Showing papers by "Chris Soulsby published in 2021"

Stable isotopes of water reveal differences in plant – soil water relationships across northern environments

Abstract: We compared stable isotopes of water in plant stem (xylem) water and soil collected over a complete growing season from five well-known long-term study sites in northern/cold regions. These spanned a decreasing temperature gradient from Bruntland Burn (Scotland), Dorset (Canadian Shield), Dry Creek (USA), Krycklan (Sweden), to Wolf Creek (northern Canada). Xylem water was isotopically depleted compared to soil waters, most notably for deuterium. The degree to which potential soil water sources could explain the isotopic composition of xylem water was assessed quantitatively using overlapping polygons to enclose respective data sets when plotted in dual isotope space. At most sites isotopes in xylem water from angiosperms showed a strong overlap with soil water; this was not the case for gymnosperms. In most cases, xylem water composition on a given sampling day could be better explained if soil water composition was considered over longer antecedent periods spanning many months. Xylem water at most sites was usually most dissimilar to soil water in drier summer months, although sites differed in the sequence of change. Open questions remain on why a significant proportion of isotopically depleted water in plant xylem cannot be explained by soil water sources, particularly for gymnosperms. It is recommended that future research focuses on the potential for fractionation to affect water uptake at the soil-root interface, both through effects of exchange between the vapour and liquid phases of soil water and the effects of mycorrhizal interactions. Additionally, in cold regions, evaporation and diffusion of xylem water in winter may be an important process.

Quantifying the effects of land use and model scale on water partitioning and water ages using tracer-aided ecohydrological models

Abstract: . Quantifying how vegetation mediates water partitioning at different spatial and temporal scales in complex, managed catchments is fundamental for long-term sustainable land and water management. Estimations from ecohydrological models conceptualising how vegetation regulates the interrelationships between evapotranspiration losses, catchment water storage dynamics, and recharge and runoff fluxes are needed to assess water availability for a range of ecosystem services and evaluate how these might change under increasing extreme events, such as droughts. Currently, the feedback mechanisms between water and mosaics of different vegetation and land cover are not well understood across spatial scales, and the effects of different scales on the skill of ecohydrological models needs to be clarified. We used the tracer-aided ecohydrological model EcH 2 O-iso in an intensively monitored 66 km2 mixed land use catchment in northeastern Germany to quantify water flux–storage–age interactions at four model grid resolutions (250, 500, 750, and 1000 m ). This used a fusion of field (including precipitation, soil water, groundwater, and stream isotopes) and remote sensing data in the calibration. Multicriteria calibration across the catchment at each resolution revealed some differences in the estimation of fluxes, storages, and water ages. In general, model sensitivity decreased and uncertainty increased with coarser model resolutions. Larger grids were unable to replicate observed streamflow and distributed isotope dynamics in the way smaller pixels could. However, using isotope data in the calibration still helped constrain the estimation of fluxes, storage, and water ages at coarser resolutions. Despite using the same data and parameterisation for calibration at different grid resolutions, the modelled proportion of fluxes differed slightly at each resolution, with coarse models simulating higher evapotranspiration, lower relative transpiration, increased overland flow, and slower groundwater movement. Although the coarser resolutions also revealed higher uncertainty and lower overall model performance, the overall results were broadly similar. The study shows that tracers provide effective calibration constraints on larger resolution ecohydrological modelling and help us understand the influence of grid resolution on the simulation of vegetation–soil interactions. This is essential in interpreting associated uncertainty in estimating land use influence on large-scale “blue” (ground and surface water) and “green” (vegetation and evaporated water) fluxes, particularly for future environmental change.

Quantifying the effects of urban green space on water partitioning and ages using an isotope-based ecohydrological model

