Rain splash of soil grains as a stochastic advection‐dispersion process, with implications for desert plant‐soil interactions and land‐surface evolution

Abstract

We formulate soil grain transport by rain splash as a stochastic advection‐dispersion process. By taking into account the intermittency of grain motions activated by raindrop impacts, the formulation indicates that gradients in raindrop intensity, and thus grain activity (the volume of grains in motion per unit area) can be as important as gradients in grain concentration and surface slope in effecting transport. This idea is confirmed by rain splash experiments and manifest in topographic roughening via mound growth beneath desert shrubs. The formulation provides a framework for describing transport and dispersal of any soil material moveable by rain splash, including soil grains, soil‐borne pathogens and nutrients, seeds, or debitage. As such it shows how classic models of topographic “diffusion” reflect effects of slope‐dependent grain drift, not diffusion, and it highlights the role of rain splash in the ecological behavior of desert shrubs as “resource islands.” Specifically, the growth of mounds beneath shrub canopies, where differential rain splash initially causes more grains to be splashed inward beneath the protective canopy than outward, involves the “harvesting” of nearby soil material, including nutrients. Mounds thus represent temporary storage of soil derived from areas surrounding the shrubs. As the inward grain flux associated with differential rain splash is sustained over the shrub lifetime, mound material is effectively sequestered from erosional processes that might otherwise move this material downslope. With shrub death and loss of the protective canopy, differential rain splash vanishes and the mound material is dispersed to the surrounding area, again subject to downslope movement.

Critiques of the seismic hypothesis and the vegetation stabilization hypothesis for the formation of Mima mounds along the western coast of the U.S.

“…According to this line of reasoning, plants can lead to the formation of mounds by trapping sediment and focusing aggradation (e.g., coppice dunes) or by locally inhibiting erosion and thereby forming mounds through the removal of material between patches of vegetation. Some Mima-like mounds may form through these processes (e.g., Furbish et al, 2009;Cramer and Barger, 2014), but several lines of observational evidence indicate that these mechanisms cannot explain the genesis of the mounds that we studied in the western U.S.…”

Impact of small-scale vegetation structure on tephra layer preservation

“…5b). In addition to aeolian processes, the differential impact of rain-splash under variable canopy cover may influence tephra layer morphology at metre-scales11.…”

“…Recent research has demonstrated that vegetation structure can influence mean tephra layer thickness on a landscape scale111. Given that most land surfaces have some sort of vegetation cover, a greater understanding of the interaction between plants and tephra deposits would have global utility in the interpretation of tephra layers and their origins.…”

Impact of consecutive extreme rainstorm events on particle transport: Case study in a Sonoran Desert range, western USA

“…The reason for such small drainages is an attempt to capture grus being detached from ridge crests, whether by raindrops (Furbish et al, 2009) or rain-impacted flow (Parsons et al, 1998), and then transported a few meters by the inter rill overland flow (Parsons et al, 1991). Field observations during intense rainstorms indicated that these traps captured grus detached by rainsplash and moved short distances by subsequent overland flow.…”

“…Field observations during precipitation events indicate that short-distance transportation of a few meters or less occurred from inter rill flow after detachment (Parsons et al, 1991). Direct observations of detachment did not involve high-speed photography, and thus a determination could not be made whether ejection was from raindrops (Erpul et al, 2005;Furbish et al, 2009) or rain-impacted flow (Parsons et al, 1998). Table 2 Percent porosity of granitic materials in pits dug near sediment traps A1-7, B1-7, C1-7, and D1-7.…”