Asymmetry of weathering‐limited hillslopes: the importance of diurnal covariation in solar insolation and temperature

Abstract

Hillslope asymmetry, i.e. variation in hillslope form as a function of slope aspect and/or mean solar insolation, has been documented in many climates and geologic contexts. Such patterns have the potential to help us better understand the hydrologic, ecologic, and geomorphologic processes and feedbacks operating on hillslopes. Here we document asymmetry in the fraction of hillslope relief accommodated by cliffs in weathering‐limited hillslopes of drainage basins incised into the East Kaibab Monocline (northern Arizona) and Raplee Ridge Monocline (southern Utah) of the southern Colorado Plateau. We document that south‐ and west‐facing hillslopes have a larger proportion of hillslope relief accommodated by cliffs compared with north‐ and east‐facing hillslopes. Cliff abundance correlates positively with mean solar insolation and, by inference, negatively with soil/rock moisture. Solar insolation control of hillslope asymmetry is an incomplete explanation, however, because it cannot account for the fact that the greatest asymmetry occurs between southwest‐ and northeast‐facing hillslopes rather than between south‐ and north‐facing hillslopes in the study sites. Modeling results suggest that southwest‐facing hillslopes are more cliff‐dominated than southeast‐facing hillslopes of the same mean solar insolation in part because potential evapotranspiration rates, which control the soil/rock moisture that drives weathering, are controlled by the product of solar insolation and a nonlinear function of surface temperature, together with the fact that southwest‐facing hillslopes receive peak solar insolation during warmer times of day compared with southeast‐facing hillslopes. The dependence of water availability on both solar insolation and surface temperature highlights the importance of the diurnal cycle in controlling water availability, and it provides a general explanation for the fact that vegetation cover tends to exhibit the greatest difference between northeast‐ and southwest‐facing hillslopes in the Northern Hemisphere and between southeast‐ and northwest‐facing hillslopes in the Southern Hemisphere. Copyright © 2017 John Wiley & Sons, Ltd.

Climate Dictates Magnitude of Asymmetry in Soil Depth and Hillslope Gradient

“…Our results from the eastern uplands support these observations. The fact that asymmetry persists across a variety of geological structures points to climate, and insolation in particular, as being the underlying cause (Pelletier & Swetnam, 2017;Poulos et al, 2012;Yetemen et al, 2015). The fact that asymmetry persists across a variety of geological structures points to climate, and insolation in particular, as being the underlying cause (Pelletier & Swetnam, 2017;Poulos et al, 2012;Yetemen et al, 2015).…”

Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks

“…There is growing evidence under a wide range of climates that hillslope diffusivity (which quantifies the efficiency of sediment transport) increases with mean‐annual precipitation (Hanks, ; Hurst et al ., ; Richardson, ) such that hillslope diffusivities tend to be largest in humid forested ecosystems that undergo more bioturbation than semiarid grass and shrub dominated ecosystems (Martin, ; Gabet and Dunne, ; Roering, ; Hughes et al ., ). Besides regional climate, field studies have revealed aspect‐modulated micro‐climate controls on hillslope diffusion (West et al ., ) and topographic attributes related to bedrock weathering rates (Burnett et al ., ; Pelletier and Swetnam, ). For example, West et al .…”

Hillslope response under variable microclimate

“…North‐facing slopes compared to south‐facing slopes tend to remain relatively moist with relatively dense vegetation cover over the year (Burnett et al ., ; McGuire et al ., ; Ebel et al, ; Pelletier et al ., ). This results in south‐facing slopes tending to have relatively higher rates of weathering and somewhat different soil‐forming processes (Pelletier and Swetnam, ; Pelletier et al ., ). Vegetation contributes to the development of a thick soil profile consisting of organic‐rich horizons (A‐horizons) with relatively fine‐grained texture and well‐developed structure (Kay and Lal, ; Bullard and McDonald, ).…”

“…Understanding these interactions is central to modelling sediment flux, erosion, rates of fluvial incision, rate at which sediment is delivered to depositional basins and landscape evolution (Sklar and Dietrich, ; Duller et al ., ; Tucker and Hancock, ; Armitage et al, ; Pelletier et al ., ; McGuire et al, ; Regmi et al ., ). Numerous studies have recognized that aspect‐controlled variation in microclimate can drive differences in moisture and energy balance which influence hydrology, soil and vegetation development, sediment transport and ultimately the hillslope morphology in various climates (Kessler et al ., ; Istanbulluoglu et al, ; Poulos et al, ; McGuire et al ., ; Yetemen et al, , ; Pelletier and Swetnam, ; Inbar et al, ; Pelletier et al ., ). However, this mechanism and associated responses, particularly mass movement and erosion, are not well understood in highly disturbed coastal ecosystems.…”