Field observations of soil hydrological flow path evolution over 10 millennia

Abstract

Abstract. Preferential flow strongly controls water flow and transport in soils. It is ubiquitous but difficult to characterize and predict. This study addresses the occurrence and the evolution of preferential flow during the evolution of landscapes and here specifically during the evolution of hillslopes. We targeted a chronosequence of glacial moraines in the Swiss Alps to investigate how water flow paths evolve along with the soil-forming processes. Dye tracer irrigation experiments with a Brilliant Blue FCF solution (4 g L−1) were conducted on four moraines of different ages (30, 160, 3000, and 10 000 years). At each moraine, three dye tracer experiments were conducted on plots of 1.5 m ×1.0 m. The three plots at each moraine were characterized by different vegetation complexities (low, medium, and high). Each plot was further divided into three equal subplots for the application of three different irrigation amounts (20, 40, and 60 mm) with an average irrigation intensity of 20 mm h−1. The day after the experiment five vertical soil sections were excavated, and the stained flow paths were photographed. Digital image analysis was used to derive average infiltration depths and flow path characteristics such as the volume and surface density of the dye patterns. Based on the volume density, the observed dye patterns were assigned to specific flow type categories. The results show a significant change in the type of preferential flow paths along the chronosequence. The flow types change from a rather homogeneous matrix flow in coarse material with high conductivities and a sparse vegetation cover at the youngest moraine to a heterogeneous infiltration pattern at the medium-age moraines. Heterogeneous matrix and finger flow are dominant at these intermediate age classes. At the oldest moraine only macropore flow via root channels was observed in deeper parts of the soil, in combination with a very high water storage capacity of the organic top layer and low hydraulic conductivity of the deeper soil. In general, we found an increase in water storage with increasing age of the moraines, based on our observations of the reduction in infiltration depth as well as laboratory measurements of porosity. Preferential flow is, however, not only caused by macropores, but especially for the medium-age moraine, it seems to be mainly initiated by soil surface characteristics (vegetation patches and microtopography).

Long‐Term Changes in Runoff Generation Mechanisms for Two Proglacial Areas in the Swiss Alps II: Subsurface Flow

“…However, sprinkling experiments on grassland hillslopes in pre‐Alpine catchments of the Swiss Alps showed large variations in the chemical composition of SSF, depending on whether SSF was directly fed by rainfall via preferential flow paths (little mixing and small pre‐event water fractions) or if SSF came from saturated parts of the soil (enhanced mixing and larger pre‐event water fractions) (Kienzler & Naef, 2008). Dye staining experiments on the 10‐ky moraine showed that the pronounced root network (Figure 2a) induced lateral macropore flow (arrows #3 and #5 in Figure 1; Hartmann, Semenova, et al., 2020). SSF likely occurred through these preferential flow pathways, as has been observed for many other hillslopes (e.g., Anderson et al., 2009; McDonnell, 1990; Uchida et al., 2005; Weiler & Naef, 2003).…”

“…This study and the accompanying study on OF (Maier & van Meerveld, 2021) are a step to establish a data basis to test and help to improve landscape evolution models. It provides valuable information on the characteristics of the moraines (see also Hartmann, Semenova, et al., 2020; Maier et al., 2020; cf. top part of Figure 1) and thus the parameters for these models, as well as information on the hydrological responses to evaluate the models.…”

Long‐Term Changes in Runoff Generation Mechanisms for Two Proglacial Areas in the Swiss Alps I: Overland Flow

“…Instead, the calculated effective infiltration rates were higher for the higher intensity sprinkling experiments (Figure 9). This is most likely due to the spatial variability in infiltration rates caused by macropore flow (Hartmann, Semenova, Weiler, & Blume, 2020; Maier et al., 2020). In addition, Alpine rose on the 10‐ and 3‐ky moraine may possibly have provoked hydrophobic topsoil conditions for the first (i.e., LI) experiments (Table 1; Ellenberg & Leuschner, 2010).…”

Vertical root distribution and biomass allocation along proglacial chronosequences in Central Switzerland

“…Glacier site, A. Hartmann et al (2020) found different flow types at the different moraines, ranging from matrix flow at younger terrain ages to macropore flow via root channels at the oldest moraine.…”