Recharge observations indicate strengthened groundwater connection to surface fluxes

Groundwater is an invaluable global resource, but its long-term viability as a resource for consumption, agriculture, and ecosystems depends on precipitation recharging aquifers. How much precipitation recharges groundwaters varies enormously across Earth's surface, but recharge rates are uncertain because field observations are sparse and modeled global estimates remain largely unvalidated. Here we show that recharge is predictable as a simple function of climatic aridity — the ratio of long-term potential evapotranspiration to precipitation — using a global synthesis of measured recharge of 5237 sites across six continents. We use this relationship to estimate long-term recharge globally outside of permafrost regions. Our estimates double previous global estimates and are more consistent with empirical observations. These revised higher estimates of global groundwater recharge imply that much more groundwater must contribute to evapotranspiration and streamflow than previously represented in global water cycle depictions or global hydrological and Earth system models.

New species of xeromorphic Banksia (Proteaceae) foliage and Banksia-like pollen from the late Eocene of Western Australia

Banksia microphylla leaf fossils and Banksieaeidites zanthus pollen are newly described from late Eocene lignite of the Zanthus-11 borehole, drilled east of Norseman in Western Australia. The leaf fossils are the first known in Banksia to show extreme narrowness (<1.5 mm wide) combined with the xeromorphic trait of margins rolled onto the lower surface so that the diffusely placed stomata are exposed to the outside environment only via grooves on each side of a thick, abaxial midrib. Both this Banksia leaf type and another with encrypted stomata evolved before the widespread initiation of severe climatic aridity in the late Neogene, likely in regions of edaphic infertility and periodic water stress. New interpretations of leaf morphology and foliar evolutionary pathways in Banksia are proposed. Banksia microphylla probably belongs to subgenus Spathulatae, where it strongly resembles many species in the large, wholly Western Australian clade that includes most species in section Oncostylis, series Abietinae. Banksieaeidites zanthus is morphologically consistent with Banksia pollen, and its extremely small size also suggests placement in Spathulatae. The new fossils and other evidence from Zanthus-11 indicate the local presence of quite open, sclerophyll vegetation with conifers, which was unlikely to have been frequently burnt.

Shapes of river networks

River network scaling laws describe how their shape varies with their size. However, the regional variation of this size-dependence remains poorly understood. Here we show that river network scaling laws vary systematically with the climatic aridity index. We find that arid basins do not change their proportions with size, while humid basins do. To explore why, we study an aspect ratio L ⊥ / L ∥ between basin width L ⊥ and basin length L ∥ . We find that the aspect ratio exhibits a dependence on climate and argue that this can be understood as a structural consequence of the confluence angle. We then find that, in humid basins, the aspect ratio decreases with basin size, which we attribute to a common hydrogeological hierarchy. Our results offer an explanation of the variability in network scaling exponents and suggest that the absence of self-similarity in humid basins can be understood as a morphological expression of subsurface processes.