Showing papers by "Doerthe Tetzlaff published in 2023"

Integrating urban water fluxes and moving beyond impervious surface cover: A review

Abstract: Though urban areas represent a small fraction of global land cover, they have an outsized impact on hydrological processes. Within these areas, the pathways that water follows are fundamentally transformed by the disturbance of soils, land cover, vegetation, topography, and built infrastructure. While progress has been made across many cities to quantify interactions between hydrological processes and the urban environment, many fundamental questions remain unanswered. In this article, we review the state of urban hydrologic science, with an eye towards identifying gaps in our understanding of how water flows through built landscapes. Our review focuses on key topics within urban hydrology related to water quantity, including runoff and streamflow generation, soils and soil water, groundwater, vegetation, and climate. We also describe some of the challenges and opportunities within the field of urban hydrology that we envision will drive future work and collaboration.

Upscaling Tracer‐Aided Ecohydrological Modeling to Larger Catchments: Implications for Process Representation and Heterogeneity in Landscape Organization

Abstract: Stable isotopes of water are ideal tracers to integrate into process‐based models, advancing ecohydrological understanding. Current tracer‐aided ecohydrological modeling is mostly conducted in relatively small‐scale catchments, due to limited tracer data availability and often highly damped stream isotope signals in larger catchments (>100 km2). Recent model developments have prioritized better spatial representation, offering new potential for advancing upscaling in tracer‐aided modeling. Here, we adapted the fully distributed EcH2O‐iso model to the Selke catchment (456 km2, Germany), incorporating monthly sampled isotopes from seven sites between 2012 and 2017. Parameter sensitivity analysis indicated that the information content of isotope data was generally complementary to discharge and more sensitive to runoff partitioning, soil water and energy dynamics. Multi‐criteria calibrations revealed that inclusion of isotopes could significantly improve discharge performance during validations and isotope simulations, resulting in more reasonable estimates of the seasonality of stream water ages. However, capturing isotopic signals of highly non‐linear near‐surface processes remained challenging for the upscaled model, but still allowed for plausible simulation of water ages reflecting non‐stationarity in transport and mixing. The detailed modeling also helped unravel spatio‐temporally varying patterns of water storage‐flux‐age interactions and their interplay under severe drought conditions. Embracing the upscaling challenges, this study demonstrated that even coarsely sampled isotope data can be of value in aiding ecohydrological modeling and consequent process representation in larger catchments. The derived innovative insights into ecohydrological functioning at scales commensurate with management decision making, are of particular importance for guiding science‐based measures for tackling environmental changes.

Enhancing urban runoff modelling using water stable isotopes and ages in complex catchments

Abstract: Increased urbanization, coupled with the projected impacts of climatic change, mandates further evaluation of the impact of urban development on water flow paths to guide sustainable land‐use planning. Though the general urbanization impacts of increased storm runoff peaks and reduced baseflows are well known; how the complex, non‐stationary interaction of the dominant water fluxes within dynamic urban water stores sustain streamflow regimes over longer periods of time is less well quantified. In particular, there is a challenge in how hydrological modelling should integrate the juxtaposition of rapid and slower flow pathways of the urban ‘karst’ landscape and different approaches need evaluation. In this context, we utilized hydrological and water stable isotope datasets within a modelling framework that combined the commonly used Hydrologic Engineering Center Hydrological Modelling System (HEC‐HMS) urban runoff model along with a simple hydrological tracer module and transit time modelling to evaluate the spatial and temporal variation of water flow paths and ages within a heavily urbanized 217 km2 catchment in Berlin, Germany. Deeper groundwater was the primary flow component in the upper reaches of the catchment within fewer urbanized regions, while the addition of wastewater effluent in the mid‐reaches of the catchment was the dominant water supply to sustain baseflow in the lower main stem stream, with additional direct storm runoff and shallow subsurface contributions in the more urbanized lower reaches. Water ages from each modelling approach mirrored flow contributions and water age mixing potential in subsurface storage; with older average water and lower young water contributions in less urbanized sub‐catchments and younger average water and higher young water contributions in more urbanized regions. The results from the first step towards more integrated tracer‐aided hydrologic modelling tools for similar peri‐urban catchments, given the potential limitations of simpler model frameworks. The results have broader implications for assessing the uncertainty in evaluating urban impacts on hydrological function under environmental change.

