We examined the storage dynamics and isotopic composition of soil water over 12 months in three hydropedological units in order to understand runoff generation in a montane catchment. The units form ...classic catena sequences from freely draining podzols on steep upper hillslopes through peaty gleys in shallower lower slopes to deeper peats in the riparian zone. The peaty gleys and peats remained saturated throughout the year, while the podzols showed distinct wetting and drying cycles. In this region, most precipitation events are <10 mm in magnitude, and storm runoff is mainly generated from the peats and peaty gleys, with runoff coefficients (RCs) typically <10%. In larger events the podzolic soils become strongly connected to the saturated areas, and RCs can exceed 40%. Isotopic variations in precipitation are significantly damped in the organic‐rich soil surface horizons due to mixing with larger volumes of stored water. This damping is accentuated in the deeper soil profile and groundwater. Consequently, the isotopic composition of stream water is also damped, but the dynamics strongly reflect those of the near‐surface waters in the riparian peats. “pre‐event” water typically accounts for >80% of flow, even in large events, reflecting the displacement of water from the riparian soils that has been stored in the catchment for >2 years. These riparian areas are the key zone where different source waters mix. Our study is novel in showing that they act as “isostats,” not only regulating the isotopic composition of stream water, but also integrating the transit time distribution for the catchment.
Key Points
Hillslope connectivity is controlled by small storage changes in soil units
Different catchment source waters mix in large riparian wetland storage
Isotopes show riparian wetlands set the catchment transit time distribution
For effective catchment management and intervention in hydrological systems a process-based understanding of hydrological connectivity is required so that: i) conceptual rather than solely empirical ...understanding drives how systems are interpreted; and ii) there is an understanding of how continuous flow fields develop under different sets of environmental conditions to enable managers to know when, where and how to intervene in catchment processes successfully. In order to direct future research into process-based hydrological connectivity this paper: i) evaluates the extent to which different concepts of hydrological connectivity have emerged from different approaches to measure and predict flow in different environments; ii) discusses the extent to which these different concepts are mutually compatible; and iii) assesses further research to contribute to a unified understanding of hydrological processes. Existing research is categorised into five different approaches to investigating hydrological connectivity: i) evaluating soil–moisture patterns (soil–moisture connectivity); ii) understanding runoff patterns and processes on hillslopes (flow-process connectivity); iii) investigating topographic controls (terrain-connectivity) including the impact of road networks on hydrological connectivity and catchment runoff; iv) developing models to explore and predict hydrological connectivity; and v) developing indices of hydrological connectivity. Analysis of published research suggests a relationship between research group, approach, geographic setting and the interpretation of hydrological connectivity. For further understanding of hydrological connectivity our knowledge needs to be developed using a range of techniques and approaches, there should be common understandings between researchers approaching the concept from different perspectives, and these meanings need to be communicated effectively with those responsible for land management.
The complex interactions of runoff generation processes underlying the hydrological response of streams remain not entirely understood at the catchment scale. Extensive research has demonstrated the ...utility of tracers for both inferring flow path distributions and constraining model parameterizations. While useful, the common use of linearity assumptions, i.e. time invariance and complete mixing, in these studies provides only partial understanding of actual process dynamics. Here we use long-term (<20 yr) precipitation, flow and tracer (chloride) data of three contrasting upland catchments in the Scottish Highlands to inform integrated conceptual models investigating different mixing assumptions. Using the models as diagnostic tools in a functional comparison, water and tracer fluxes were then tracked with the objective of exploring the differences between different water age distributions, such as flux and resident water age distributions, and characterizing the contrasting water age pattern of the dominant hydrological processes in the three study catchments to establish an improved understanding of the wetness-dependent temporal dynamics of these distributions. The results highlight the potential importance of partial mixing processes which can be dependent on the hydrological functioning of a catchment. Further, tracking tracer fluxes showed that the various components of a model can be characterized by fundamentally different water age distributions which may be highly sensitive to catchment wetness history, available storage, mixing mechanisms, flow path connectivity and the relative importance of the different hydrological processes involved. Flux tracking also revealed that, although negligible for simulating the runoff response, the omission of processes such as interception evaporation can result in considerably biased water age distributions. Finally, the modeling indicated that water age distributions in the three study catchments do have long, power-law tails, which are generated by the interplay of flow path connectivity, the relative importance of different flow paths as well as by the mixing mechanisms involved. In general this study highlights the potential of customized integrated conceptual models, based on multiple mixing assumptions, to infer system internal transport dynamics and their sensitivity to catchment wetness states.
