Climate, physical landscapes, and biota interact to generate heterogeneous hydrologic conditions in space and over time, which are reflected in spatial patterns of species distributions. As these species distributions respond to rapid climate change, microrefugia may support local species persistence in the face of deteriorating climatic suitability. Recent focus on temperature as a determinant of microrefugia insufficiently accounts for the importance of hydrologic processes and changing water availability with changing climate. Where water scarcity is a major limitation now or under future climates, hydrologic microrefugia are likely to prove essential for species persistence, particularly for sessile species and plants. Zones of high relative water availability - mesic microenvironments - are generated by a wide array of hydrologic processes, and may be loosely coupled to climatic processes and therefore buffered from climate change. Here, we review the mechanisms that generate mesic microenvironments and their likely robustness in the face of climate change. We argue that mesic microenvironments will act as species-specific refugia only if the nature and space/time variability in water availability are compatible with the ecological requirements of a target species. We illustrate this argument with case studies drawn from California oak woodland ecosystems. We posit that identification of hydrologic refugia could form a cornerstone of climate-cognizant conservation strategies, but that this would require improved understanding of climate change effects on key hydrologic processes, including frequently cryptic processes such as groundwater flow.

This study investigates the extent of the rain-snow transition zone across the complex terrain of the western United States for both late 20th century climate and projected changes in climate by the mid-21st century. Observed and projected temperature and precipitation data at 4 km resolution were used with an empirical probabilistic precipitation phase model to estimate and map the likelihood of snow versus rain occurrence. This approach identifies areas most likely to undergo precipitation phase change over the next half century. At broad scales, these projections indicate an average 30% decrease in areal extent of winter wet-day temperatures conducive to snowfall over the western United States. At higher resolution scales, this approach identifies existing and potential experimental sites best suited for research investigating the mechanisms linking precipitation phase change to a broad array of processes, such as shifts in rain-on-snow flood risk, timing of water resource availability, and ecosystem dynamics.

Enhanced understanding of subsurface water storage will improve prediction of future impacts of climate change, including drought, forest mortality, wildland fire, and strained water security. Previous research has examined the importance of plant‐accessible water in soil, but in upland landscapes within Mediterranean climates, soil often accounts for only a fraction of subsurface water storage. We draw insights from previous research and a case study of the Southern Sierra Critical Zone Observatory to define attributes of subsurface storage; review observed patterns in their distribution; highlight nested methods for estimating them across scales; and showcase the fundamental processes controlling their formation. We review observations that highlight how forest ecosystems subsist on lasting plant‐accessible stores of subsurface water during the summer dry period and during multiyear droughts. The data suggest that trees in these forest ecosystems are rooted deeply in the weathered, highly porous saprolite or saprock, which reaches up to 10–20 m beneath the surface. This review confirms that the system harbors large volumes of subsurface water and shows that they are vital to supporting the ecosystem through the summer dry season and extended droughts. This research enhances understanding of deep subsurface water storage across landscapes and identifies key remaining challenges in predicting and managing response to climate and land use change in mountain ecosystems of the Sierra Nevada and in other Mediterranean climates worldwide. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Extremes Water and Life > Nature of Freshwater Ecosystems

Abstract. The phase of precipitation when it reaches the ground is a first-order driver of hydrologic processes in a watershed. The presence of snow, rain, or mixed-phase precipitation affects the initial and boundary conditions that drive hydrological models. Despite their foundational importance to terrestrial hydrology, typical phase partitioning methods (PPMs) specify the phase based on near-surface air temperature only. Our review conveys the diversity of tools available for PPMs in hydrological modeling and the advancements needed to improve predictions in complex terrain with large spatiotemporal variations in precipitation phase. Initially, we review the processes and physics that control precipitation phase as relevant to hydrologists, focusing on the importance of processes occurring aloft. There is a wide range of options for field observations of precipitation phase, but there is a lack of a robust observation networks in complex terrain. New remote sensing observations have the potential to increase PPM fidelity, but generally require assumptions typical of other PPMs and field validation before they are operational. We review common PPMs and find that accuracy is generally increased at finer measurement intervals and by including humidity information. One important tool for PPM development is atmospheric modeling, which includes microphysical schemes that have not been effectively linked to hydrological models or validated against near-surface precipitation-phase observations. The review concludes by describing key research gaps and recommendations to improve PPMs, including better incorporation of atmospheric information, improved validation datasets, and regional-scale gridded data products. Two key points emerge from this synthesis for the hydrologic community: (1) current PPMs are too simple to capture important processes and are not well validated for most locations, (2) lack of sophisticated PPMs increases the uncertainty in estimation of hydrological sensitivity to changes in precipitation phase at local to regional scales. The advancement of PPMs is a critical research frontier in hydrology that requires scientific cooperation between hydrological and atmospheric modelers and field scientists.

ABSTRACT. Recent mandates in the United States require federal agencies to incorporate climate change science into land management planning efforts. These mandates target possible adaptation and mitigation strategies. However, the degree to which climate change is actively being considered in agency planning and management decisions is largely unknown. We explored the usefulness of climate change science for federal resource managers, focusing on the efficacy of potential adaptation strategies and barriers limiting the use of climate change science in adaptation efforts. Our study was conducted in the northern Rocky Mountains region of the western United States, where we interacted with 77 U.S. Forest Service and Bureau of Land Management personnel through surveys, semistructured interviews, and four collaborative workshops at locations across Idaho and Montana. We used a mixed-methods approach to evaluate managers' perceptions about adapting to and mitigating for climate change. Although resource managers incorporate general language about climate change in regional and landscape-level planning documents, they are currently not planning on-the-ground adaptation or mitigation projects. However, managers felt that their organizations were most likely to adapt to climate change through use of existing management strategies that are already widely implemented for other non climate-related management goals. These existing strategies, (e.g., thinning and prescribed burning) are perceived as more feasible than new climate-specific methods (e.g., assisted migration) because they already have public and agency support, accomplish multiple goals, and require less anticipation of the future timing and probability of climate change impacts. Participants reported that the most common barriers to using climate change information included a lack of management-relevant climate change science, inconsistent agency guidance, and insufficient time and resources to access, interpret, and apply current climate science information to management plans.