Synoptic water isotope surveys to understand the hydrology of large intensively managed catchments

Abstract: Understanding the hydrology of large catchments has always been a major challenge due to the spatial heterogeneity of climate, geology, topography, soils and land use. However, precise knowledge of water cycling, storage and losses across large scale catchments is required to sustainably meet growing water demands and adjust water management strategies. This study used four large-scale, seasonal synoptic surveys in 2021 to characterize the spatio-temporal patterns of water isotopes (stable water isotopes and radioisotope tritium at natural abundance) to better understand the complex hydrology of the intensively managed 10,000 km2 catchment of the River Spree in Germany. Apart from the upper headwaters, the hydrology of the Spree is heavily regulated by reservoir releases, pumped minewater discharges, extensive wetlands and lakes, water abstractions and urban drainage. Moreover, the catchment is drought-sensitive with potential evapotranspiration (∼800 mm a year) often exceeding annual rainfall (∼600 mm). This is reflected in the isotopic composition of river water: In the steeper, upper headwaters, the river has a “flashy” rainfall-runoff response, with isotopes plotting close to the Global Meteoric Water Line (GMWL) consistent with dominant groundwater sources, but showing more influence by rainfall in winter. However, flows in the midstream parts are complex due to artificially enhanced baseflows and attenuated high flows from extensive reservoir releases and pumped groundwater releases from dewatering of mines. The reservoir waters are isotopically heavier and reflect the effects of open water evaporation. Fractionation effects strengthen downstream as managed wetland areas in the Spreewald and natural lakes further enhance evaporation and attenuate flows. Water abstractions for drinking water and to sustain the Oder-Spree canal reduce flows further. In Berlin, sources of urban runoff and waste water discharges reduce the effects of fractionation, though samples still plot below the meteoric water line. Seasonally, the effects of evaporation in the lower river network are strongest in summer and autumn, though they remain in winter and spring, indicating a large memory effect due to long mean travel times within the river system. Tritium variability along the river reflects inputs of younger and older water in different parts of the river system; though the influence of pumped groundwater means that the mean age of stream water (i.e., time elapsed since rainfall) in the lower river is likely to be >50 years. Climate change, together with increased population and economic growth in Berlin, is likely to increase pressures on the Spree system in future. In the coming years, the anticipated termination of the lignite mining will lead to a reduction in artificial inputs into surface water, which in turn may exacerbate water stress. We demonstrated that longer-term, synoptic isotope studies are valuable in better understanding the hydrology of such complex, large-scale, heavily modified river systems in terms of insights into water sources and mixing processes; thus, providing an evidence base for more sustainable management in the future.

Quantifying heterogeneity in ecohydrological partitioning in urban green spaces through the integration of empirical and modelling approaches

Abstract: Abstract Urban green spaces (UGS) can help mitigate hydrological impacts of urbanisation and climate change through precipitation infiltration, evapotranspiration and groundwater recharge. However, there is a need to understand how precipitation is partitioned by contrasting vegetation types in order to target UGS management for specific ecosystem services. We monitored, over one growing season, hydrometeorology, soil moisture, sapflux and isotopic variability of soil water under contrasting vegetation (evergreen shrub, evergreen conifer, grassland, larger and smaller deciduous trees), focussed around a 150-m transect of UGS in northern Scotland. We further used the data to develop a one-dimensional model, calibrated to soil moisture observations (KGE’s generally > 0.65), to estimate evapotranspiration and groundwater recharge. Our results evidenced clear inter-site differences, with grassland soils experiencing rapid drying at the start of summer, resulting in more fractionated soil water isotopes. Contrastingly, the larger deciduous site saw gradual drying, whilst deeper sandy upslope soils beneath the evergreen shrub drained rapidly. Soils beneath the denser canopied evergreen conifer were overall least responsive to precipitation. Modelled ecohydrological fluxes showed similar diversity, with median evapotranspiration estimates increasing in the order grassland (193 mm) < evergreen shrub (214 mm) < larger deciduous tree (224 mm) < evergreen conifer tree (265 mm). The evergreen shrub had similar estimated median transpiration totals as the larger deciduous tree (155 mm and 128 mm, respectively), though timing of water uptake was different. Median groundwater recharge was greatest beneath grassland (232 mm) and lowest beneath the evergreen conifer (128 mm). The study showed how integrating observational data and simple modelling can quantify heterogeneities in ecohydrological partitioning and help guide UGS management.

