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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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