Next phase in RIBUST WP 2: runoff experiments

Working package 2 consists of two columns: soil core sampling of vegetated filter strips (VFS) and an experimental approach. For the latter, we conducted artificial runoff experiments using grassland plots (2 x 5 m) that were subjected to artificial runoff. The water was applied through an overflow tank and was spiked with bromide, to be able to distinguish it from autochthonous water in the soil, as well as phosphate, to mimic nutrient enriched agricultural runoff. Our main interest was on how concentrated runoff (i.e., due to flow convergence in the field or at the field edge) affects flow characteristics and VFS performance. To this end, we used three different widths of the overflow tank and analysed total water budget and flow velocities, as well as bromide and phosphate concentrations at the end of the plots.

With this straight-forward, yet elaborate setup we can analyse several runoff characteristics that have an effect on buffer performance, for instance if the soil is able to take up phosphorus (P) or if P is being released. (C) BAW-IKT / Ramler

The outdoor experiments are completed, currently the data is processed and analysed. We expect that flow concentrations have a significant effect on runoff characteristics, e.g., less infiltration and, in turn, a higher amount of runoff water, accompanied by higher flow velocities and less time for soil/water interaction processes. All these aspects would have direct consequences for VFS performance. However, flow concentration is rarely accounted for in VFS design guidelines and recommendations.

Runoff water was applied to the plots using overflow tanks of different width. (C) BAW-IKT / Ramler
Also drones were used in a preliminary experiment to check if thermal imaging can be used to depict flow path development. (C) BAW-IKT / Ramler

Combing undisturbed soil monoliths – a preliminary trial for runoff experiments

Artificial runoff experiments are a part of working package 2, in which we seek to analyse the response of buffer strip soils to different runoff scenarios. To improve the meaningfulness of the data, we plan to combine undisturbed soil monoliths to larger soil plots, taking advantage of both the flexibility of indoor experiments and the realism of undisturbed soils. To our knowledge, this has not been done before. Therefore, we started a preliminary trial to ascertain that combining blocks of soil does not interfere with the runoff characteristics.

Taking undisturbed soil monoliths is a strenuous task. All monoliths were taken back-to-back from the same spot. (C) BAW-IKT / Ramler

To this end, we took six undisturbed soil monoliths from a buffer strip. Half of them were cut in the middle and then recombined. The monoliths were then used for a runoff experiment, during which the blocks received a constant flow from a tank, with water spiked with phosphorus (P) and salt tracers. Most water left the monoliths as surface runoff, but substantial amounts were also recorded as drainage water (passing through the soil body, probably due to the natural macropore network) and bypass water (e.g. water that leaves the monolith at its sides). Interestingly, we noticed an enrichment of the surface runoff with P, which means that the buffer soil acted as a P source at it surface, rather than a sink.

Three out of six monoliths were cut in half and recombined. Prior to the experiment, all monoliths were saturated with water to have similar soil moisture conditions. (C) BAW-IKT / Ramler

We found no significant differences between cut and uncut monoliths. In fact, it was apparent that the inherent variability between the monoliths (due to the inevitable spatial heterogeneity of the soil) was much larger than any effect that the cutting could have had. We conclude that a careful combination of soil monoliths is a valid procedure and plan to further pursue this approach for the main runoff experiment starting next year.

Experimental set-up. Water was collected from different soil compartments (surface runoff, interflow, drainage, and bypass water) using a custom-built steel-frame. An overflow tank provided a constant flow of runoff water. (C) BAW-IKT / Ramler