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Subsurface plant‐accessible water in mountain ecosystems with a Mediterranean climate

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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
Study site locations and cross‐sectional depictions of the Southern Sierra CZO and its ecosystems. The four focal sites of the CZO span an elevation and climatic gradient from low to middle elevations on the western flank of the Sierra Nevada of California, USA. Watersheds situated on the western flank provide surface water and groundwater recharge that support agricultural, municipal, and industrial water demands in the San Joaquin Valley and across California
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Conceptual overview of key integrated physical–biological–chemical processes in the critical zone in need of quantitative investigation through new predictive coupled‐modeling frameworks that span temporal scales from days to millennia
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Examples from the Southern Sierra CZO showing quantitative estimates for actual stores and storage capacities of subsurface water integrated across the entire regolith depth at an individual site for (a) a single instant in time and (b) across several years; (c) across elevation; and (d) across the landscape. There is insufficient data to accurately estimate plant‐accessible water storage capacity or dry‐season available water at the 2000 m site (c)
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Geophysical estimates of depth‐integrated total porosity (which is a minimum due to assumptions about saturation) across the three lowest‐elevation measurement sites of the Southern Sierra CZO (see text for methods). Total integrated porosity, in cm, quantifies the depth‐integrated volume‐per‐unit‐area (i.e., the effective thickness) of void space in the subsurface below any given 1 cm2 area on the land surface. Similar total porosity data for the 2700 m site is not available
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Types and scales of measurement that can be integrated to understand the temporal and spatial variations of subsurface water‐related attributes in mountain ecosystems
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Conceptual model of critical zone architecture (left), fracture density (middle), and their relation to changes in subsurface attributes (right), both dynamic (blue) and static (earth tones). Processes (upper right) that control a decrease in water availability (from right to left) and define these attributes of storage capacity and stores of subsurface water. The attributes can be defined as follows: dry season water drawdown (DSWD) is the amount of water lost from storage during the dry season; dry season available water (DSAW) is the amount of plant‐accessible water in storage at the beginning of the dry season; plant accessible water storage capacity (PAWSC) is the amount of void space available to hold water that is accessible to plants, and depends on rooting depth; available water storage capacity (AWSC) is the amount of void space available to hold plant‐accessible water at amounts below field capacity and above wilting point, regardless of presence or not of rhizosphere; and total porosity (TP) is the total void space
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Water and Life > Nature of Freshwater Ecosystems
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