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Existing studies on the impacts of climate change on groundwater recharge are either global or basin/location-specific. The global studies lack the specificity to inform decision making, while the local studies do little to clarify potential changes over large regions (major river basins, states, or groups of states), a scale often important in the development of water policy. An analysis of the potential impact of climate change on groundwater recharge across the western United States (west of 100° longitude) is presented synthesizing existing studies and applying current knowledge of recharge processes and amounts. Eight representative aquifers located across the region were evaluated. For each aquifer published recharge budget components were converted into four standard recharge mechanisms: diffuse, focused, irrigation, and mountain-systems recharge. Future changes in individual recharge mechanisms and total recharge were then estimated for each aquifer. Model-based studies of projected climate-change effects on recharge were available and utilized for half of the aquifers. For the remainder, forecasted changes in temperature and precipitation were logically propagated through each recharge mechanism producing qualitative estimates of direction of changes in recharge only (not magnitude). Several key patterns emerge from the analysis. First, the available estimates indicate average declines of 10-20% in total recharge across the southern aquifers, but with a wide range of uncertainty that includes no change. Second, the northern set of aquifers will likely incur little change to slight increases in total recharge. Third, mountain system recharge is expected to decline across much of the region due to decreased snowpack, with that impact lessening with higher elevation and latitude. Factors contributing the greatest uncertainty in the estimates include: (1) limited studies quantitatively coupling climate projections to recharge estimation methods using detailed, process-based numerical models; (2) a generally poor understanding of hydrologic flowpaths and processes in mountain systems; (3) difficulty predicting the response of focused recharge to potential changes in the frequency and intensity of extreme precipitation events; and (4) unconstrained feedbacks between climate, irrigation practices, and recharge in highly developed aquifer systems.

The article titled "Thank You to Our 2023 Reviewers" expresses gratitude to the individuals who reviewed manuscripts for the journal Water Resources Research (WRR) in 2023. The efforts of these reviewers have contributed to the high quality and impact of the journal's papers and the overall research in the field of water science. In total, 2,809 individuals provided 4,453 peer reviews, with 354 individuals reviewing three or more papers. The editors, associate editors, and authors of WRR sincerely thank all the reviewers and anticipate the publication of exciting advances in 2024. [Extracted from the article]

Dual‐porosity models are often used to describe solute transport in heterogeneous media, but the parameters within these models (e.g., immobile porosity and mobile/immobile exchange rate coefficients) are difficult to identify experimentally or relate to measurable quantities. Here, we performed synthetic, pore‐scale millifluidics simulations that coupled fluid flow, solute transport, and electrical resistivity (ER). A conductive‐tracer test and the associated geoelectrical signatures were simulated for four flow rates in two distinct pore‐scale model scenarios: one with intergranular porosity, and a second with an intragranular porosity also defined. With these models, we explore how the effective characteristic‐length scale estimated from a best‐fit dual‐domain mass transfer (DDMT) model compares to geometric aspects of the flow field. In both model scenarios we find that: (1) mobile domains and immobile domains develop even in a system that is explicitly defined with one domain; (2) the ratio of immobile to mobile porosity is larger at faster flow rates as is the mass‐transfer rate; and (3) a comparison of length scales associated with the mass‐transfer rate (Lα) and those associated with calculation of the Peclet number (LPe) show LPe is commonly larger than Lα. These results suggest that estimated immobile porosities from a DDMT model are not only a function of physically mobile or immobile pore space, but also are a function of the average linear pore‐water velocity and physical obstructions to flow, which can drive the development of immobile porosity even in single‐porosity domains. [ABSTRACT FROM AUTHOR]

Two major barriers hinder the holistic understanding of subsurface critical zone (CZ) evolution and its impacts: (a) an inability to measure, define, and share information and (b) a societal structure that inhibits inclusivity and creativity. In contrast to the aboveground portion of the CZ, which is visible and measurable, the bottom boundary is difficult to access and quantify. In the context of these barriers, we aim to expand the spatial reach of the CZ by highlighting existing and effective tools for research as well as the "human reach" of CZ science by expanding who performs such science and who it benefits. We do so by exploring the diversity of vocabularies and techniques used in relevant disciplines, defining terminology, and prioritizing research questions that can be addressed. Specifically, we explore geochemical, geomorphological, geophysical, and ecological measurements and modeling tools to estimate CZ base and thickness. We also outline the importance of and approaches to developing a diverse CZ workforce that looks like and harnesses the creativity of the society it serves, addressing historical legacies of exclusion. Looking forward, we suggest that to grow CZ science, we must broaden the physical spaces studied and their relationships with inhabitants, measure the "deep" CZ and make data accessible, and address the bottlenecks of scaling and data‐model integration. What is needed—and what we have tried to outline—are common and fundamental structures that can be applied anywhere and used by the diversity of researchers involved in investigating and recording CZ processes from a myriad of perspectives. Plain Language Summary: The "critical zone" is the zone of the Earth from treetops to belowground water. It is where crops are grown, water is drawn for drinking and industry, and waste generated by humans ends up. Understanding how deep this section of the Earth is, and how it is changing, is key to being able to determine how impactful changes in land use or climate will be to human systems. That said, it is difficult to get below‐ground information, and scientists in different subfields of the Earth sciences define the depth of the critical zone differently. Here, we describe the tools and language we use to make those decisions. We also note that Earth scientists are not a diverse group, which means we miss out on the ideas, solutions, and impacts that those from historically excluded groups might have to address the big problems humans must solve. To advance critical zone science, we need to broaden the physical areas we study, access to these sites, and the inclusion and sense of belonging of people studying them. We also need to measure deep into the Earth where we can, make our data accessible, and tackle scientific obstacles in integrating data and models. Key Points: CZ depth is defined by multiple processes key to human existence, yet is undermeasured and thus poorly understoodScientific jargon can make defining and communicating CZ processes difficult to stakeholders and even scientists in other fieldsLegacies in western science, upon which CZ science is based, can be countered to promote progress in the Earth sciences [ABSTRACT FROM AUTHOR]

Visualising subsurface processes in hydrogeology and building intuition for how these processes are controlled by changes in forcing is hard for many undergraduate students. While numerical modelling is one way to help undergraduate students explore outcomes of multiple scenarios, many codes are not user-friendly with respect to defining domains, boundary conditions, and coupling processes, and numerical modelling exercises are also often disconnected from systems that the students understand, limiting their ability to extrapolate what they have learned for other situations. Here, we test the hypothesis that hydrogeology students will better estimate rates of groundwater flow and contaminant transport and the magnitudes of the parameters that control flow and transport by linking physical and numerical models. We present an exercise that links physical and numerical modelling of fluid flow and solute transport using 2-D "ant farm" sand tanks with parallel models in COMSOL Multiphysics. The sand tank exercises provide students with a way to visualise subsurface flow and transport processes, while COMSOL allows them to explicitly pull apart the mathematics associated with these systems and build intuition for their solutions. Given coupled experimentation and numerical exercises, we find that students will connect processes that they see in the laboratory with the outcomes of numerical models, and the post-exercise tests indicate that they have an improved understanding of: (1) the magnitude and importance of properties and parameters that control flow and transport and (2) the simplifications made in numerical models of physical systems. (Contains 1 table and 6 figures.)