The interaction of flow regimes and nutrient fluxes on the water quality and ecosystem health of a clear, freshwater wetland

Across the globe, the hydrology and ecology of wetland systems have been altered by anthropogenic activities, sometimes leading to regime shift or even ecosystem collapse. Often, it is not only the impact of one stressor, but the combination of multiple stressors interacting that ultimately leads to adverse ecological impact in wetland systems. However, because of the difficulty in measuring the combined, dynamic effects of multiple stressors, relatively few studies estimate the relative importance of multiple stressors on wetland ecosystems. We combined controlled laboratory and field experiments with a modeling exercise to examine the relative importance of flow and nutrient loads on the resilience of a clear, groundwater-fed wetland dominated by macrophytes. We examined the potential for a combination of lower inflow and higher nutrient loads to increase phytoplankton growth and reduce light availability, culminating in a reduction in macrophyte growth due to the shading of the phytoplankton. This combination of events could result in a collapse of this endemic ecosystem, including local extinction of several endangered species. We found that the resilience of the macrophyte-dominated wetlands is maintained by preserving high flow even under increasing phosphorus concentrations. Nutrient availability increases as flow decreases, favoring pelagic algal development and inducing a shift in the ecosystem conditions. This shows that focusing only on input nutrient levels, as is often done in open waters of concern, is not sufficient to preserve the native ecosystem and highlights the need to consider multiple factors when assessing anthropogenic impacts on wetlands.Margaret Shanafield, Anna Rigosi, Yang Liu and Justin Brooke

LPMLE3 : a novel 1-D approach to study water flow in streambeds using heat as a tracer

We introduce LPMLE3, a new 1-D approach to quantify vertical water flow components at streambeds using temperature data collected in different depths. LPMLE3 solves the partial differential equation for coupled water flow and heat transport in the frequency domain. Unlike other 1-D approaches it does not assume a semi-infinite halfspace with the location of the lower boundary condition approaching infinity. Instead, it uses local upper and lower boundary conditions. As such, the streambed can be divided into finite subdomains bound at the top and bottom by a temperature-time series. Information from a third temperature sensor within each subdomain is then used for parameter estimation. LPMLE3 applies a low order local polynomial to separate periodic and transient parts (including the noise contributions) of a temperature-time series and calculates the frequency response of each subdomain to a known temperature input at the streambed top. A maximum-likelihood estimator is used to estimate the vertical component of water flow, thermal diffusivity, and their uncertainties for each streambed subdomain and provides information regarding model quality. We tested the method on synthetic temperature data generated with the numerical model STRIVE and demonstrate how the vertical flow component can be quantified for field data collected in a Belgian stream. We show that by using the results in additional analyses, nonvertical flow components could be identified and by making certain assumptions they could be quantified for each subdomain. LPMLE3 performed well on both simulated and field data and can be considered a valuable addition to the existing 1-D methods

Active heat pulse sensing of 3-D-flow fields in streambeds

© Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License.Profiles of temperature time series are commonly used to determine hyporheic flow patterns and hydraulic dynamics in the streambed sediments. Although hyporheic flows are 3-D, past research has focused on determining the magnitude of the vertical flow component and how this varies spatially. This study used a portable 56-sensor, 3-D temperature array with three heat pulse sources to measure the flow direction and magnitude up to 200 mm below the water–sediment interface. Short, 1 min heat pulses were injected at one of the three heat sources and the temperature response was monitored over a period of 30 min. Breakthrough curves from each of the sensors were analysed using a heat transport equation. Parameter estimation and uncertainty analysis was undertaken using the differential evolution adaptive metropolis (DREAM) algorithm, an adaption of the Markov chain Monte Carlo method, to estimate the flux and its orientation. Measurements were conducted in the field and in a sand tank under an extensive range of controlled hydraulic conditions to validate the method. The use of short-duration heat pulses provided a rapid, accurate assessment technique for determining dynamic and multi-directional flow patterns in the hyporheic zone and is a basis for improved understanding of biogeochemical processes at the water–streambed interface

