Form and function in hillslope hydrology: characterization of subsurface flow based on response observations

Abstract

Abstract. The phrase form and function was established in architecture and biology and refers to the idea that form and functionality are closely correlated, influence each other, and co-evolve. We suggest transferring this idea to hydrological systems to separate and analyze their two main characteristics: their form, which is equivalent to the spatial structure and static properties, and their function, equivalent to internal responses and hydrological behavior. While this approach is not particularly new to hydrological field research, we want to employ this concept to explicitly pursue the question of what information is most advantageous to understand a hydrological system. We applied this concept to subsurface flow within a hillslope, with a methodological focus on function: we conducted observations during a natural storm event and followed this with a hillslope-scale irrigation experiment. The results are used to infer hydrological processes of the monitored system. Based on these findings, the explanatory power and conclusiveness of the data are discussed. The measurements included basic hydrological monitoring methods, like piezometers, soil moisture, and discharge measurements. These were accompanied by isotope sampling and a novel application of 2-D time-lapse GPR (ground-penetrating radar). The main finding regarding the processes in the hillslope was that preferential flow paths were established quickly, despite unsaturated conditions. These flow paths also caused a detectable signal in the catchment response following a natural rainfall event, showing that these processes are relevant also at the catchment scale. Thus, we conclude that response observations (dynamics and patterns, i.e., indicators of function) were well suited to describing processes at the observational scale. Especially the use of 2-D time-lapse GPR measurements, providing detailed subsurface response patterns, as well as the combination of stream-centered and hillslope-centered approaches, allowed us to link processes and put them in a larger context. Transfer to other scales beyond observational scale and generalizations, however, rely on the knowledge of structures (form) and remain speculative. The complementary approach with a methodological focus on form (i.e., structure exploration) is presented and discussed in the companion paper by Jackisch et al. (2017).

Dissolved Organic Matter Chemistry and Transport Along an Arctic Tundra Hillslope

“…the residence time of water within the mineral matrix (Angermann et al, 2017;Atchley et al, 2015;Rodhe & Seibert, 2011;Zastruzny et al, 2017). Conceptual model depicting the downslope transport and transformation of dissolved organic matter (DOM) through Arctic hillslopes.…”

“…However, shifts in vegetation and root exudate deposition, as well as chromatographic fractionation (coiled arrow), influence DOM structural diversity, which increases during downslope export and is greatest along the thawing mineral-permafrost interface. the residence time of water within the mineral matrix (Angermann et al, 2017;Atchley et al, 2015;Rodhe & Seibert, 2011;Zastruzny et al, 2017). As a result, sustained, lateral dispersion may partially offset rapid export along preferential flow paths in low-gradient catchments and help shape the molecular composition of materials delivered to surface waters (Harms & Jones, 2012;Kawahigashi et al, 2004;Vonk et al, 2015;Ward & Cory, 2015).…”

How can we model subsurface stormflow at the catchment scale if we cannot measure it?

“…Hydrogeophysical methods may have the potential to identify subsurface flow paths (e.g., Angermann et al, ; Binley et al, ) or, at least, soil heterogeneity (Martini et al, ), but their use is mostly limited to the hillslope scale (Vereecken et al, ). Here, subsurface flow paths are more likely to be connected over shorter rather than longer distances, which leads to higher effective flow velocities (e.g., Anderson, Weiler, Alila, & Hudson, ; Wienhöfer & Zehe, ).…”

“…Hydrogeophysical methods may have the potential to identify subsurface flow paths (e.g., Angermann et al, 2017;Binley et al, 2015) or, at least, soil heterogeneity (Martini et al, 2017), but their use is mostly limited to the hillslope scale (Vereecken et al, 2015).…”

Characterizing hydrological processes in a semiarid rangeland watershed: A hydrogeophysical approach

“…Other geophysical tools such as seismic refraction and ground-penetrating radar (GPR) can provide valuable information regarding subsurface structure (Holbrook et al, 2014;McClymont, Hayashi, Bentley, & Liard, 2012;Mills, 1990;Neal, 2004;Parsekian, Singha, Minsley, Holbrook, & Slater, 2015). GPR has also been used in a time-lapse capacity to examine subsurface hydrologic function (Allroggen, van Schaik, & Tronicke, 2015;Angermann et al, 2017;Nyquist, Toran, Pitman, Guo, & Lin, 2018).…”