[1] Permeability, the ease of fluid flow through porous rocks and soils, is a fundamental but often poorly quantified component in the analysis of regional-scale water fluxes. Permeability is difficult to quantify because it varies over more than 13 orders of magnitude and is heterogeneous and dependent on flow direction. Indeed, at the regional scale, maps of permeability only exist for soil to depths of 1-2 m. Here we use an extensive compilation of results from hydrogeologic models to show that regional-scale (>5 km) permeability of consolidated and unconsolidated geologic units below soil horizons (hydrolithologies) can be characterized in a statistically meaningful way. The representative permeabilities of these hydrolithologies are used to map the distribution of near-surface (on the order of 100 m depth) permeability globally and over North America. The distribution of each hydrolithology is generally scale independent. The near-surface mean permeability is of the order of ∼5 × 10 −14 m 2 . The results provide the first global picture of near-surface permeability and will be of particular value for evaluating global water resources and modeling the influence of climate-surface-subsurface interactions on global climate change. Citation: Gleeson, T

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[1] This study uses numerical simulations to define the salient controls on regional groundwater flow in 3-D mountainous terrain by systematically varying topographic and hydrogeologic variables. Topography for idealized multiple-basin mountainous terrain is derived from geomatic data and literature values. Water table elevation, controlled by the ratio of recharge to hydraulic conductivity, largely controls the distribution of recharged water into local, regional, and perpendicular flow systems, perpendicular flow being perpendicular to the regional topographic gradient. Both the relative (%) and absolute (m 3 /d) values of regional flow and perpendicular flow are examined. The relationship between regional flow and water table elevation is highly nonlinear. With lower water table elevations, relative and absolute regional flow dramatically increase and decrease, respectively, as the water table is lowered further. However, for higher water table elevations above the top of the headwater stream, changes in water table elevation have little effect on regional flow. Local flow predominates in high water table configurations, with regional and perpendicular flow <15% and <10%, respectively, of total recharge in the models tested. Both the relative and the maximum absolute regional flow are directly controlled by the degree of incision of the mountain drainage network; the elevation of mountain ridges is considerably less important. The percentage of the headwater stream with perennial streamflow is a potentially powerful indicator of regional flow in all water table configurations and may be a good indicator of the susceptibility of mountain groundwater systems to increased aridity.

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Water tables at regional to continental scales can be classified into two distinct types: recharge‐controlled water tables that are largely disconnected from topography and topography‐controlled water tables that are closely tied to topography. We use geomatic synthesis of hydrologic, geologic and topographic data sets to quantify and map water‐table type over the contiguous United States using a dimensionless criterion introduced by Haitjema and Mitchell‐Bruker (2005), called the water‐table ratio, which differentiates water‐table type. Our analysis indicates that specific regions of the United States have broadly contiguous and characteristic water‐table types. Water‐table ratio relates to water‐table depth and the potential for regional groundwater flow. In regions with recharge‐controlled water tables, for example the Southwest or Rocky Mountains, USA, water‐tables depths are generally greater and more variable and regional groundwater flow is generally more important as a percentage of the watershed budget. Water‐table depths are generally shallow and less variable, and regional groundwater flow is limited in areas with topography‐controlled water tables such as the Northeast USA. The water‐table ratio is a simple but powerful criterion for evaluating regional groundwater systems over broad areas.