Mark Olaf Cuthbert

Despite growing acceptance of the importance of groundwater use by hominins, there has been little hydrogeological investigation into the controls on groundwater availability as relevant to their evolution and geographic dispersal. This study integrates available fossil and stone tool data with hydrogeological landscape reconstruction and conceptualisation to develop understanding and constraints on the earliest use of groundwater by hominins and early migrations between Africa and Eurasia. It is proposed that the first bipedal human ancestors, appearing in the fossil record in Central and East Africa between 6 and 7 Ma, learned from other mammals how to find and access shallow groundwater in ephemeral streambeds. The first evidence of hominin associations with groundwater discharge from springs is from around 2–4.5 Ma in the East African Rift System (EARS), these locations likely being critically stable hydro-refugia within the driest parts of precession-dominated climatic cycles. The first migrating hominin species appears to have been Homo erectus , with those in the EARS leaving in waves as small bands from around 1.9 Ma at the end of a humid period in the Arabian Peninsula. It is proposed that the most hydrologically plausible routes linking the EARS to Eurasia are via Red Sea crossings and then through the Arabian Peninsula moving along the strike of regional aquifers which would have provided perennial groundwater even over long climatically arid periods. The paper demonstrates the fundamental importance of groundwater for early hominin communities during a critical period in the evolution of human physiology and consciousness.

The spatial and temporal resolution of hydrological models determine model performance and affect water partitioning into fluxes and water storage changes. This is especially important in dryland regions which exhibit dynamic hydrological processes (both spatially and temporally) driven by highly variable and heterogeneous rainfall events, and potential evapotranspiration which exceeds annual precipitation rates. However, it is still unclear how varying model resolutions may affect performance and introduce uncertainty in water balance partitioning. Here, we use a parsimonious dryland‐specific process‐based hydrological model, DRYP, to assess the impact of temporal and spatial resolution on the water balance fluxes and stores in a dryland basin. To achieve this, we performed numerical experiments using three model grid sizes (300 m, 1 km, 5 km) and three time‐steps (15 min, 1 h, 1 day) with four different built‐in infiltration equations (Philips, Green Ampt, Shaake, Upscaled Green Ampt). The analysis was performed in the Walnut Gulch Experimental Watershed, Arizona, USA, where long‐term, high‐resolution datasets are available for both model calibration and evaluation. Results showed that sub‐daily time steps combined with spatial resolution of less than 1‐km grid size yield the best representations (Nash Sutcliffe index, NSE > 0.55) of hydrological fluxes and stores such as streamflow, soil moisture, and actual evapotranspiration. Model parameterization also indicates that coarser spatial and temporal resolutions require lower effective saturated hydraulic conductivity, reflecting the smoothing of rainfall variability associated with spatial and temporal aggregation. The analysis also highlights the importance of near‐channel processes, such as transmission losses in ephemeral streambeds and the role of the riparian zone in providing water for plants and for groundwater recharge. These results have important implications for the quantification of water partitioning in dryland environments in response to rainstorms, and provide guidance for how to parameterize large‐scale models, which requires careful consideration to represent key hydrological processes of interest to the modeller.

Groundwater may be the most important substance on Earth that most people have never heard of, yet it has quietly influenced the evolution of human culture since its earliest beginnings. The predominant physicalist ontology of modernity has created such a powerful conceptual representation of groundwater, that seemingly incredible benefits have accrued to society globally via our ability to understand and relate to it in this mode. However, this modern ontological lens goes unnoticed and unannounced in most groundwater research. This study aims to broaden existing narratives about what groundwater was, is and could be, by exploring how groundwater has come to be perceived, understood and defined by WEIRD (Western, educated, industrialised, rich, democratic) societies in which the author is embedded. By considering the co-evolution of four fields of human-groundwater development, namely - behaviour, systems, psychology, and culture - this paper explores how originally participatory ways of perceiving and knowing groundwater gave way to increasingly abstracted propositional modes of knowledge. Eventually many humans came to believe that the abstract symbols and modelled quantities which they used to represent reality were actually reality itself. Thus ‘modern groundwater’ is now commonly defined and related to as a resource, a ‘thing’ devoid of inherent meaning or value, separate from ourselves, which can be manipulated to our own ends. Greater epistemic reflectivity, and a pivot to working metaphysically with groundwater as a relational process, may lead to more effective responses to some of the global predicaments we currently face.

