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Abstract Changes in the properties of rainfall distributions at sub‐daily scales are key to assessing soil erosion rates under climate transition. However, such changes are difficult to detect and model, especially over landscape evolution timescales. In this contribution, we validate a new catchment‐scale landscape evolution model against event‐scale runoff and sediment records. Through multi‐century numerical experiments, we also show that changes in the sub‐daily rainfall distribution, like those observed under modern climate change, can increase soil erosion rates by 40% but cannot be accurately inferred from changes in the average event properties and total rainfall. We quantify erosion and topographic trajectories associated with plausible changes in the sub‐daily rainfall distribution, highlighting scenarios in which shifting tail properties impact landscape evolution, at times, contrary to expectations based on changes in total rainfall. 

Abstract Large dendritic valley networks observed on Mars present a paleoclimate paradox. Geologic observations of Noachian units on Mars reveal a global extent of valley networks, which are believed to have been formed through incisions made by flowing water. However, most climate models predict global surface temperatures too far below the freezing point of water to support an active hydrological system. Conflicting observations and models have led to disparate theories for the climate of early Mars. In this work, we surveyed a large region of the cratered southern highlands to identify the location, elevation, and distribution of observed valley heads. These valley head locations were compared to landscape evolution simulations in which the spatial distribution of runoff was varied. The measured valley head distributions were compared to predictions from landscape evolution models for two end‐member hypotheses: (a) a warm wet climate that supported spatially distributed precipitation, and (b) surface runoff from ice cap margins, as envisioned by the Late Noachian Icy Highland model (LNIH). The observed elevation distribution in valley heads is consistent with the prediction of precipitation‐fed models, and inconsistent with models in which runoff derives exclusively from a single line‐source of high‐elevation ice‐melt. The results support the view that it is unlikely for ice caps to be the sole source of water and are consistent with the hypothesis that precipitation significantly contributed to valley network formation on ancient Mars. 

Abstract Features of landscape morphology—including slope, curvature, and drainage dissection—are important controls on runoff generation in upland landscapes. Over long timescales, runoff plays an essential role in shaping these same features through surface erosion. This feedback between erosion and runoff generation suggests that modeling long‐term landscape evolution together with dynamic runoff generation could provide insight into hydrological function. Here we examine the emergence of variable source area runoff generation in a new coupled hydro‐geomorphic model that accounts for water balance partitioning between surface flow, subsurface flow, and evapotranspiration as landscapes evolve over millions of years. We derive a minimal set of dimensionless numbers that provide insight into how hydrologic and geomorphic parameters together affect landscapes. Across the parameter space we investigated, model results collapsed to a single inverse relationship between the dimensionless relief and the ratio of catchment quickflow to discharge. Furthermore, we found an inverse relationship between the Hillslope number, which describes topographic relief relative to aquifer thickness, and the proportion of the landscape that was variably saturated. While the model generally produces fluvial topography visually similar to simpler landscape evolution models, certain parameter combinations produce wide valley bottom wetlands and non‐dendritic, trellis‐like drainage networks, which may reflect real conditions in some landscapes where aquifer gradients become decoupled from topography. With these results, we demonstrate the power of hydro‐geomorphic models for generating new insights into hydrological processes, and also suggest that subsurface hydrology may be integral for modeling aspects of long‐term landscape evolution. 

Significance Soil mixing occurs when sediment and organic matter are moved by biotic processes (burrowing mammals and invertebrates, root growth, and decay) and abiotic processes (movement of water and granular creep). Soil mixing plays a key role in carbon storage, nutrient and contaminant dispersion, and hillslope sediment transport, yet we lack quantitative information about soil mixing and its variation with depth in a soil profile. Here we present evidence showing that soil mixing systematically varies with depth in the soil. We also find that soils appear to be mixed similarly across climate zones, suggesting that soil mixing operates in a similar way over a range of different environments.