Catch up on sediments to understand particulate phosphorus processes

PhD student Hannah Wenng and co-authors recently published a paper where they study catchment processes of particle mobilisation and transport. In arable land, phosphorus losses are often closely linked to sediment losses, which means that a better understanding of the sediment-runoff processes is important to select the most optimal and targeted mitigation measures to reduce eutrophication.

PhD student Hannah Wenng used an automated sampler to calibrate the sensor data with particle and phosphorus concentrations of water samples. Ph: Marianne Bechmann.

Hannah installed turbidity sensors in two Norwegian agricultural streams under the JOVA Programme (Skuterud and Mørdre) and combined the turbidity data with data on water discharge. The advantage of using such sensors is the high frequency of the measurements, which makes it possible to study how particles move in the streams during storm events, at both raising and falling water discharges. Turbidity correlated well with suspended solids and particulate phosphorus in the streams. With her co-workers she also calculated an index for connectivity, which simplified can be expressed as the likelihood of water and particles to be transported to the stream from different parts of the catchment area with different soil tillage. Moreover, within the JOVA Programme the agricultural activities are recorded each year, including dates for fertilizing, sowing, and harvesting. Used in combination, these data allowed the authors to study how water discharge and agricultural management affect the processes of mobilisation and transport of particles and particle associated phosphorus in the catchments.  

The results showed that the dominant concentration-discharge pattern was described by a clockwise pattern in both catchments (see figure below). This can indicate that areas close to or in the stream gave the highest contribution of particles. The most important factor for increasing turbidity was water discharge, but also soil water storage capacity and rain intensity played a role. In addition, it was observed that when two events followed each other, the turbidity of the second peak was lower than the first. This can be due to depletion of the available sediments for transport.

In all four seasons, the turbidity increased during increasing water discharge and reached its peak while the discharge was still increasing. This gives a clockwise pattern in what is known as a hysteresis pattern. From the paper by Wenng et al (2021).

Intense soil tillage with little vegetation cover gave the highest turbidity concentrations. However, other links between agricultural management and in-stream data were difficult to interpret, possible because many processes interact with each other in these catchments.

This is PhD student Hannah’s second paper as the main author, although she has been co-author of other Biowater related papers. Her first paper as main author was published in the special issue of Biowater in Ambio 49(11) 2020.

Wenng, H., Barneveld, R., Bechmann, M., Marttila, H., Krogstad, T. & Skarbøvik, E. 2021. Sediment transport dynamics in small agricultural catchments in a cold climate: A case study from Norway. Agriculture, Ecosystems & Environment, Vol 317, https://doi.org/10.1016/j.agee.2021.107484

Feature photo: From the Skuterud Creek catchment area. By Hannah Wenng.