Long-term records of combined stream flow and water chemistry can be an invaluable source of information on changes in the quantity and quality of water resources. To understand the effect of ...hydroclimate and water management on the heavily urbanized Panke catchment in Berlin, Germany, an extensive search, collation and digitization of historic data from various sources was undertaken. This integrated a unique 66-year spatially distributed record of stream water quality, a 21-year record of groundwater quality and a 31-year stream flow record. These data were analysed in the context of hydroclimatic variability, as well as the history and technological evolution of water resource management in the catchment. To contextualize the effect of droughts, “average” and wet years the Standard Precipitation Index (SPI) was applied. As upstream sites have been less regulated by human impacts, the flow regime is most sensitive to changes in hydroclimatic conditions, while downstream sites are more influenced by wastewater effluents, urban storm drains and inter-basin transfers for flood alleviation. However, at all sites, a general increase in maximum event discharge was observed until a recent drought, starting in 2018. In general, water quality in the catchment has gradually improved as a result of management change and increasingly effective wastewater treatment, though in some places legacy and/or contemporary urban and rural groundwater contamination may be affecting the stream. Hydroclimatic changes, particularly drought years can affect water quality classes, and alter the chemostatic/dynamic behaviour of catchment export patterns. These insights from the Panke catchment underline the importance of strategic adaptation and improvement of water treatment and water resource management in order to enhance the quality of urban water courses. It also demonstrates the importance of long-term integrated data sets.
Changes in water quality related to hydroclimate. Display omitted
•Hydrological, spatial distributed long term time series for urban catchments.•Water management development during 1985–1995 drastically improved water quality.•Hydroclimate impacts stream connectivity and subsequently water quality.•The catchment becomes increasingly chemodynamic during dry conditions.
Abstract Long‐term data are crucial for understanding ecological responses to climate and land use change; they are also vital evidence for informing management. As a migratory fish, Atlantic salmon ...are sentinels of both global and local environmental change. This paper reviews the main insights from six decades of research in an upland Scottish stream (Girnock Burn) inhabited by a spring Atlantic salmon population dominated by multi‐sea‐winter fish. Research began in the 1960s providing a census of returning adults, juvenile emigrants and in‐stream production of Atlantic salmon. Early research pioneered new monitoring techniques providing new insights into salmon ecology and population dynamics. These studies underlined the need for interdisciplinary approaches for understanding salmon interactions with physical, chemical and biological components of in‐stream habitats at different life‐stages. This highlighted variations in catchment‐scale hydroclimate, hydrology, geomorphology and hydrochemistry as essential to understanding freshwater habitats in the wider landscape context. Evolution of research has resulted in a remarkable catalogue of novel findings underlining the value of long‐term data that increases with time as modelling tools advance to leverage more insights from “big data”. Data are available on fish numbers, sizes and ages across multiple life stages, extending over many decades and covering a wide range of stock levels. Combined with an unusually detailed characterization of the environment, these data have enabled a unique process‐based understanding of the controls and bottlenecks on salmon population dynamics across the entire lifecycle and the consequences of declining marine survival and ova deposition. Such powerful datasets, methodological enhancements and the resulting process understanding have informed and supported the development of fish population assessment tools which have been applied to aid management of threatened salmon stocks at large‐catchment, regional and national scales. Many pioneering monitoring and modelling approaches developed have been applied internationally. This history shows the importance of integrating discovery science with monitoring for informing policy development and assessing efficacy of management options. It also demonstrates the need to continue to resource long‐term sites, which act as a focus for inter‐disciplinary research and innovation, and where the overall value of the research greatly exceeds the costs of individual component parts.