Abstract: . The acceleration of urbanization requires sustainable, adaptive management strategies for land and water use in cities. Although the effects of buildings and sealed surfaces on urban runoff generation and local climate are well known, much less is known about the role of water partitioning in urban green spaces. In particular, little is quantitatively known about how different vegetation types of urban green spaces (lawns, parks, woodland, etc.) regulate partitioning of precipitation into evaporation, transpiration and groundwater recharge and how this partitioning is affected by sealed surfaces. Here, we integrated field observations with advanced, isotope-based ecohydrological modelling at a plot-scale site in Berlin, Germany. Soil moisture and sap flow, together with stable isotopes in precipitation, soil water and groundwater recharge, were measured over the course of one growing season under three generic types of urban green space: trees, shrub and grass. Additionally, an eddy flux tower at the site continuously collected hydroclimate data. These data have been used as input and for calibration of the process-based ecohydrological model Ec H2O -iso. The model tracks stable isotope ratios and water ages in various stores (e.g. soils and groundwater) and fluxes (evaporation, transpiration and recharge). Green water fluxes in evapotranspiration increased in the order shrub ( 381 ± 1 mm ) grass ( 434 ± 21 mm ) trees ( 489 ± 30 mm), mainly driven by higher interception and transpiration. Similarly, ages of stored water and fluxes were generally older under trees than shrub or grass. The model also showed how the interface between sealed surfaces and green space creates edge effects in the form of “infiltration hotspots”. These can both enhance evapotranspiration and increase groundwater recharge. For example, in our model, transpiration from trees increased by ∼ 50 % when run-on from an adjacent sealed surface was present and led to groundwater recharge even during the growing season, which was not the case under trees without run-on. The results form an important basis for future upscaling studies by showing that vegetation management needs to be considered within sustainable water and land use planning in urban areas to build resilience in cities to climatic and other environmental change.

Using isotopes to understand landscape‐scale connectivity in a groundwater‐dominated, lowland catchment under drought conditions

Abstract: Correspondence Lukas Kleine, Humboldt-Universität zu Berlin and IGB Leibniz Institute of Freshwater Ecology and Inland Fisheries, Mueggelseedamm 310, 12587 Berlin, Germany. Email: l.kleine@igb-berlin.de Abstract The Demnitzer Millcreek catchment (DMC), is a 66 km long-term experimental catchment located 50 km SE of Berlin. Monitoring over the past 30 years has focused on hydrological and biogeochemical changes associated with deintensification of farming and riparian restoration in the low-lying landscape dominated by rain-fed farming and forestry. However, the hydrological function of the catchment, which is closely linked to nutrient fluxes and highly sensitive to climatic variability, is still poorly understood. In the last 3 years, a prolonged drought period with below-average rainfall and above-average temperatures has resulted in marked hydrological change. This caused low soil moisture storage in the growing season, agricultural yield losses, reduced groundwater recharge, and intermittent streamflows in parts of an increasingly disconnected channel network. This paper focuses on a two-year long isotope study that sought to understand how different parts of the catchment affect ecohydrological partitioning, hydrological connectivity and streamflow generation during drought conditions. The work has shown the critical importance of groundwater storage in sustaining flows, basic in-stream ecosystem services and the dominant influence of vegetation on groundwater recharge. Recharge was much lower and occurred during a shorter window of time in winter under forests compared to grasslands. Conversely, groundwater recharge was locally enhanced by the restoration of riparian wetlands and storage-dependent water losses from the stream to the subsurface. The isotopic variability displayed complex emerging spatio-temporal patterns of stream connectivity and flow duration during droughts that may have implications for in-stream solute transport and future ecohydrological interactions between landscapes and riverscapes. Given climate projections for drier and warmer summers, reduced and increasingly intermittent streamflows are very likely not just in the study region, but in similar lowland areas across Europe. An integrated land Received: 25 September 2020 Accepted: 27 April 2021 DOI: 10.1002/hyp.14197

Effects of streamflow isotope sampling strategies on the calibration of a tracer-aided rainfall-runoff model

Abstract: Leverhulme Trust (GrantNumber(s): RPG-2018 -375) European Research Council VeWa project (GA335910) (GrantNumber(s): GA335910) Open access via Wiley Agreement

Structural changes to forests during regeneration affect water flux partitioning, water ages and hydrological connectivity: Insights from tracer-aided ecohydrological modelling