Drought propagation in dryland environments is often conceptualised as a sequential transfer of meteorological deficits into soil moisture, streamflow, and groundwater drought with the propagation rate controlled by catchment and aquifer properties. However, quantifying the spatial and temporal propagation of rainfall deficits remains challenging because few hydrological models explicitly connect climate forcing to all relevant hydrological stores and fluxes, or represent their interactions across spatial scales. Here, we investigate how meteorological drought propagates through overland flow, streamflow, and soil moisture into groundwater storage across dryland catchments in the Horn of Africa. We hypothesise that drought propagation will be related to aridity, catchment scale, and lateral groundwater connectivity. To investigate these ideas, we use the 1 km resolution DRYland water Partition (DRYP) hydrological model to simulate daily water-balance components across the Horn of Africa from 2000–2023. To examine the influence of basin size, catchments are delineated using a range of stream orders (4–8); for example, stream order 5 yields ~1,300 basins with a mean area of ≈900 km² spanning hyper-arid to humid conditions. We focus on regionally widespread meteorological drought events defined using SPEI-12 (derived from CHIRPS rainfall and hPET); catchment-mean SPEI is used to identify basins experiencing prolonged meteorological drought (> 12 months). We then use basin-scale total water storage anomaly (TWSA) and the standardised total water storage index (STWSI) to explore groundwater dynamics in basins experiencing prolonged drought. Our analysis reveals substantial spatial heterogeneity in groundwater response to meteorological drought. Even during prolonged drought events, many basins exhibit neutral or increasing TWSA trends, with coherent spatial clusters of both declining and increasing storage observed within the same drought period. These results demonstrate that meteorological drought does not consistently translate into groundwater drought, even over multi-year timescales. We explore how aridity, basin size, and lateral groundwater contributions can decouple groundwater dynamics from atmospheric water deficits, leading to enhanced drought resilience in some regions (and longer recovery in others). This work highlights the limitations of assuming fixed drought-propagation pathways in drylands and demonstrates the value of high-resolution modelling for improving drought monitoring and water-resource management under increasing climate variability.

Groundwater recharge is one of the least constrained hydrological fluxes in dryland catchments, particularly where thick vadose zones, ephemeral streamflow, and high evaporative demand decouple rainfall from aquifer replenishment. Recharge is commonly attributed to rare, high-magnitude floods, yet this perspective rarely accounts for event sequencing and vadose-zone memory. Here, we synthesise multi-year hydrometric observations and process-based simulations from an ephemeral dryland stream in the arid zone of Australia (Fowlers Gap, NSW) to show that temporal clustering of moderate streamflow events can enable focused recharge where thick vadose zones impose strong percolation thresholds. Field data indicate that several historic floods produced substantial vadose-zone wetting but no sustained groundwater response, whereas sequences of closely spaced, moderate flows generated delayed, yet persistent water-table rise beneath the channel.Numerical simulations demonstrate that event clustering progressively wets the vadose zone, suppresses evapotranspiration losses, and non-linearly increases unsaturated hydraulic conductivity, resulting in sufficient flow via the streambed to measurably recharge the aquifer. These results show that groundwater recharge in dryland catchments emerges from the interaction between event sequencing, vadose-zone properties, transmission losses, and evapotranspiration, rather than from rainfall or streamflow magnitude alone. Under projected climate change, shifts toward more intense but less frequent rainfall may reduce recharge by disrupting event clustering, even where total precipitation remains unchanged. Explicitly accounting for vadose-zone memory and event sequencing is therefore essential for recharge estimation, model calibration, and dryland water-resource assessments.