Integrated ecohydrological hydrometric and stable water isotope data of a drought-sensitive mixed land use lowland catchment

Abstract: Abstract. Data from long-term experimental catchments are the foundation of hydrological sciences and are crucial for benchmarking process understanding, observing trends and natural cycles, and being prerequisites for testing predictive models. Integrated data sets which capture all compartments of our landscapes are particularly important in times of land use and climate change. Here, we present ecohydrological data measured at multiple spatial scales which allow differentiation of “blue” water fluxes (which maintain streamflow generation and groundwater recharge) and “green” water fluxes (which sustain vegetation growth). There are two particular unique aspects to this data set: (a) we measured water stable isotopes in the different landscape compartments (i.e. in precipitation, surface water, soil, groundwater, and plant water), and (b) we conducted this monitoring during the extreme drought of 2018 in central Europe. Stable water isotopes are so useful in hydrology as they provide “fingerprints” of the pathways water took when moving through a catchment. Thus, isotopes allow one to evaluate the dynamic relationships between water storage changes and fluxes, which is fundamental to understanding how catchments respond to hydroclimate perturbations or abrupt land use conversion. Second, as we provide the data until 2020, one can also investigate recovery of water stores and fluxes after extreme droughts. Last but not least, lowland headwaters are often understudied systems despite them providing important ecosystem services such as groundwater and drinking water provision and management for forestry and agriculture. The data are available at https://doi.org/10.18728/igb-fred-826.3 (Dämpfling, 2023).

Using spectral and thermal UAS data to infer the influence of shaded and unshaded urban vegetation on evapotranspiration and land surface temperature

Abstract: As the urban population has become predominant globally, heat stress and its negative consequences on human health have grown due to increasingly dense and artificial environments. Urban green infrastructures (UGI) mitigate heat stress by providing cooling services through evapotranspiration (ET) and by blocking solar radiation through shading. Even though ET is a crucial component of urban water and energy regimes, our understanding of the role of vegetation on urban water cycling is still poor when observed through remote sensing. To better understand the seasonal and diurnal variability of ET from urban vegetation, a comprehensive sampling campaign combining an unmanned aerial aircraft system (UAS) and field-based measurements in an urban ecohydrological research observatory in Berlin (Germany) was conducted. The sampling was undertaken throughout an entire growing period (from April to October 2019) to characterize the seasonality of both climatic drivers and phenological effects on ET. Three vegetation types were sampled in the study site&#160; (grassland, forest, and shrubs).&#160;Field-based measurements included sap flow and stomatal conductance (LI-6800 gas exchange), to capture monthly and diurnal dynamics of transpiration, leaf area index (LAI), grassland vegetation height as well as soil moisture. Soil moisture and sap flow were available at hourly resolution while LAI, stomatal conductance and vegetation height were measured at monthly intervals. The images were captured by UAS flights with multi-spectral (Tetracam MCA) and thermal (Flir Tau 2) cameras on a monthly basis and, on some dates, at multiple times during the day to capture diel variability. UAS data were divided into shaded and unshaded areas within the three vegetation classes. ET estimates from UAS observations were derived using the inference method based on vegetation indices (VI) as described in (Nouri et al., 2013), Eddy flux data was used to validate modeled ET and also provided hydroclimatic data .&#160;Results showed clear differences for ET and land surface temperature (LST) between vegetation classes throughout the year, with trees and shrubs showing lower overall temperatures and higher ET estimations than grassland during the observation period. The influence of shadow on modeled ET and observed LST also became apparent for all classes, especially when multiple UAS observations were taken during a single day. Shaded areas exhibited lower overall LST and ET than non-shaded areas, with the starkest contrast exhibited for grassland where shaded areas showed up to 50% lower LST and estimated ET was reduced by up to 25%. Both ET and LST showed correlation to the measured sap flow and stomata conductance at both diurnal and seasonal temporal scale.Our findings provide important insights into the influence of&#160; different urban vegetation types in both ET and LST with respect to shaded and unshaded surfaces. Our study also highlights the importance of a detailed understanding of UGI characteristics and its cooling potential for further improvements in urban green management. Moreover, it could improve models of the urban water cycle and is important for upscaling ET to a broader city scale.

