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Groundwater is the largest available store of global freshwater , upon which more than two billion people rely 2. It is therefore important to quantify the spatiotemporal interactions between groundwater and climate. However, current understanding of the global scale sensitivity of groundwater systems to climate change 3,4-as well as the resulting variation in feedbacks from groundwater to the climate system 5,6-is limited. Here, using groundwater model results in combination with hydrologic datasets, we examine the dynamic timescales of groundwater system responses to climate change. We show that nearly half of global groundwater fluxes could equilibrate with recharge variations due to climate change on human (~100 year) timescales, and that areas where water tables are most sensitive to changes in recharge are also those that have the longest groundwater response times. In particular, groundwater fluxes in arid regions are shown to be less responsive to climate variability than in humid regions. Adaptation strategies must therefore account for the hydraulic memory of groundwater systems which can buffer climate change impacts on water resources in many regions, but may also lead to a long, but initially hidden, legacy of anthropogenic and climatic impacts on river flows and groundwater dependent ecosystems.

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We use a minimally invasive, shallow geophysical technique to image the structure of the criti cal zone from surface to bedrock (0-20 m) in two small drainages within the Boulder Creek Criti cal Zone Observatory (BcCZO). Shallow seismic refracti on (SSR) surveys provide a three-dimensional network of two-dimensional cross-secti ons (termed quasi-3D) of criti cal zone compressional wave velocity (V p ) structure within each catchment, yielding a spati al descripti on of the current criti cal zone structure. The two catchments, Betasso and Gordon Gulch, represent contrasti ng geomorphic histories within the Front Range: Betasso shows hillslope response to a late Cenozoic increase in fl uvial incision of Boulder Creek, while Gordon Gulch represents more steady erosion. The mean depth to fresh bedrock in both catchments is roughly 15 m. Unique subsurface features in each catchment refl ect acti ve geomorphic processes not suggested by similariti es in mean interface depths. Betasso contains thick disaggregated materials high in the drainage that are nearly absent near the outlet. This presumably refl ects the impact of base-level lowering, which we suggest has progressed roughly 500 to 1000 m up into the catchment. Aspect-driven diff erences in the subsurface within each catchment add complexity and overprint the broader geomorphic signals. Shallow seismic refracti on subsurface structure models will guide future investi gati ons of criti cal zone processes from landscape to hydrologic modeling and are invaluable as connecti ons between ti me-consuming point measurements of physical, chemical, and biological processes.Abbreviati ons: BcCZO, Boulder Creek Criti cal Zone Observatory; ERT, electrical resisti vity tomography; GPR, ground-penetrati ng radar; Ma, megaannum; quasi-3D, a three-dimensional grouping of two-dimensional surveys; SSR, shallow seismic refracti on.Shallow seismic refracti on allows minimally invasive, broad spatial investigations of the subsurface (Leopold et al., 2008b). We use this geophysical technique to pair surficial evidence of geomorphic processes with physical and chemical weathering signatures interpreted from SSR surveys.Th e interactions between weathering and transport processes sculpt terrestrial landscapes. Hillslope processes that include downslope movement of material by rain splash, frost creep, biological activity, and other gravity-driven mechanisms serve to redistribute material that is freed from the underlying bedrock by weathering processes. Th e hillslopes are in turn coupled to adjacent streams, which serve as their boundary conditions. Th is highly coupled geomorphic system therefore encompasses the majority of hydrological, geochemical, and biological activities that can all change in effi ciency and in dominance through time.Recently, the term critical zone (CZ) has been applied to the shallow terrestrial environment spanning from the lowest extent of groundwater to the top of the vegetation canopy (Brantley et al., 2007;Anderson et al., 2007) (Fig. 1). Th e defi nition...

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The northwest Pacific Ocean is a hot spot for sea level rise and increasing frequency of stronger storms. It is where Supertyphoon Haiyan formed, the strongest storm to hit land, which provided a window into the hydrologic impacts of an extreme storm. Through detailed documentation of flood levels, groundwater table elevations and salinity, electrical resistivity, and modeling, we found that Haiyan's storm surge reached 7 m above sea level along Samar Island, Philippines, which led to contamination of crucial aquifers by infiltrating seawater. A contaminated surficial aquifer will take years to recover. Groundwater in an underlying deeper aquifer saw widespread contamination immediately after the storm, but here salinity has decreased significantly after 8 months. However, this deeper aquifer remains vulnerable to seawater slowly percolating through the surficial aquifer. As warmer seas generate more powerful storms, the vulnerability of aquifers to persistent contamination from intense storm surges is a growing concern for coastal communities.

[1] Intertidal zones are spatially complex and temporally dynamic environments. Coastal groundwater discharge, including submarine groundwater discharge, may provide stabilizing conditions for intertidal zone permeable sediments. In this study, we integrated detailed time series temperature observations, porewater pressure measurements, and two-dimensional electrical resistivity tomography profiles to understand the coupled hydraulic-thermal regime of a tropical sandy intertidal zone in a fringing coral reef lagoon (Rarotonga, Cook Islands). We found three heating patterns across the 15 m study transect over tidal and diel periods: (1) a highly variable thermal regime dominated by swash infiltration and changes in saturation state in the upper foreshore with net heat import into the sediment, (2) a groundwater-supported underground stable, cool region just seaward of the intertidal slope break also importing heat into the subsurface, and (3) a zone of seawater recirculation that sustained consistently warm subsurface temperatures that exported heat across the sediment-water interface. Simple calculations suggested thermal conduction as the main heat transport mechanism for the shallow intertidal sediment, but deeper and/or multidimensional groundwater flow was required to explain temperature patterns beyond 20 cm depth. Temperature differences between the distinct hydrodynamic zones of the foreshore site resulted in significant thermal gradients that persisted beyond tidal and diel periods. The thermal buffering of intertidal zones by coastal groundwater systems, both at surface seeps and in the shallow subsurface, can be responsible for thermal refugia for some coastal organisms and hotspots for biogeochemical reactions.