Students projects

Context: in soil hydrology, we study the movement of water through the Swiss forest soils to understand how resilient the forest ecosystems are to prolonged summer drought periods. A process important for the survival of vegetation is the storage volume that soil provides to forests.

Objective: Progress on robust and accurate numerical methods for solving the Richards equation is still made. An overview of methods can be found in Maina and Ackerer (2017). An application showcasing simulations of another Julia-implemented water balance model is presented by Pollacco (2022). Our objective is to test alternative algorithms in the context of LWFBrook90.jl. Examples of these algorithms could be: (Celia, 1990), (Ross, 2003), (Tubini and Rigon, 2022), or (Li et al., 2021).

Expected work:

• Get familiar with the Julia package jl (used internally by LWFBrook90.jl)
• Assess compatibility of the alternative algorithms with jl
• Implement 1 or 2 algorithms in a sandbox example or directly into jl
• Run synthetic examples and/or real world simulations to evaluate computational efficiency
• Merge the developed code into the package jl

Detailed project: Bernhard MA Richards Suggestion V1 (pdf. format)

References:

Bittelli, M., Campbell, G. S., and Tomei, F.: Soil Physics with Python: Transport in the Soil–Plant–Atmosphere System, Oxford University Press, 2015.

Radcliffe, D. E., Simunek, J., and Simunek, J.: Soil Physics with HYDRUS : Modeling and Applications, CRC Press, https://doi.org/10.1201/9781315275666, 2018.

Celia, M. A., Bouloutas, E. T., and Zarba, R. L.: A general mass-conservative numerical solution for the unsaturated flow equation, Water Resour. Res., 26, 1483–1496, https://doi.org/10.1029/WR026i007p01483, 1990.

Maina, F. H. and Ackerer, P.: Ross scheme, Newton–Raphson iterative methods and time-stepping strategies for solving the mixed form of Richards’ equation, Hydrol. Earth Syst. Sci., 21, 2667–2683, https://doi.org/10.5194/hess-21-2667-2017, 2017.

Pollacco, J. A. P., Fernández-Gálvez, J., Ackerer, P., Belfort, B., Lassabatere, L., Angulo-Jaramillo, R., Rajanayaka, C., Lilburne, L., Carrick, S., and Peltzer, D. A.: HyPix: 1D physically based hydrological model with novel adaptive time-stepping management and smoothing dynamic criterion for controlling Newton–Raphson step, Environ. Model. Softw., 153, 105386, https://doi.org/10.1016/j.envsoft.2022.105386, 2022.

Ross, P. J.: Modeling Soil Water and Solute Transport—Fast, Simplified Numerical Solutions, Agron. J., 95, 1352–1361, https://doi.org/10.2134/agronj2003.1352, 2003.

Tubini, N. and Rigon, R.: Implementing the Water, HEat and Transport model in GEOframe (WHETGEO-1D v.1.0): algorithms, informatics, design patterns, open science features, and 1D deployment, Geoscientific Model Development, 15, 75–104, https://doi.org/10.5194/gmd-15-75-2022, 2022.

Li, Z., Özgen-Xian, I., and Maina, F. Z.: A mass-conservative predictor-corrector solution to the 1D Richards equation with adaptive time control, J. Hydrol., 592, 125809, https://doi.org/10.1016/j.jhydrol.2020.125809, 2021.

Please contact Fabian Bernhard (WSL) for more information and to discuss the project in more details (French, English, German)

Context: wind and winter cooling are known to induce currents along the lateral slopes of large lakes. These currents are of great importance for the development of the lake ecological state, because they affect thermal exchange and the transport of nutrients, pollutants and oxygen. To study the dynamics of these currents, several sets of instruments have been deployed by the Ecological Engineering Laboratory (ECOL) (www.epfl.ch/labs/ecol/) on the northern shore of Lake Geneva. They measure continuously the velocity and temperature over the whole water column. Recently, a new turbulence platform was developed, and is currently collecting data with high spatial and temporal resolution in the same area. (For more information, see the following link: research project); a view of the platform can be seen here: youtube turbulator).

Objective: analyze the above data, in particular the turbulence data, which should provide estimates of water mixing and entrainment. Depending on the student’s interests, the project could focus on seasonal variability or on specific events, such as strong wind events. In addition, results of numerical simulations of the general lake circulation are available to help interpret the near-shore data. Participation in field campaigns can also be envisioned. The student will gain valuable insights into the hydrodynamics of large lakes, data processing, data analysis and turbulence.

Please contact Dr. François Mettra (ECOL) for more information and to more precisely define an appropriate project ([email protected]).  The project and supervision can be done in English or French.

Context: Diel vertical migration (DVM) of zooplankton, considered as “one of the most profound coordinated movements of organisms on the planet” (Häfker et al. 2017), is a ubiquitous phenomenon in marine and freshwater pelagic zones (Meester 2009). Zooplankton synchronously descend from the near-surface water layer during daylight hours, then ascend at night, but the reverse can also occur (Cohen and Forward 2019). The depth to which zooplankton descend varies depending on the organism and environmental conditions (light intensity, daylength, season, temperature, dissolved oxygen, buoyancy, currents, food, predation or internal clocks). In Lake Geneva, previous measurements showed that DVM can reach up to 150-m depth, however very little is known about its driving mechanisms and controlling factors.

In order to fill the gap in DVM knowledge in Lake Geneva, a set of instruments is currently deployed in the deepest part of the lake the Ecological Engineering Laboratory (ECOL; https://www.epfl.ch/labs/ecol/). This set consists in a mooring line measuring currents, echo intensity, temperature, oxygen and light along the water column, in additional light measurements at other locations and depths in the lake, and in zooplankton images and samples (species identification and abundance).

