Earth System Models (ESMs) are essential tools for understanding and predicting global change, but they cannot explicitly resolve hillslope‐scale terrain structures that fundamentally organize water, energy, and biogeochemical stores and fluxes at subgrid scales. Here we bring together hydrologists, Critical Zone scientists, and ESM developers, to explore how hillslope structures may modulate ESM grid‐level water, energy, and biogeochemical fluxes. In contrast to the one‐dimensional (1‐D), 2‐ to 3‐m deep, and free‐draining soil hydrology in most ESM land models, we hypothesize that 3‐D, lateral ridge‐to‐valley flow through shallow and deep paths and insolation contrasts between sunny and shady slopes are the top two globally quantifiable organizers of water and energy (and vegetation) within an ESM grid cell. We hypothesize that these two processes are likely to impact ESM predictions where (and when) water and/or energy are limiting. We further hypothesize that, if implemented in ESM land models, these processes will increase simulated continental water storage and residence time, buffering terrestrial ecosystems against seasonal and interannual droughts. We explore efficient ways to capture these mechanisms in ESMs and identify critical knowledge gaps preventing us from scaling up hillslope to global processes. One such gap is our extremely limited knowledge of the subsurface, where water is stored (supporting vegetation) and released to stream baseflow (supporting aquatic ecosystems). We conclude with a set of organizing hypotheses and a call for global syntheses activities and model experiments to assess the impact of hillslope hydrology on global change predictions.

Soil biota generates carbon that exports vertically to the atmosphere (CO₂) and transports laterally to streams and rivers (dissolved organic and inorganic carbon, DOC and DIC). These processes, together with chemical weathering, vary with flow paths across hydrological regimes; yet an integrated understanding of these interactive processes is still lacking. Here we ask: How and to what extent do subsurface carbon transformation, chemical weathering, and solute export differ across hydrological and subsurface structure regimes? We address this question using a hillslope reactive transport model calibrated using soil CO₂ and water chemistry data from Fitch, a temperate forest at the ecotone boundary of the Eastern temperate forest and mid‐continent grasslands in Kansas, USA. Model results show that droughts (discharge at 0.08 mm/day) promoted deeper flow paths, longer water transit time, carbonate precipitation, and mineralization of organic carbon (OC) into inorganic carbon (IC) (∼98% of OC). Of the IC produced, ∼86% was emitted upward as CO₂ gas and ∼14% was exported laterally as DIC into the stream. Storms (8.0 mm/day) led to carbonate dissolution but reduced OC mineralization (∼88% of OC) and promoted DOC production (∼12% of OC) and lateral fluxes of IC (∼53% of produced IC). Differences in shallow‐versus‐deep permeability contrasts led to smaller difference (<10%) than discharge‐induced differences and were most pronounced under wet conditions. High permeability contrasts (low vertical connectivity) enhanced lateral fluxes. Model results generally delineate hillslopes as active CO₂ producers and vertical carbon transporters under dry conditions, and as active DOC producers and lateral carbon transporter under wet conditions.

This study investigates causes behind correlations between snow and terrain properties in a 27 km² mountain watershed. Whereas terrain correlations reveal where snow resides, the physical processes responsible for correlations can be ambiguous. We conducted biweekly snow surveys at small transect scales to provide insight into late‐season correlations at the basin scale. The evolving parameters of transect variograms reveal the interplay between differential accumulation and differential ablation that is responsible for correlations between snow and terrain properties including elevation, aspect, and canopy density. Elevation‐induced differential accumulation imposes a persistent source of varariabity at the basin scale, but is not sufficient to explain the elevational distribution of snow water equivalent (SWE) on the ground. Differential ablation, with earlier and more frequent ablation at lower elevations, steepens the SWE‐elevation gradient through the season. Correlations with aspect are primarily controlled by differences in solar loading. Aspect related redistribution of precipitation by wind, however, is important early in the season. Forested sites hold more snow than nonforested sites at the basin scale due to differences in ablation processes, while open areas within forested sites hold more snow than covered areas due to interception. However, as the season progresses energetic differences between open and covered areas within forested sites cause differences induced by interception to diminish. Results of this study can help determine which accumulation and ablation processes must be represented explicitly and which can be parameterized in models of snow dynamics.