Establishing ecological thresholds and targets for groundwater management

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Establishing ecological thresholds and targets for groundwater management

Establishing ecological thresholds and targets for groundwater management

Nature Water  (2024) see full paper attached

Abstract

Groundwater is critical for many ecosystems, yet groundwater requirements for dependent ecosystems are rarely accounted for during water and conservation planning. Here we compile 38 years of Landsat-derived normalized difference vegetation index (NDVI) to evaluate groundwater-dependent vegetation responses to changes in depth to groundwater (DTG) across California. To maximize applicability, we standardized raw NDVI and DTG values using  Z  scores to identify groundwater thresholds, groundwater targets and map potential drought refugia across a diversity of biomes and local conditions. Groundwater thresholds were analysed for vegetation impacts where  Z NDVI dropped below −1.  Z DTG thresholds and targets were then evaluated with respect to groundwater-dependent vegetation in different condition classes and rooting depths.  Z NDVI scores were applied statewide to identify potential drought refugia supported by groundwater. Our approach provides a simple and robust methodology for water and conservation practitioners to support ecosystem water needs so biodiversity and sustainable water-management goals can be achieved.

Main

Groundwater is a critical component of many aquatic and terrestrial ecosystems, but its role in sustaining ecosystem function and stability is rarely acknowledged in conservation and water-management planning globally1,2,3. Increasing evidence over recent decades has linked groundwater pumping and other water-use practices to adverse impacts on groundwater-dependent species, habitats and ecosystems through declines in streamflow4,5,6 and groundwater levels7. Moreover, rates of groundwater pumping are likely to intensify as global warming increases the frequency and severity of extreme drought events8,9,10. Subsequent groundwater declines will amplify adverse ecosystem impacts because groundwater is an essential buffer in meeting higher evapotranspiration demands under drought11,12,13. Groundwater-related adverse impacts range in severity from water stress (individual scale) to habitat loss (population scale) to, in the worst-case scenario, ecosystem collapse (system scale). Whereas some impacts may be reversible, others may result in the permanent loss of species and/or habitats. To meet global biodiversity and sustainable water-management goals, while preserving cultural values associated with these ecosystems, ecosystem water requirements need to be identified and quantified1,14,15. However, ecological thresholds and targets for groundwater are not well defined due to deficiencies in data, ecohydrologic understanding, legal protections or political will in water-management agencies2,16,17,18,19. Even where targets exist, traditional approaches to adaptive resource management may not be appropriate due to high levels of uncertainty and the potential severity and permanence of impacts from groundwater-management actions20. Thus, a practical approach for quantifying ecosystem groundwater requirements across a wide range of ecosystem and local conditions is needed to support conservation action, water-management and water-allocation decisions.

Groundwater-dependent ecosystems (GDEs) are comprised of species and habitats that rely on groundwater for some or all of their water needs. These ecosystems are incredibly diverse, existing across above- and below-ground aquatic and terrestrial realms and supporting a wide range of species and niche habitats19. Whereas groundwater can support subterranean ecosystems (for example, in caves, aquifers)3, our study focuses on terrestrial groundwater-dependent ecosystems with perennial vegetation, including riparian woodlands that are reliant on groundwater occurring on or near Earth’s surface. Rather than measuring the functional biological responses of all taxonomic groups within these ecosystems, here we target the dominant groundwater-dependent vegetation (‘phreatophyte’) species and monitor their sensitivity to changes in groundwater availability. Phreatophytes are prominent members of many groundwater-dependent ecosystems and are not only good indicators of near-surface groundwater conditions7 but are also considered ‘foundation’ species within their communities21. Woody phreatophytes, such as the dominant trees in riparian ecosystems, provide structure, biomass, material flows and microclimate regulation for many dependent species. Canopy stratification and vertical complexity creates diverse habitats that support associated fauna species including birds, mammals, fish and insects. Phreatophytes can also be monitored using remote sensing methods to detect responses over large areas. Phreatophytes are thus appropriate and practical indicators for evaluating ecosystem groundwater requirements across a region, improving the applicability of ecosystem groundwater needs assessments in water-use and planning decisions.

Ecosystem groundwater needs assessments require science-based ecological  thresholds  that quantify the transition between functionally stable and detrimental ecosystem states and science-based ecological  targets  that quantify ecosystem states that have the physiological capacity to deal with natural range of hydrologic variability14. Previous studies have demonstrated that both the rate of water table decline and absolute groundwater depth can trigger declines in vegetation health12,13. This is because phreatophyte species can often tolerate or adapt to gradual changes in groundwater levels, but if these changes are prolonged or abrupt, it can result in a range of functional or ecological responses ranging in severity from declines in productivity, recruitment and mortality22,23,24,25. Thus, groundwater thresholds and targets must take into consideration the rate, magnitude and duration of the groundwater change and how these relate to either temporary or long-term impacts on an ecosystem26,27. Therefore, determining ecosystem groundwater requirements depends upon understanding groundwater-level thresholds and targets required to sustain groundwater-dependent ecosystems, so that management actions can augment supplies (for example, managed aquifer recharge projects) or reduce demand (for example, pumping restrictions) to maintain ecological function and sustain species and their communities.

The challenge is that vegetation responses to groundwater changes can vary substantially over time and space due to natural seasonal and interannual groundwater fluctuations, species-specific tolerance and adaptations to water stress and the existence of supplemental water sources such as surface water, irrigation return flow and treated wastewater effluent28. Furthermore, the effects of groundwater changes can vary even within species due to local vegetation density and biomass that influence demand and individuals’ adaptation to site-specific hydrology and climate during establishment and growth. These uncertainties are further complicated by the impacts of climate change on the water cycle and vegetation, as well as existing groundwater data gaps in most regions7,28.

To address this challenge, we utilize  Z  scores as a means to standardize vegetation responses and groundwater levels for quantifying ecological thresholds and targets relative to a historic baseline that can be consistently applied across a diversity of ecosystems and local environmental conditions (Fig. 1).  Z  scores are a well-established statistical metric to indicate how many standard deviations an individual observation value is from the mean and are useful in comparing values from distributions with different units and ranges. Thus,  Z  scores calculated from the range of location-specific values enable us to track ecological responses to groundwater levels that fluctuate above and below local baseline conditions, which can vary greatly due to vegetation species composition and density, aquifer properties, natural climate and surface water flow and groundwater regimes.

Fig. 1: Schematic diagrams of  Z  score and ecosystem water needs assessments.

 

figure 1

a , Landscape schematic diagram illustrating how normalized difference vegetation index (NDVI) and depth-to-groundwater (DTG) fluctuations can vary temporally and spatially across the landscape depending on land surface elevation and landscape positioning. Groundwater depths and associated NDVI values are site specific and provided here for illustrative purposes. Most vegetation are adapted to natural fluctuations in DTG (for example, within −1 to +1 standard deviation (σ) over a baseline period), but if DTG exceeds these natural fluctuations, vegetation can become adversely impacted.  b , DTG naturally fluctuates over time, but drought events and intensive groundwater pumping can cause groundwater levels to extend far below the natural range of variability observed over time.  c ,  Z  scores can be used to standardize groundwater levels as an alternative metric for quantifying ecological thresholds and targets across environments with variable local conditions.

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