Objective: you will be analysing the above-mentioned datasets, and investigating the potential influence of light, temperature and currents on the DVM variability. Depending on your interests and timing, the project could focus on the changes of migration rates at the seasonal scale or at the event scale (few days). Some Matlab codes will already be available for the data pre-treatment.

You will gain valuable insights into the hydrological and biological dynamics of large lakes, and into the processing and analysis of datasets from various instruments.

Note that this project is available for consideration for the fall semester (2022) only. Please contact Dr. Violaine Piton (ECOL) for additional information ([email protected]). The project can be supervised in English or French.

References:

Cohen, J. H., and R. B. Forward. 2019. Vertical Migration of Aquatic Animals, p. 546–552. In Encyclopedia of Animal Behavior. Elsevier.

Häfker, N. S., B. Meyer, K. S. Last, D. W. Pond, L. Hüppe, and M. Teschke. 2017. Circadian clock involvement in zooplankton diel vertical migration. Curr. Biol. 27: 2194–2201. doi:10.1016/j.cub.2017.06.025

Meester, L. D. 2009. Diel Vertical Migration, p. 651–658. In G.E. Likens [ed.], Encyclopedia of Inland Waters. Academic Press.

Context: Convective cooling is the main process to cool and de-stratify lakes after the warm season. Heat loss at the air-water interface cools the lake surface water. As this thin and cold surface layer is denser than the layers below, the water column near the surface becomes unstable, resulting in the formation of convective plumes. This process is essential for mixing surface layers, eroding the vertical temperature gradients, and bringing dissolved oxygen towards the deep lake. However, it has been poorly studied in details at the air-water interface (where organized temperature patterns are visible) or in the near-surface water (where plumes develop). Furthermore, the thermal patterns that form during convective cooling at the surface of lakes and oceans are rarely observed in the field [1], with most studies being conducted in laboratories or using numerical simulations [2, 3]. Below the surface, a study conducted in Lake Geneva using a submarine was able to give rough estimations of the size of these plumes [4]. Yet, a solid understanding of the relationship between such surface convective patterns and both atmospheric (e.g., wind speed and direction, air-water heat exchange) and water-side (e.g., near-surface temperature and velocity) parameters is still lacking.

To gain a more thorough insight into convective cooling, an experimental
study was conducted in Lake Geneva with the aim of obtaining detailed
measurements in both the air and water side during strong nighttime cooling conditions. The ZiviCat, an autonomous floating platform, and an infrared camera mounted on a balloon, both developed at ECOL, were used for 2 nighttime campaigns during the winter of 2021-2022. Additional observations from moorings and a CTD probe complete this extensive data set.

A detailed description of the instruments and study site can be found under
the following two links: ECOL equipment and Bois Chamblard Foundation.
The two following sub-projects aim to perform the first analyses and interpretations, one concerning the thermal patterns at the air-water interface and air-side processes, the other focusing on the convective plumes.

Subproject I: This part focuses on the surface thermal patterns that appear during nighttime cooling. Specifically, we are interested in the spatial scales of these patterns, as well as whether there is any correlation between the surface thermal structures and atmospheric parameters above them.

Objective of subproject I includes (1) general data analysis/visualization, primarily for preliminary analysis and identifying time periods of greatest
interest, (2) basic image processing of the thermal images, and (3) time
series analyses of the air-side parameters.

Subproject II: The second project mostly concerns the convective plume formation below the air-water interface. Similarly to the first part, it is interesting to know how near-surface temperature and currents are related to thermal patterns on the surface. Using both Lagrangian and Eulerian measurements, the student will identify spatiotemporal characteristics of the observed convective plumes.

Objective of subproject II includes data analysis on (1) stationary mooring data (temperature and current measurements) and (2) the spatiotemporal recordings of the ZiviCat, mainly near-surface temperature and currents.

We seek two motivated students interested in physics, hydrodynamics,
data analysis, and of course, Lake Geneva, and encourage them to work collaboratively and share insight and ideas since they are approaching the same physical phenomenon from different perspectives.

Also, to ensure a rapid and successful start of the projects, the students will work with existing data analysis scripts. Therefore, a basic knowledge of Python or a comparable programming language is necessary. The projects are supervised by two postdocs and one PhD student

For more information, please contact Mehrshad Foroughan
([email protected])

References:

George O. Marmorino et al. “Airborne imagery of ocean mixed-layer
convective patterns”. In: Deep-Sea Research Part I: Oceanographic Research
Papers 56.3 (2009), pp. 435–441. doi: 10.1016/j.dsr.2008.
10.007.

R. A. Handler, G. B. Smith, and R. I. Leighton. “The thermal structure
of an air-water interface at low wind speeds”. In: Tellus, Series A:
Dynamic Meteorology and Oceanography 53.2 (2001), pp. 233–244. doi:
10.3402/tellusa.v53i2.12187.

R. I. Leighton, G. B. Smith, and R. A. Handler. “Direct numerical
simulations of free convection beneath an air–water interface at low
Rayleigh numbers”. In: Physics of Fluids 15.10 (Oct. 2003), pp. 3181–
3193. doi: 10.1063/1.1606679.

S. A. Thorpe. “Observations of the thermal structure of a lake using
a submarine”. In: Limnology and Oceanography 44.6 (1999), pp. 1575–
1582. issn: 1939-5590. doi: 10.4319/lo.1999.44.6.1575.