Surface Energy Budgets of Arctic Tundra During Growing Season

This study analyzed summer observations of diurnal and seasonal surface energy budgets across several monitoring sites within the Arctic tundra underlain by permafrost. In these areas, latent and sensible heat fluxes have comparable magnitudes, and ground heat flux enters the subsurface during short summer intervals of the growing period, leading to seasonal thaw. The maximum entropy production (MEP) model was tested as an input and parameter parsimonious model of surface heat fluxes for the simulation of energy budgets of these permafrost‐underlain environments. Using net radiation, surface temperature, and a single parameter characterizing the thermal inertia of the heat exchanging surface, the MEP model estimates latent, sensible, and ground heat fluxes that agree closely with observations at five sites for which detailed flux data are available. The MEP potential evapotranspiration model reproduces estimates of the Penman‐Monteith potential evapotranspiration model that requires at least five input meteorological variables (net radiation, ground heat flux, air temperature, air humidity, and wind speed) and empirical parameters of surface resistance. The potential and challenges of MEP model application in sparsely monitored areas of the Arctic are discussed, highlighting the need for accurate measurements and constraints of ground heat flux.Plain Language SummaryGrowing season latent and sensible heat fluxes are nearly equal over the Arctic permafrost tundra regions. Persistent ground heat flux into the subsurface layer leads to seasonal thaw of the top permafrost layer. The maximum energy production model accurately estimates the latent, sensible, and ground heat flux of the surface energy budget of the Arctic permafrost regions.Key PointThe MEP model is parsimonious and well suited to modeling surface energy budget in data‐sparse permafrost environmentsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/150560/1/jgrd55584.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/150560/2/jgrd55584_am.pd

Emerging Themes and Future Directions of Multi-Sector Nexus Research and Implementation

Water, energy, and food are all essential components of human societies. Collectively, their respective resource systems are interconnected in what is called the “nexus”. There is growing consensus that a holistic understanding of the interdependencies and trade-offs between these sectors and other related systems is critical to solving many of the global challenges they present. While nexus research has grown exponentially since 2011, there is no unified, overarching approach, and the implementation of concepts remains hampered by the lack of clear case studies. Here, we present the results of a collaborative thought exercise involving 75 scientists and summarize them into 10 key recommendations covering: the most critical nexus issues of today, emerging themes, and where future efforts should be directed. We conclude that a nexus community of practice to promote open communication among researchers, to maintain and share standardized datasets, and to develop applied case studies will facilitate transparent comparisons of models and encourage the adoption of nexus approaches in practice

River-aquifer interactions in a semiarid environment investigated using point and reach measurements

A critical hydrological process is the interaction between rivers and aquifers. However, accurately determining this interaction from one method alone is difficult. At a point, the water exchange in the riverbed can be determined using temperature variations over depth. Over the river reach, differential gauging can be used to determine averaged losses or gains. This study combines these two methods and applies them to a 34 km reach of a semiarid river in eastern Australia under highly transient conditions. It is found that high and low river flows translate into high and low riverbed Darcy fluxes, and that these are strongly losing during high flows, and only slightly losing or gaining for low flows. The spatial variability in riverbed Darcy fluxes may be explained by riverbed heterogeneity, with higher variability at greater spatial scales. Although the river-aquifer gradient is the main driver of riverbed Darcy flux at high flows, considerable uncertainty in both the flux magnitude and direction estimates were found during low flows. The reach-scale results demonstrate that high-flow events account for 64% of the reach loss (or 43% if overbank events are excluded) despite occurring only 11% of the time. By examining the relationship between total flow volume, river stage and duration for in-channel flows, we find the loss ratio (flow loss/total flow) can be greater for smaller flows than larger flows with similar duration. Implications of the study for the modeling and management of connected water resources are also discussed. Key Points Losing riverbed fluxes under high flows and approximately neutral under low flows Event driven riverbed fluxes dominate reach losses Smaller events can have higher loss ratio than larger event

