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A numerical study on the influence of mìcrotopography on raìnfall-runoff-infiltration partitioning
(2017)
Microtopographic features, although minute relative to the hillslope scales, are not insignificant in terms of runoff generation, rain-runoff-infiltration partitioning and overall hillslope hydrological signals. As-sessing the effects of such small scale features, arguably requires mathematical models that can cope with microtopraphic complexity to adequately represent surface water dynamics, which in turn deter-mine hydrological signals at the hillslope scale. In this work, rain-runoff simulations are perfomed with a 2D shallow water model on a rectangular domain representing a hillslope with an idealized 2D sinusoidal microtopography. Several combinations of slope, wavelength and amplitudes were used to create over 500 surfaces on which simulations were performed in order to assess their hydrological response in terms of rainfallrunoff-infiltration partitioning. The results were analysed through several dimensionless indices which allow to observe the dependency of characteristic hydrological responses to mi-crotopography properties. They reveal a complex dependency of hydrological signatures to surface microtopography. In particular, the results show that the fraction of rainfall that results in infiltration is increased following a particular non-linear dependency on surface smoothness. Additionally, hydrograph properties and surface flow connectivity also show emerging patterns in response to microtopography.
Microtopography (MT) can govern runoff dynamics as a net result of local heterogeneities in the flow paths and ponding. This in turn controls the development of the surface water layer that connects and flows downslope. It is therefore important to understand which microtopographic features affect runoff generation dynamics and its macroscopic—hillslope scale—hydrological signatures (e.g., hydrographs, runoff and infiltration volumes). In this study, we numerically solve 2D overland flow from a single rain pulse on 1,460 idealized hillslopes with different slopes and sinusoidal microtopographies and different infiltration capacities. We assess hydrodynamic distributions, hydrographs and hydrological indices to assess the effects of MT and infiltration on the (local) hydrodynamic and (larger scale) hydrologic responses in terms of surface runoff regimes. The results show that MT enhances infiltration and that infiltration and runoff depend in a strong non-linear way on slope and the properties of MT. Three regimes of influence of MT were identified: one in which MT plays a negligible role but there is a high sensitivity to the infiltration capacity curve, a second regime in which hydrological partitioning is highly sensitive to MT and the infiltration capacity curve, and a third regime in which MT increases infiltration, but the response is insensitive to particular features, and more affected by the average slopes. The regimes are the product of the interplay between small (MT) and large scale (slope) properties. Furthermore, the results suggest that hydrological signatures can be interpreted and explained by the spatiotemporal variation of surface connectivity.
In terrestrial systems limited by water availability the spatial distribution of vegetation
can self-organize into a mosaic of vegetated patches and bare soil. Spatially extensive
competition for water and short-range facilitation underpin many models that describe the
process of vegetation pattern formation. Earlier studies investigating this self-organized
patchiness have largely considered smooth landscapes. However, topographic
variations can significantly alter the redistribution of surface water flow and therefore
the pattern-forming process. Here, we consider how microtopographic variations, at
the scale of individual plants, alters self-organized vegetation patterns with the use of a
simple ecohydrological model. We show that increasing microtopography can induce a
change from banded vegetation, oriented across the slope, to irregular drainage patterns,
oriented in the downslope direction. The mechanism responsible is shown to be a change in
the spatial redistribution of infiltration around plants and plant patches. Only small
increases in microtopography are required to cause banded systems with weak facilitation
to change to downslope-oriented patterns. When non-periodic boundary conditions were
considered, band orientation tended to become oblique to the topographic contour and in
some circumstances their migration upslope ceased. These results suggest that diffusive
sediment transport processes may be essential for the maintenance of regular periodic
vegetation patterns, which implies that erosion may be critical for understanding the
susceptibility of these ecosystems to catastrophic shifts.