To assess the influence of storage dynamics and nonlinearities in hydrological connectivity on time‐variant stream water ages, we used a new long‐term record of daily isotope measurements in ...precipitation and streamflow to calibrate and test a parsimonious tracer‐aided runoff model. This can track tracers and the ages of water fluxes through and between conceptual stores in steeper hillslopes, dynamically saturated riparian peatlands, and deeper groundwater; these represent the main landscape units involved in runoff generation. Storage volumes are largest in groundwater and on the hillslopes, though most dynamic mixing occurs in the smaller stores in riparian peat. Both streamflow and isotope variations are generally well captured by the model, and the simulated storage and tracer dynamics in the main landscape units are consistent with independent measurements. The model predicts that the average age of stream water is ∼1.8 years. On a daily basis, this varies between ∼1 month in storm events, when younger waters draining the hillslope and riparian peatland dominates, to around 4 years in dry periods when groundwater sustains flow. This variability reflects the integration of differently aged water fluxes from the main landscape units and their mixing in riparian wetlands. The connectivity between these spatial units varies in a nonlinear way with storage that depends upon precipitation characteristics and antecedent conditions. This, in turn, determines the spatial distribution of flow paths and the integration of their contrasting nonstationary ages. This approach is well suited for constraining process‐based modeling in a range of northern temperate and boreal environments.
Key Points:
Stream water age is estimated by modeling high‐resolution isotope time series
Storage dynamics drive nonlinear spatial patterns of hydrologic connectivity
Connectivity variations determine nonstationary stream water ages
We combined high‐frequency dissolved organic matter fluorescence (FDOM) data with stable isotope observations to identify the sources and ages of runoff that cause temporal variability in dissolved ...organic carbon (DOC) within a peat‐dominated Scottish catchment. FDOM was strongly correlated (r2 ∼ 0.8) with DOC, allowing inference of a 15 min time series. We captured 34 events over a range of hydrological conditions. Along with marked seasonality, different event responses were observed during summer depending on dry or wet antecedent conditions. The majority of events exhibited anticlockwise hysteresis as a result of the expansion of the riparian saturation zone, mobilizing previously unconnected DOC sources. Water ages from the main runoff sources were extracted from a tracer‐aided hydrological model. Particularly useful were ages of overland flow, which were negatively correlated with DOC concentration. Overland flow age, which ranged between 0.2 and 360 days, reflected antecedent conditions, with younger water generally mobilizing the highest DOC concentrations in summer events. During small events with dry antecedent conditions, DOC response was proportionally higher due to the displacement and mixing of small volumes of previously unconnected highly concentrated riparian soil waters by new precipitation. During large events with wet antecedent conditions, the riparian saturation zone expands to organic layers on the hillslopes causing peaks in DOC. However, these peaks were limited by dilution and supply. This study highlights the utility of linking high‐frequency DOC measurements with other tracers, allowing the effects of hydrologic connectivity and antecedent conditions on delivery of DOC to streams to be assessed.
Key Points
Stream DOC dynamics are strongly linked to variations in the age of source waters
Nonstationary flow‐DOC relationships reflect changing hydrologic connectivity
FDOM is a reliable proxy for long‐term, high‐resolution DOC time series
•Assessing hydrochemistry in montane stream during 10year return period drought.•Well-mixed near surface sources of stream flow during wet conditions.•Highly dynamic baseflow reflects evolving ...contribution of diverse montane groundwater.•Heterogonous groundwater stores show differences in bedrock/drift characteristics.•Stream water consists of deeper, older groundwater sources in lower catchment.
