My first blog entry looks back at a year of gathering data from the field. Last week was supposedly the last fieldwork days for this year but despite being planned weeks ago, the fieldwork was cancelled due to snow falling over the study site overnight. It is not uncommon that some days in the field would get cancelled or postponed due to (un)foreseen circumstances. This marks the start of a short break from fieldwork for my i-CONN PhD project as the year 2021 ends. Throughout 2021, I have been doing different kinds of fieldwork with the guidance and support from the Human Impact and Connectivity (HI-CONN) sub-working group at the University of Vienna: acquisition of topographic data (UAV surveys), soil and sediment sampling, and field appraisals.
I have done Unmanned Aerial Vehicles (UAV) surveys to capture the topography of specific hillslopes in my study area at multiple times to monitor changes in elevation through time, indicating sediment erosion or deposition. UAVs capture ground data using a downward-facing sensor. Prior to flying the the UAV, a Global Navigation Satellite System (GNSS) survey is also conducted on the ground to get the exact location of markers (called ground control points or GCPs) that will be visible in the aerial photographs (Fig. 1). Images taken by the UAV are georeferenced to the GCPs in post-processing and can readily be used to create orthophotographs or can be further processed using Structure-from-Motion imaging techniques to create high-resolution 3D digital surface models (DSMs).
Figure 1. Equipment and setup used in UAV surveys.
Field mapping was conducted to have an inventory of notable ground features that show erosion, transport, and deposition of sediments. The figures below show different kinds of features I have mapped for an inventory of in-stream (dis-)connecting features, i.e. large wood that can attenuate the flow of sediments (Fig. 2) and lateral (dis-)connectivity features (such as sediment entry points; Fig. 3). Thereby, features mapped from aerial surveys can also be verified through field mapping and can provide additional information, e.g. for ground truthing and performance assessment of upcoming model simulations.
Figure 2. A channel-spanning large wood accumulation blocking the flow of water and attenuating the passage of fine sediments serves as a (dis-)connectivity hotspot within the river.
Figure 3. Connectivity between the source areas and the channel network is facilitated by sediment entry points.
Soil and sediment sampling
Finally, we also collected samples of soils and sediments from the field. For soil samples, we collected topsoil from different portions of a site as subsamples. These subsamples were mixed to produce one composite sample representative for the whole site. If there were evidences of erosion or exposed subsoil, these were sampled as well. This was done for different land use types (i.e. cultivated, uncultivated, urban areas, channel banks, roads) to look at their respective contributions as sediment sources (Fig. 4a). Suspended river sediments, on the other hand, were collected using Phillips samplers (Phillips et al., 2000), placed near the confluence of major tributaries from each subcatchment with the main channel, as well as near the outlet of the whole catchment (Fig. 4b). Data from both, the sediment sources and the river sediment, will be used for sediment fingerprinting as part of my next steps, which will help in providing insights on sediment connectivity patterns within the catchment.
Figure 4. Soil and sediment sampling in the field. a.) Sampling of exposed subsoil. b.) Installation of Phillips samplers within streams.
At the moment, I still need to collect more data, from the UAV surveys up to soil and sediment sampling. But each of these activities are worth it since I become familiar with this catchment a bit more every time I go out to the field and experience the nuances of doing fieldwork in the such settings. These will all be useful when I go deeper into understanding the system through modeling and network analysis.