Groundwater is a central component of the Earth system. However, our understanding of how it is dynamically interlinked with the atmosphere, hydrosphere, cryosphere, biosphere, geosphere, and anthroposphere remains limited. In the pursuit of understanding groundwater dynamics across diverse global settings, we present GROW (the global-scale integrated GROundWater package). This analysis-ready, quality-controlled dataset combines depth to groundwater and level time series from 55 countries, 91% from North America, India, Europe, and Australia, with associated Earth system variables. The dataset contains >200,000 time series with either daily, monthly, or yearly temporal resolution, accompanied by 36 time series or static attributes of meteorological, hydrological, geophysical, vegetation, and anthropogenic variables (e.g., precipitation, drainage density, rock type, NDVI, land use). 34 data flags regarding well features (e.g., coordinates and country), as well as time series characteristics (e.g., gap fraction or autocorrelation), facilitate quick data filtering. GROW provides a foundation for understanding large-scale groundwater processes in space and time, as well as for calibrating and evaluating models that simulate groundwater dynamics within the Earth system.

Groundwater is the primary freshwater resource in many of the world’s drylands, sustaining millions of people and supporting agriculture and ecosystems where surface water is scarce or unreliable. Recharge in these regions is highly episodic and occurs mainly through ephemeral streams (i.e., focused recharge), yet the mechanisms that determine whether surface flows contribute to aquifer replenishment remain poorly constrained. A common assumption is that large floods dominate recharge, but evidence from long-term monitoring is limited and inconclusive. We combine a unique hydrogeological monitoring dataset from the arid zone (Fowlers Gap in western New South Wales, Australia) with numerical modelling of vadose zone processes to assess the controls on focused groundwater recharge. Our results show that even extreme floods that overtopped piezometers did not produce measurable recharge at the water table. In contrast, significant recharge occurred only during a temporal cluster of moderate flow events in 2022. Numerical simulations confirm that temporal flow clustering produces longer periods of ephemeral streamflow, which progressively wet the vadose zone, overcome evapotranspiration-driven moisture deficits, and increase relative hydraulic conductivity, enabling percolation to the water table. Isolated floods, by contrast, largely saturate only shallow sediments and water is subsequently lost to evapotranspiration. By explicitly incorporating evapotranspiration, our modelling provides a more realistic representation of dryland recharge dynamics and highlights the roles of antecedent conditions and vadose zone properties. These findings demonstrate that recharge is not governed by rainfall totals or intensity alone, but critically depends on the timing and sequence of storm events. The implications for climate change assessments and water management are substantial, as projected shifts toward more intense but less frequent rainfall may reduce opportunities for clustering and thereby limit groundwater replenishment. Process-based modelling and event-scale analyses are therefore essential for reliable recharge projections and sustainable groundwater management in drylands.

The increasing demand for freshwater resources due to population growth, economic development, and climate change, requires more accurate representation and quantification of the key components of the water balance at relevant scales that goes beyond catchment domains. To address this need, we present DRYP 2.0 (DRYland water Partition model), a new version of a parsimonious, process-based, spatially distributed hydrological model (DRYP). DRYP 2.0 introduces several new capabilities, including the hydrological representation of small ephemeral ponds and large lakes, multiple interacting hydrogeological domains within a single-layer groundwater model, and vegetation canopy interception and evaporation to better capture the effects of vegetation on hydrology across different climatic gradients. Computational performance has also been enhanced through more efficient algorithms that reduce simulation time for long runs and/or over large spatial domains. We demonstrate these advances using high-resolution (1 km, 1 h) simulations over the Horn of Africa Dryland region (2,000,000 km²) as well as through various synthetic numerical tests. The results highlight the ability of the model, even without calibration, to reproduce global remote sensing data such as soil moisture, actual evapotranspiration, and total water storage, while also significantly reducing computation time. Furthermore, the explicit inclusion of multiple hydrogeological domains reveals important impacts on water table depth, with implications for improving global-scale simulations of the water balance.