Unsaturated subsurface flow: How it evolves in the first 10 000 years after landscape initialization

Form and function are two major characteristics describing hydrological systems and can also be helpful in the context of understanding and analyzing unsaturated flow. Whereas the term “form” summarizes the structure and properties of the system, the term “function” represents the hydrological response. Form parameters such as structural and hydraulic soil properties, but also vegetation cover, have a major influence on the subsurface hydrological response. The combination of different soil and surface properties affects the formation of subsurface hydrological flow paths and their interaction and feedbacks can lead to the formation of preferential flow paths that are difficult to characterize and predict. Little is known about how these characteristics co-evolve over time and how form impacts function in young hydrological systems.

We systematically investigated how form and function evolved during the first 10 millennia of landscape development. We analyzed two hillslope choronosequences in glacial forelands in the Swiss Alps, one developed from siliceous and one from calcareous parent material.
Variables describing form were studied in terms of soil properties and vegetation characteristics obtained by vegetation mapping, extensive soil sampling and laboratory analyses. Variables describing hydrologic function include soil water response times, soil water storage, dominant flow path types, and the frequency of preferential flow paths which were obtained by Brilliant Blue dye tracer irrigation experiments and sprinkling experiments with deuterium. A principal component analysis and clustering were used to identify how form features relate to specific functions.

Our investigation revealed differences in the evolution of form and function between the two different parent materials. At the calcareous site, a change in flow types with increasing moraine age was observed from a rather homogeneous matrix flow to heterogeneous matrix and finger-like flow. However, the high buffering capacity of the calcareous soil leads to less soil formation and fast, vertical subsurface water transport dominates the water redistribution even after more than 10 000 years of landscape evolution. At the siliceous parent material accumulation of organic material with high water storage capacity and podsolization was observed between 3 000 and 10 000 years of landscape development. Under these conditions water redistribution is dominated by vertical subsurface water transport via matrix flow only at young age classes (< 3 000 years). After 10 000 years of soil development vertical subsurface water transport below the organic top layer mainly takes place via macropore flow, and water storage in the organic surface layer and lateral subsurface water transport become the major components controlling water redistribution.

We found that in general the increase in preferential flow frequency is caused by soil development and is further enhanced by an increase in above ground biomass, organic matter and micro topography. Form parameters driving the evolution of subsurface function differ between the two contrasting geologies which highlights the importance of the parent material for landscape development.