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Hillslope asymmetry is often attributed to differential eco-hydro-geomorphic processes resulting from aspect-related differences in insolation. At midlatitudes, polar facing hillslopes are steeper, wetter, have denser vegetation, and deeper soils than their equatorial facing counterparts. We propose that at regional scales, the magnitude in insolation-driven hillslope asymmetry is sensitive to variations in climate, and investigate the fire-prone landscapes in southeastern Australia to evaluate this hypothesis. Patterns of asymmetry in soil depth and landform were quantified using soil depth measurements and topographic analysis across a contemporary rainfall gradient. Results show that polar facing hillslopes are steeper, and have greater soil depth, than equatorial facing slopes. Furthermore, we show that the magnitude of this asymmetry varies systematically with aridity index, with a maximum at the transition between water and energy limitation, suggesting a possible long-term role of climate in hillslope development.

Plain Language Summary In midlatitudes, hillslopes facing the pole (polar) tend to be steeper, wetter, and have denser vegetation and deeper soils than hillslope facing the equator (equatorial). This asymmetry is often attributed to different insolation regime between the opposing hillslopes, which cause differences in eco-hydro-geomorphic process that control long-term hillslope evolution. In this study we propose that at regional scales, the magnitude of asymmetry in soil depth and hillslope gradient is sensitive to variations in climate. We investigate this hypothesis by topographic analyses and measurements of soil depth across a contemporary rainfall gradient in southeastern Australia. Our Results show that polar facing hillslopes are steeper, and have deeper soils, than equatorial facing ones. Furthermore, magnitude of asymmetry in both soil depth and hillslope gradient varies nonlinearly with aridity index, with a maximum within a transition between water- and energy-limited climatic domains. Landscape analysis shows that dry systems experience higher erosion rates, supporting evidence of postfire debris flows in contemporary dry systems. Based on published data, we propose that these patterns are likely a result of thresholds in erosion, caused by climatically driven feedback between vegetation, fire, and soil properties.