We monitored changing groundwater-surface water interactions during a drought with a 10year return period in a 3.2km2 catchment in the Scottish Highlands. The montane catchment is underlain by granite and metasediments and has extensive cover of diverse drift deposits (70%), which are up to 40m deep. Flat valley bottom areas fringing the stream channel are characterised by deep peat soil (0.5–4m deep) which covers about 20% of the catchment and receive drainage from upslope areas. The drought resulted in small declines in soil moisture and groundwater levels in the valley bottom wetlands, but marked, rapid declines on steeper upland slopes. These coincided with gradual decreases in discharge; however, the chemical and isotopic composition of reduced stream flows showed both temporal and spatial variation. Synoptic hydrogeochemical surveys were carried out on four occasions as flows declined. Each survey repeated sampling of 26 sites along the 3km long stream network. Samples were analysed for major anions, cations and water isotopes. Initial surveys just after the last winter rain showed relatively homogenous stream chemistry, consistent with dominant near-surface drainage from acidic riparian peat soils. Stream chemistry became increasingly enriched with weathering-derived solutes (e.g. alkalinity, Ca2+, Mg2+, etc.) as flows declined and groundwater dominance of flow increased. However, these changes showed marked spatial variability implying geochemical differences in the bedrock geology and the distribution of storage in drift deposits. Temporal dynamics inferred heterogeneous montane groundwater bodies contributed to flows differentially during the recession. Isotope data indicated that in places the stream was also influenced by evaporative losses from the surface of the peat soils. The largest sources of groundwater appear to be located in the drift in the lower catchment where the most marked increase in weathering-derived ions occurred, and depleted, non-fractionated isotope signatures implied deeper inflows.
Spatially explicit knowledge of the origins of water resources for ecosystems and rivers is challenging when using tracer data alone. We use simulations from a spatially distributed model calibrated ...by extensive ecohydrological data sets in a small, energy‐limited catchment, where hillslope‐riparian dynamics are broadly representative of humid boreal headwater catchments that are experiencing rapid environmental transition. We hypothesize that in addition to wetness status, landscape heterogeneity modulates the water pathways that sustain ecosystem function and streamflows. Simulations show that catchment storage inversely controls stream water ages year‐round, but only during the drier seasons for transpiration and soil evaporation. The ages of these evaporative outputs depend much less on wetness status in the oft‐saturated riparian soils than on the freely draining hillslopes that subsidize them. This work highlights the need to consider local dynamics and time‐changing lateral heterogeneities when interpreting the ages, and thus the vulnerability, of water resources feeding streams and ecosystems in landscapes.
Plain Language Summary
Knowing how much time water spends in a landscape (its “age”) helps understanding how water travels through it. These dynamics inform of the stability of water resources for ecosystems and societies, and of their vulnerabilities under climate and land use changes. Water ages may vary depending on how wet or dry a location gets between seasons and years. We thus need to learn more about the demographics (“how much and how old?”) of the water used by plants, evaporated from soils, and flowing in streams, but it is often impossible to monitor the heterogeneity of water pathways within landscapes. Addressing this challenge, we used a numerical model built upon coupling ecohydrological processes and that maps landscape locations. We adjusted this model using multiple data sets in a catchment representative of humid boreal environments where climate and vegetation are rapidly changing. We found markedly different aging patterns between water escaping the system through the plants, soils, and stream, depending on water storage status. This changing duration of water movement also differs between the catchment as a whole and its parts. This method can be used to better understand the multiple ways in which water moves through landscapes, in current and future conditions.
Key Points
Age since precipitation displays inverse storage effect in stream, but not transpiration and soil evaporation, in a humid northern catchment
Hysteresis between storage and the age of transpired water suggests cross‐season carryover, despite weak hydroclimatic seasonality
Downslope water subsidies result in valley bottom having weaker storage‐age relationships than seen in freely draining hillslopes
PDF
