Solar radiation as a global driver of hillslope asymmetry: Insights from an ecogeomorphic landscape evolution model

Abstract

Observations at the field, catchment, and continental scales across a range of arid and semiarid climates and latitudes reveal aspect-controlled patterns in soil properties, vegetation types, ecohydrologic fluxes, and hillslope morphology. Although the global distribution of solar radiation on earth's surface and its implications on vegetation dynamics are well documented, we know little about how variation of solar radiation across latitudes influence landscape evolution and resulting geomorphic difference. Here, we used a landscape evolution model that couples the continuity equations for water, sediment, and aboveground vegetation biomass at each model element in order to explore the controls of latitude and mean annual precipitation (MAP) on the development of hillslope asymmetry (HA). In our model, asymmetric hillslopes emerged from the competition between soil creep and vegetation-modulated fluvial transport, driven by spatial distribution of solar radiation. Latitude was a primary driver of HA because of its effects on the global distribution of solar radiation. In the Northern Hemisphere, north-facing slopes (NFS), which support more vegetation cover and have lower transport efficiency, get steeper toward the North Pole while south-facing slopes (SFS) get gentler. In the Southern Hemisphere, the patterns are reversed and SFS get steeper toward the South Pole. For any given latitude, MAP is found to have minor control on HA. Our results underscore the potential influence of solar radiation as a global control on the development of asymmetric hillslopes in fluvial landscapes.

The Influence of Lithology on the Forms of Hillslopes and Drainage Basins in Soil Mantled Landscapes

“…While the configuration of drainage networks displays non-random components in a variety of landscapes surveyed here, it is only in sites in weak rocks where a strong north-south asymmetry in the density and length of low-order tributaries is observed. Past studies have focused on the role of contrasts in vegetation with aspect in driving asymmetry in channel networks (Istanbulluoglu et al, 2008;McGuire et al, 2014;Yetemen et al, 2015) and these certainly drive differences in landscape evolution. However, using remotely-sensed proxies for vegetation productivity, I demonstrate that all the surveyed sites are formed in climates that favor contrasts in vegetation with aspect.…”

“…Aspect-dependent differences in landscape morphology are not uncommon, and barring tectonic or geologic control are typically attributed to variations in the relative intensity of different geomorphic processes across the microclimatic gradients that arise on opposing slopes due to variations in incoming solar radiation (Bass, 1929;Istanbulluoglu et al, 2008;McGuire et al, 2014;West et al, 2014;Yetemen et al, 2015). Early work was divided on the cause of topographic asymmetry in the Gabilan…”

“…The production of asymmetric topography as a result of aspect-controlled microclimates might be viewed as the result of one of two general mechanisms. In one model, referred to here as locally driven asymmetry, differences in the relative intensity of different erosion and transport processes operating at the hillslope scale produce the majority of the asymmetry seen in landscape morphology (Bass, 1929;Istanbulluoglu et al, 2008;McGuire et al, 2014;West et al, 2014;Yetemen et al, 2015). Alternatively, these initial contrasts in erosion may yield aspect-dependent discrepancies in the flux of sediment to trunk channels, favoring lateral migration of trunk channels in a particular direction and greatly exacerbating asymmetry (Melton, 1960).…”

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

“…A better understanding of the causes of hillslope asymmetry has the potential to help identify the dominant processes and feedback mechanisms acting on hillslopes, since spatial variations in solar insolation affect different processes in different ways. The aspect and gradient of a hillslope, in addition to the presence/absence of surrounding topography that cause shading, controls solar insolation and hence local soil temperatures and evapotranspiration rates (Grayson et al ., ; Williams et al ., ; Gutiérrez‐Jurado and Vivoni, , b; Hinckley et al ., ; Yetemen et al ., , b). If precipitation rates are approximately uniform at a study site, higher potential evapotranspiration rates on south‐facing hillslopes result in less soil/rock moisture available to drive many weathering (defined herein as the breakdown of bedrock into transportable material) and sediment transport processes.…”

Ecohydrology in space and time: functioning and changes in the streamflow generation of catchments

“…Over the long-term, these soil physical properties have been primarily developed in response to the moisture availability dictated by the catchment water and energy balances (Berry et al, 2006;Yetemen et al, 2015).…”