Emerging Themes and Future Directions of Multi-Sector Nexus Research and Implementation

Water, energy, and food are all essential components of human societies. Collectively, their respective resource systems are interconnected in what is called the “nexus”. There is growing consensus that a holistic understanding of the interdependencies and trade-offs between these sectors and other related systems is critical to solving many of the global challenges they present. While nexus research has grown exponentially since 2011, there is no unified, overarching approach, and the implementation of concepts remains hampered by the lack of clear case studies. Here, we present the results of a collaborative thought exercise involving 75 scientists and summarize them into 10 key recommendations covering: the most critical nexus issues of today, emerging themes, and where future efforts should be directed. We conclude that a nexus community of practice to promote open communication among researchers, to maintain and share standardized datasets, and to develop applied case studies will facilitate transparent comparisons of models and encourage the adoption of nexus approaches in practice

Emerging Themes and Future Directions of Multi-Sector Nexus Research and Implementation

Water, energy, and food are all essential components of human societies. Collectively, their respective resource systems are interconnected in what is called the “nexus”. There is growing consensus that a holistic understanding of the interdependencies and trade-offs between these sectors and other related systems is critical to solving many of the global challenges they present. While nexus research has grown exponentially since 2011, there is no unified, overarching approach, and the implementation of concepts remains hampered by the lack of clear case studies. Here, we present the results of a collaborative thought exercise involving 75 scientists and summarize them into 10 key recommendations covering: the most critical nexus issues of today, emerging themes, and where future efforts should be directed. We conclude that a nexus community of practice to promote open communication among researchers, to maintain and share standardized datasets, and to develop applied case studies will facilitate transparent comparisons of models and encourage the adoption of nexus approaches in practice

Field comparison of methods for estimating groundwater discharge by evaporation and evapotranspiration in an arid-zone playa

© 2015 Elsevier B.V. Evaporative losses typically play a substantial role in the water balances of arid regions. However, they are often poorly understood due to low flux rates and difficulty in direct measurement. We compared six field methods to quantify groundwater discharge due to evaporative and evapotranspirative fluxes from Stirling Swamp, a playa in central Australia; Bowen ratio-energy balance (BREB), maximum entropy production (MEP), chloride and stable isotope profiling, change in groundwater level, and 14C profiles within the aquifer. The latter method has not been previously used to determine groundwater discharge. Evaporative groundwater discharge estimates varied between 0 and 300mm/y, partly due to variability in spatial and temporal scales captured by the individual methods. Within playa systems where evapotranspiration within the soil is negligible but the depth to groundwater is small, land surface energy balances were found to have the advantage of integrating over hundreds of metres, and when upscaled to annual estimates they agreed well with expected evaporative flux values. Soil profile methods yielded a wide range of results depending on the values of several constants that must be assumed, and the assumption of steady state was found to be a disadvantage. Groundwater methods also had the advantage of integrating over some distance within the aquifer; however, advective transport in the subsurface may have led to under-estimation of evaporative flux with these methods

Impacts of nonuniform flow on estimates of vertical streambed flux

[1] The use of inverse one-dimensional (1-D) analytical methods for estimating vertical stream-aquifer exchange flux is now commonplace. However, the application of such simple models can lead to significant errors in estimates of vertical exchange flux where the model assumptions are violated in real systems. An idea that is gaining acceptance in the literature is that the presence of nonvertical flow is such a violation. However, it is shown here that nonvertical flow by itself will not necessarily lead to errors in vertical flux estimation but rather that significant errors can stem from nonuniform (convergent/divergent) flow fields and/or hydrodynamic dispersion even within uniform flow fields. Nonuniform flow may also be expected, in some cases, to create discrepancies between flux estimates made on the basis of vertical head gradient measurements and those made using 1-D analytical heat tracer methods. Significant differences are observed in the estimates of heat-derived fluxes obtained by the amplitude ratio and phase-shift time-series methods when convergent and divergent flows are apparent. Such differences may potentially be used to infer that convergent or divergent flow is occurring and that a 1-D analysis is inappropriate