Abstract

Abstract. The study deals with the identification and characterization of rapid subsurface flow structures through pedo- and geo-physical measurements and irrigation experiments at the point, plot and hillslope scale. Our investigation of flow-relevant structures and hydrological responses refers to the general interplay of form and function, respectively. To obtain a holistic picture of the subsurface, a large set of different laboratory, exploratory and experimental methods was used at the different scales. For exploration these methods included drilled soil core profiles, in situ measurements of infiltration capacity and saturated hydraulic conductivity, and laboratory analyses of soil water retention and saturated hydraulic conductivity. The irrigation experiments at the plot scale were monitored through a combination of dye tracer, salt tracer, soil moisture dynamics, and 3-D time-lapse ground penetrating radar (GPR) methods. At the hillslope scale the subsurface was explored by a 3-D GPR survey. A natural storm event and an irrigation experiment were monitored by a dense network of soil moisture observations and a cascade of 2-D time-lapse GPR trenches. We show that the shift between activated and non-activated state of the flow paths is needed to distinguish structures from overall heterogeneity. Pedo-physical analyses of point-scale samples are the basis for sub-scale structure inference. At the plot and hillslope scale 3-D and 2-D time-lapse GPR applications are successfully employed as non-invasive means to image subsurface response patterns and to identify flow-relevant paths. Tracer recovery and soil water responses from irrigation experiments deliver a consistent estimate of response velocities. The combined observation of form and function under active conditions provides the means to localize and characterize the structures (this study) and the hydrological processes (companion study Angermann et al., 2017, this issue).

Highlights

  • 1.1 Form–function relationship in hydrological sciencesFrom a general perspective the interplay of processes and spatial structures (Grayson and Blöschl, 2001) manifests itself as patterns in dynamics (Sivapalan, 2005) and selforganization (Zehe et al, 2013)

  • This interplay can be expressed as a form–function relationship, which is addressed in many disciplines

  • In abstract terms the relation of form and function is fundamental for the concept that we can predict the behavior of a system under different forcings by knowing its constructive properties

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Summary

Introduction

From a general perspective the interplay of processes and spatial structures (Grayson and Blöschl, 2001) manifests itself as patterns in dynamics (Sivapalan, 2005) and selforganization (Zehe et al, 2013) This interplay can be expressed as a form–function relationship, which is addressed in many disciplines. The hydraulic conductivity curve relates the pore size distribution and the interconnectedness of the pores to the conductance/release function of water depending on the wetting state. These are classic examples of form–function relations at the Darcy scale. In either case determining topology and connectivity (form) and understanding their implication for soil water transport (function) is seen as the “forefront of multiphase flow research” (Armstrong et al, 2016)

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