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.

One of the principal questions in hydrology is how and when water leaves the critical zone storage as either stream flow or evapotranspiration. We investigated subsurface water storage and storage selection of the Southern Sierra Critical Zone Observatory (California, USA) within the age‐ranked storage selection framework, constrained by a novel combination of cosmogenic radioactive and stable isotopes: tritium, sodium‐22, sulfur‐35, and oxygen‐18. We found a significant positive correlation between tritium and stream flow rate and between sulfur‐35 and stream flow rate, indicating that the age distribution of stream flow varies with stream flow rate. Storage selection functions that vary with stream flow rate are better able to reproduce tritium concentrations in stream flow than functions that are constant in time. For the Southern Sierra Critical Zone, there is a strong preference to discharge the oldest water in storage during dry conditions but only a weak preference for younger water during wet conditions. The preference of evapotranspiration for younger water, constrained by oxygen‐18 in stream water, is essential to parameterize subsurface storage but needs to be confirmed by isotopic or other investigations of evapotranspiration. This is the first study to illustrate how a combination of cosmogenic radioactive isotopes reveals the hydrochronology and water storage dynamics of catchments, constrains the subsurface architecture of the critical zone, and provides insight into landscape evolution.

In the western United States, climate warming poses a unique threat to water and snow hydrology because much of the snowpack accumulates at temperatures near 0 °C. As the climate continues to warm, much of the region's precipitation is expected to switch from snow to rain, causing flashier hydrographs, earlier inflow to reservoirs, and reduced spring and summer snowpack. This study investigates historical variability in snow to precipitation proportion (S_f) and maps areas in the western United States that have demonstrated higher S_f sensitivity to warming in the past. Projected changes in S_f under 1.1, 1.8, and 3.0 °C future warming scenarios are presented in relation to historical variability and sensitivity. Our findings suggest that S_f in this region has primarily varied based on winter temperature rather than precipitation. The difference in S_f between cold and warm winters at low‐ and mid‐elevations during 1916–2003 ranged from 31% in the Pacific Northwest to 40% in the California Sierra Nevada. In contrast, the difference in S_f between wet and dry winters was statistically not significant. Overall, in the northern Sierra, Klamath, and western slopes of the Cascade Mountains Ranges, S_f was most sensitive to temperature where winter temperature ranged between −5 to 5 °C. Results from our trend analysis show a regional shift in both S_f and signal‐to‐noise ratios during 1960–2003 as compared with 1916–2003. Our findings indicate that natural variability in S_f over 1916–2003 across all regions except for the Great Basin most closely resembles the projected 2040‐warming scenario (+1.8 °C).

Despite a multitude of small catchment studies, we lack a deep understanding of how variations in critical zone architecture lead to variations in hydrologic states and fluxes. This study characterizes hydrologic dynamics of 15 catchments of the U.S. Critical Zone Observatory (CZO) network where we hypothesized that our understanding of subsurface structure would illuminate patterns of hydrologic partitioning. The CZOs collect data sets that characterize the physical, chemical, and biological architecture of the subsurface, while also monitoring hydrologic fluxes such as streamflow, precipitation, and evapotranspiration. For the first time, we collate time series of hydrologic variables across the CZO network and begin the process of examining hydrologic signatures across sites. We find that catchments with low baseflow indices and high runoff sensitivity to storage receive most of their precipitation as rain and contain clay‐rich regolith profiles, prominent argillic horizons, and/or anthropogenic modifications. In contrast, sites with high baseflow indices and low runoff sensitivity to storage receive the majority of precipitation as snow and have more permeable regolith profiles. The seasonal variability of water balance components is a key control on the dynamic range of hydraulically connected water in the critical zone. These findings lead us to posit that water balance partitioning and streamflow hydraulics are linked through the coevolution of critical zone architecture but that much work remains to parse these controls out quantitatively.