Abstract: . Increasing rates of biodiversity loss are adding momentum to efforts seeking to restore or rewild degraded landscapes. Here, we investigated the effects of natural forest regeneration on water flux partitioning, water ages and hydrological connectivity, using the tracer-aided ecohydrological model EcH 2 O-iso. The model was calibrated using ∼ 3.5 years of diverse ecohydrological and isotope data available for a catchment in the Scottish Highlands, an area where impetus for native pinewood regeneration is growing. We then simulated two land cover change scenarios that incorporated forests at early (dense thicket) and late (old open forest) stages of regeneration, respectively. Changes to forest structure (proportional vegetation cover, vegetation heights and leaf area index of pine trees) were modelled for each stage. The scenarios were then compared to a present-day baseline simulation. Establishment of thicket forest had substantial ecohydrological consequences for the catchment. Specifically, increased losses to transpiration and, in particular, interception evaporation drove reductions in below-canopy fluxes (soil evaporation, groundwater (GW) recharge and streamflow) and generally slower rates of water turnover. The greatest reductions in streamflow and connectivity were simulated for summer baseflows and small to moderate events during summer and the autumn/winter rewetting period. This resulted from the effect of local changes to flux partitioning in regenerating areas on the hillslopes extending to the wider catchment by reducing downslope GW subsidies that help sustain summer baseflows and saturation in the valley bottom. Meanwhile, higher flows were relatively less affected, especially in winter. Despite the generally drier state of the catchment, simulated water ages suggested that the increased transpiration demands of the thicket forest could be satisfied by moisture carried over from previous seasons. The more open nature of the old forest generally resulted in water fluxes, water ages and connectivity returning towards baseline conditions. Our work implies that the ecohydrological consequences of natural forest regeneration depend on the structural characteristics of the forest at different stages of development. Consequently, future land cover change investigations need to move beyond consideration of simple forest vs. non-forest scenarios to inform sustainable landscape restoration efforts.

Using soil water isotopes to infer the influence of contrasting urban green space on ecohydrological partitioning

Abstract: . In cities around the world, urban green spaces provide a range of benefits and ecosystem services. However, recent years have shown how prolonged warm and dry periods can affect urban water resources and lead to water stress in vegetation in urban green spaces, even in temperate regions. Consequently, quantitative knowledge about ecohydrological partitioning in different types of urban green space is crucial for balancing sustainable water needs in cities during future challenges of increasing urbanization and climate warming. Using isotopic tracers in precipitation and soil water, along with conventional hydrometric measurements in a plot-scale study in Berlin, Germany, we investigated water partitioning under different generic types of urban vegetation (grassland, shrub and trees). This allowed for the assessment of urban vegetation effects on evapotranspiration, subsurface flow paths and storage during a prolonged drought period with episodic rainfall. Despite higher soil evaporation losses under urban grassland, higher interception and transpiration likely contributed to slower turnover of soil water and older groundwater recharge under urban trees. Shrub vegetation seemed to be most resilient to prolonged drought periods, with lower evapotranspiration losses. Our results contribute to a better understanding of ecohydrological partitioning under mixed urban vegetation communities and an evidence base for better adaptive management of urban water and irrigation strategies to sustainably meet the water demands of urban green spaces in the future.

Catchment Functioning Under Prolonged Drought Stress: Tracer-Aided Ecohydrological Modeling in an Intensively Managed Agricultural Catchment

Abstract: Recent extreme droughts in Europe have provided strong research foci in the hydrological community to better understand and forecast periods of water stress (Albergel et al., 2020; Bakke et al., 2020; Kleine et al., 2020; Smith et al., 2020a). In line with long-term climate change projections, the extreme drought in 2018–2019 provided a unique opportunity to investigate expected regional climate change impacts both in terms of monitoring (e.g., the TERENO observatories in Germany [Heinrich et al., 2019; Wollschläger et al., 2016]) and modeling (e.g., Samaniego et al., 2018; Smith et al., 2020a) flux-storage dynamics. As drought impacts propagate through a catchment's ecohydrological cycle (Wilhite & Glantz, 1985), increasingly dry conditions (i.e., a meteorological drought of reduced precipitation and high temperatures) typically deplete soil water storage, gradually supressing “green” (i.e., soil evaporation and vegetation transpiration) and “blue” (i.e., groundwater recharge and runoff generation) water fluxes. However, resilience to droughts and subsequent recovery patterns differ for different compartments of the ecohydrological cycle. Orth and Destouni (2018) showed that blue water fluxes generally decrease more strongly and rapidly than green water fluxes as soil moisture deficits increase. In prolonged droughts, impacts propagate to aquifers, and it generally takes a longer time for recharge fluxes to recover during rewetting periods (Orth & Destouni, 2018). Of course, the general patterns vary under specific physiographic and climate conditions. In regions where surface waters are substantially fed by deeper aquifer storage, blue water fluxes likely Abstract High spatial heterogeneity of catchment properties enhances the variability of ecohydrological responses to changing natural and anthropogenic conditions, like the European-wide droughts in 2018–2019. Based on new adaptations of a tracer-aided, process-based ecohydrological model (EcH2O-iso), we investigated drought-induced nonstationary ecohydrological behavior in a small agricultural headwater catchment (1.44 km) in central Germany. Multiple environmental time-series helped inform various aspects of catchment functioning that have been impacted by agricultural activity and changing climate conditions and helped to further constrain model calibration. Multi-criteria calibration showed that data collected during drought years were highly informative in reproducing the changes in stream water dynamics. Further, inclusion of  2H and  18O data was valuable for reducing model uncertainty and increasing confidence in simulations of greenand blue-water flux partitioning and storage-flux-age interactions. Using the best-performing calibrations, we further analyzed the high spatiotemporal variability of internal ecohydrological processes and the varying responses of fluxes and associated water ages to prolonged drought stress. Under drought conditions, modeled stream runoff contributed from deeper, older storages increased significantly after a particularly wet season, resulting in a sharp increase in stream water age. Unlike relatively minor changes in soil evaporation, seasonally typical transpiration fluxes were initially maintained in April–June but dramatically decreased as the drought further developed in July–September. Importantly, the tracer-based transpired water age was much older after April, providing a potential indicator of drought impacts and the need for precautionary management responses. Our findings are important for similar agricultural headwater ecosystems in other drought-sensitive regions.

