soil depth; steady state Received 10 June 2013; Accepted 20 December 2013 INTRODUCTION Generation of shallow subsurface flow on hillslopes is widely acknowledged as an important hydrological process governing the total catchment runoff in humid climates. Particularly, in mountain watersheds with shallow and conductive soils, steep slopes and relatively unfractured bedrock, lateral subsurface flow may be the main mechanism of storm runoff formation occurring on hillslopes (Weiler et al., 2005). Hillslope response in these environments often involves the transient saturation along the soil-bedrock interface or another layer of lower permeability and the rapid rise of a water table moving laterally as subsurface flow. Recent studies showed the importance of hillslopes as landscape units acting as main contributors to streamflow (McGlynn and McDonnell, 2003b; Penna et al., 2011). Transient patterns of hillslope subsurface stormflow are also involved in transport of solutes and nutrients to the stream (McGlynn and McDonnell, 2003a; Zhang et al., 2011) and represent a strong hydrological control on the triggering of shallow landslides (Berti and Simoni, 2011; Dhakal and Sullivan, 2012; Lanni et al., 2012a, b) and debris flow (Montgomery et al., 2002; Marchi et al., 2009). Furthermore, the development of subsurface saturation patterns and their variability in space and time are responsible for the establishment of the stream-hillslope connectivity, a process with important implications for the catchment runoff production (Jensco et al., 2009; Detty and McGuire 2010a; Hopp and McDonnell, 2011). Transient groundwater response at the hillslope scale often shows a marked spatio-temporal variability (Montgomery et al., 2002; Anderson et al., 2010; Haught and van Meerveld, 2011; Bachmair and Weiler, 2012, 2013; Dhakal and Sullivan, 2012), nonlinearities and threshold effects (Tromp-van Meerveld and McDonnell, 2006a, b; Detty and McGuire 2010a, b; Penna et al., 2011), making it difficult to identify its dominant controls. Surface and *Correspondence to: Daniele Penna, Department of Land, Environment, Agriculture and Forestry, University of Padova, viale dell’Università 16, 35020 Agripolis, Legnaro (PD), Italy. E-mail: daniele.penna@unipd.it † Current Address: Department of Environmental Systems Science, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland. HYDROLOGICAL PROCESSES Hydrol. Process. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.10140 Copyright © 2014 John Wiley & Sons, Ltd.