Coupled effects of microtopography and time-dependant infiltration capacity on rainfall-runoff-infiltration partitioning on a hillslope

The onset and generation of runoff, and the overall rainfall-runoff transformation, resulting in hillslope and catchment runoff response, are controlled by multiple interacting small-scale processes. Small scale features such as surface microtopography -small variations around the average terrain shape- can govern large scale signatures of runoff dynamics. This is the net result of local heterogeneities in the flow paths and ponding which in turn control the development of the surface water layer and how it connects and flows downslope. It is therefore relevant to understand which microtopographic features may play a governing role in runoff generation dynamics. Given that it is very difficult to assess such processes experimentally in the field, we turn to computational modelling to assess different features, hydrological conditions and the overall response.

 

In this work, we numerically solve a physically-based surface water model (based on the Zero-Intertia approximation of the shallow-water equations) on an idealised hillslope domain, forced by a single pulse of rain. To explore different topographies and microtopographies, we model 1460 surfaces, based on 10 sloping planes (from 0.1% to 10%) on which a sinusoidal microtopography of various amplitudes (from 1 to 10 cm) and wavelengths (from 15 to 200 cm) is overlaid. In a previous proof-of-concept work, we showed how these microtopograhies have an impact on rainfall-runoff-infiltration partitioning and generate different runoff regimes from disconnected flow to steady sheet flow. In this contribution, we extend our analysis to include a more realistic, time-dependent infiltration capacity, and therefore explore the effects this has in the process of ponding and establishing surface flow connectivity. We extend the number of surfaces (within the same ranges) to better observe the different runoff regimes. We quantitatively assess the results mainly in terms of the increase in total infiltration in the presence of microtopography relative to a smooth plane, and qualitatively in terms of the generated runoff regimes.

 

The results show that microtopography increases total infiltration (up to six times in our simulations) over the whole domain relative to a smooth plane and there is a strong non-linear dependency of infiltration and runoff on slope and on the ratio of the characteristic wavelength and amplitude of microtopography. Moreover, three characteristic regimes of influence of microtopography exist: one in which microtopography plays a negligible role, another in which microtopography increases infiltration, but the particular microtopography features are not very relevant, and one regime in which small changes in microtopography generate significant variations on infiltration. Such regimes are the result of the interplay between small (microtopography) and large scale (slope) system features. Finally, the results also show that the time-dependent infiltration capacity can enhance the effect of microtopography on infiltration. From a modelling perspective, these results hint that neglecting microtopography and time-dependent infiltration in hydrological modelling can lead to an underestimation of infiltration and an overestimation of runoff. The coupled analysis of spatial hydrodynamics and hydrological signatures suggests that the latter can be interpreted and explained by the spatiotemporal variations triggered by surface connectivity.