Soil ecological stoichiometry synchronously regulates stream nitrogen and phosphorus concentrations and ratios

Abstract: Whether and how to synchronously regulate stream water nitrogen (N) and phosphorus (P) concentrations and ratios is a major challenge for sustainable aquatic functions. Soil carbon (C):N:P ratios influence soil N and P stocks and biogeochemical processes that elicit subsequent substantial impacts on stream water N and P concentrations and ratios. Therefore, bridging soil and stream water with ecological stoichiometry is one of the most promising technologies for improving stream water quality. Here, we quantified the ecological stoichiometry of soil and stream water relationships across nine catchments. Soil C:P ratio was the main driver of water quality, showing negative correlations with stream water N and P concentrations, and positive correlations with the N:P ratio in P-limited catchments. We revealed that soil C:P ratios higher than 97.8 mol mol−1 are required to achieve the simultaneous regulation of stream water N and P concentrations below the eutrophication threshold and make algal growth P-limited. Furthermore, we found that the relationships between catchment landscape and soil ecological stoichiometry likely provided practical options for regulating soil ecological stoichiometry. Our work highlights that soil ecological stoichiometry can effectively indicate the amount and proportion of soil N and P losses, and can be intervened through rational landscape planning to achieve sustainable aquatic ecosystems in catchments.

Quantifying changes and trends of NO3 concentrations and concentration-discharge relationships in a complex, heavily managed, drought-sensitive river system

Abstract: Stream nitrate nitrogen (NO3) concentrations and concentration-discharge (C-Q) relationships are key indicators of water quality. However, their long-term variability and response to climate change in large-scale catchments are still poorly understood. Here, we report a synoptic survey and long-term observations (2000-2021) in the Spree catchment (10,105 km2), Germany, to identify the spatial-temporal patterns in stream NO3 concentrations and C-Q relationships, as well as the underlying environmental drivers. We found that in the context of gradual reduction of nitrate inputs, the stream NO3 concentrations had a decreasing trend, but showed different magnitudes at different spatial scales (-0.01-0.48 mg L-1 yr-1). In the upstream parts of the catchment - with a high proportion of farmland - high levels of stream NO3 concentration remained with a risk of eutrophication due to the large nitrogen legacy. Especially in winter, stream NO3 concentrations were much higher due to the groundwater export with high NO3 concentrations, a decreased dilution effect of rainfall and vegetation uptake became clear. Consequently, the large nitrogen legacy in the upper catchment resulted in NO3 concentrations not significantly changing with streamflow and showing chemostatic behavior. This was different from NO3 dynamics in the mid- and lower-catchment areas, which were positively related to streamflow, showing a chemodynamic behavior. Further, we found the export patterns (i.e. enrichment vs. dilution) of stream NO3 concentrations strongly correlated with the export regimes (i.e. chemostatic vs. chemodynamic), which were also affected by drought conditions. This is probably due to the decrease in stream depth which would enhance benthic removal leading to a decrease in stream NO3 concentrations. Reduced hydrological connectivity leads to higher spatial heterogeneity of residual NO3 in soil and drainage water, which will result in chemodynamic behavior of stream NO3 concentrations. After rewetting, the export of these high NO3 concentration in soils led to elevated stream NO3 concentrations, resulting in a positive correlation with streamflow. Our work revealed significant heterogeneity in stream NO3 concentrations and C-Q relationships at different spatial-temporal scales in large catchments. Critically, current stream NO3 concentrations and C-Q relationships are likely to respond strongly to future drought, leading to challenges for future land and water management.