Evaluating hydrograph and sedigraph characteristics as early-warning signals of soil degradation

Landscape systems my undergo abrupt transitions as a result of a gradual change in system drivers. Such regime shifts, or critical transitions, are often considered undesirable because they cause large changes in the landscape that are often irreversible. A well known regime shift in land surface systems is the shift from thick hillslope soils with high biomass to soils with almost no soil cover and low biomass. This process of land degradation is often abrupt, while it may be driven by a rather gradual increase in grazing intensity. At a certain threshold grazing intensity, biomass starts to decrease, which results in increased throughfall and runoff, causing increased runoff erosion, reducing soil thickness, which again has a negative effect on biomass growth. This positive feedback loop results in a relatively abrupt degradation at the grazing intensity threshold.

It is notably hard to detect this upcoming regime shift, because mean values of the system state variables (e.g. soil thickness, discharge, vegetation biomass) show little change before a transition occurs. This problem has sparked research focused on finding alternative properties of the system that show a more marked change before a transition is coming. It has been shown that such so-called early-warning signals exist, more specifically higher-order statistics of state variables (e.g. instead of the mean value of discharge, the variance; instead of the mean vegetation biomass the spatial variation in biomass).

In this study you will evaluate whether statistical properties of hydrographs and/or sedigraphs can be used as early-warning signals for soil degradation. This is done in a modelling study. An existing hillslope evolution model that runs over time periods of hundreds to thousands of years is used to simulate the shift from a vegetated hillslope with thick soils to a degraded hillslope. Its main output is a timeseries of hillslope geometries (topographical surface, development of gullies, regolith thickness) and vegetation coverage. This output is used as input to an event-based hydrological model that is capable of modelling the complete hydrograph for individual events. It is expected that the hydrograph properties will change in advance of the upcoming shift towards a degraded system. To analyse, this, statistical properties (e.g. peakflow, time to peak, total discharge) will be calculated of hydrographs.

This is an interesting topic if you like a fundamental approach to hydrology, with a focus on modelling. The study is quite innovative, and you will be able to position your work in a (recently emerged) large body of literature on critical shifts and early-warning signals.

Supervision: Dr Derek Karssenberg (Utrecht University)

Location: Utrecht University

Period: to be determined

Number of students: 1

Program/track: Earth Surface Hydrology or Natural Hazards and Earth Observation

Prerequisites: courses in spatio-temporal modelling, hydrology, geomorphology, and/or natural hazards (content of project can be adjusted to your background)

Contact/info: Derek Karssenberg (d.karssenberg@uu.nl)