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).
Highlights
Characterizing subsurface flow is the aim of many hydrological field and modeling studies
In hillslopes with steep slopes and structured soils, subsurface flow is controlled by high gradients and high heterogeneity of hydraulic properties of the soil, resulting in a highly heterogeneous flow field and preferential flow paths (e.g., Scaini et al, 2017)
We found that distinct preferential flow paths occurred at greater depth, which suggests that the bedrock might not be as impermeable as previously assumed (Fenicia et al, 2014)
Summary
Characterizing subsurface flow is the aim of many hydrological field and modeling studies. In hillslopes with steep slopes and structured soils, subsurface flow is controlled by high gradients and high heterogeneity of hydraulic properties of the soil, resulting in a highly heterogeneous flow field and preferential flow paths (e.g., Scaini et al, 2017). The specific challenge of investigating preferential flow lies in its manifestation across scales, its high spatial variability, and pronounced temporal dynamics. L. Angermann et al.: Temporal dynamics of preferential flow of experimental and model approaches have been proposed to investigate the issue (Beven and Germann, 1982, 2013; Šimunek et al, 2003; Gerke, 2006; Weiler and McDonnell, 2007; Köhne et al, 2009; Germann, 2014). Rapid flow in structured soils is still a challenge to current means of observation, process understanding, and modeling
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