Overland flow and dryland ecohydrology

Background: describing overland flow in patchily vegetated landscapes

Understanding shallow overland flow in dryland environments is a challenge because it involves unsteady flows over heterogeneous surfaces with contrasting surface roughness and infiltration rates.

The contrast in surface roughness between bare soil and vegetation

patches impeded run-on to vegetation, whereas the contrast in infiltration rates promotes it. I'm interested in the interplay between these factors – both as a fundamental research question – and in viewing dryland vulnerability through a hydrodynamic lens.

To model these systems, the Saint Venant Equations – also known as the shallow water equations – are a physics-based approach, describing conservation of mass and momentum.

The animations below illustrate storm-driven overland flow on patchily vegetated dryland hillslopes.

Predicting overland flow during storms involves solving the Saint Venant Equations:

This animation shows a model simulation of overland flow on a patchily vegetated dryland hill slope during a single constant-intensity rainstorm. The spatial pattern of vegetation is projected on the horizontal plane below the hillslope.

Colors in the left and center panels show water height and flow velocity as functions of time. The line on the right hand side shows the hillslope hydrograph, that is, the flow rate at the slope outlet.

Next animation illustrates influence of patchiness...

Emulation of the Saint Venant equations with machine learning

The Saint Venant equations are computationally intensive to solve in 2D and difficult to implement without background in fluid mechanics. This project used machine learning – specifically a random forest – to emulate solutions to the Saint Venant equations under a limited set of scenarios. The emulator – trained on a library of SVE simulations – could successfully reproduce spatial patterns of cumulative infiltration and overland flow velocity.

Applications include rapid landscape assessment, landscape restoration and long-term ecohydrological modeling.

Read more: Crompton, O., Sytsma, A., & Thompson, S. (2019). Emulation of the Saint Venant equations enables rapid and accurate predictions of infiltration and overland flow velocity on spatially heterogeneous surfaces. Water Resources Research, 55(8), 7108-7129.