Isotope hydrology and water sources in a heavily urbanised stream

Abstract: Complex networks of both natural and engineered flow paths control the hydrology of streams in major cities through spatio-temporal variations in connection and disconnection of water sources. We used spatially extensive and temporally intensive sampling of water stable isotopes to disentangle the hydrological sources of the heavily urbanized Panke catchment (≈ 220 km²) in the north of Berlin, Germany. The isotopic data enabled us to partition stream water sources across the catchment using a Bayesian mixing analysis. The upper part of the catchment streamflow here is dominated by groundwater from gravel aquifers underlying surrounding agricultural land. In dry summer periods, streamflow becomes intermittent; possibly as a result of local groundwater abstractions. Urban storm drainage is also an important part of runoff generation, dominating the responses to precipitation events. Although this dramatically changes the isotopic composition of the stream, it only accounts for 10-15% of annual streamflow. Moving downstream, subtle changes in sources and isotope signatures occur as catchment characteristic vary and the stream is affected by different tributary inflows. However, effluent from a wastewater treatment plant (WWTP) serving 700,000 people dominates the stream in the lower catchment where urbanisation effects are more dramatic. The associated increase in sealed surfaces downstream also reduces the relative contribution of groundwater to streamflow. The volume and isotopic composition of storm runoff is again dominated by urban drainage. As a result, only about 10% of annual runoff in the lower catchment comes from urban storm drains. The study shows the potential of stable water isotopes as inexpensive tracers in urban catchments that can provide a more integrated understanding of the complex hydrology of major cities. This offers an important evidence base for guiding the plans to develop and re-develop urban catchments to protect, restore and enhance the ecological and amenity value of these important resources.

Modelling ecohydrological feedbacks in forest and grassland plots under a prolonged drought anomaly in Central Europe 2018–2020

Abstract: Funding Information: The authors are grateful to all colleagues involved in the sample collection and infrastructure installation in the DMC (in particular H. Dampfling, J. Freymuller, H. Wang, S. Jordan, A. Douinot, A. Wieland, N. Weis, L. Kuhlemann, C. Marx, L. Lachmann, W. Lehmann). We thank D. Dubbert for support with the extensive isotope analysis, as well as Department 6 of the IGB (in particular T. Rossoll) for help with the sampling, measurement equipment and insights to the long-term catchment infrastructure and background. We are thankful for trustful collaboration with B. Bosel and technical support by the WLV (Wasser und Landschaftspflegeverband Untere Spree). Contributions from Soulsby were supported by the Leverhulme Trust's ISO-LAND project (RPG-2018-375). Two anonymous reviewers are thanked for constructive comments.

A longer-term perspective on soil moisture, groundwater and stream flow response to the 2018 drought in an experimental catchment in the Scottish Highlands

Abstract: Acknowledgements: Contributions from CS were supported by the Leverhulme Trust through the ISO-LAND project (RPG 2018 375). The authors acknowledge funding from the European Research Council (project GA 335910 VeWa).