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  • Articles  (4,281)
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  • 1
    Publication Date: 2018-03-16
    Description: Soil salinity affects plant transpiration and growth through two main pathways: the osmotic effect of salt in the soil (osmotic stress; analogous to water stress), and the toxic effect of salt within the plant (ionic stress; salt-specific). However, the drastic and sudden reduction of transpiration exhibited by most species in response to an increase of salinity in the root zone is mainly associated with the osmotic phase, while ionic stress appears at a later time, causing the premature senescence of leaves and the reduction of the plant photosynthetic area. To better investigate the effects of salinity on plant-water relations, we introduce a parsimonious soilplant-atmosphere continuum (SPAC) model accounting for both salt-exclusion at the root level and osmoregulation – i.e. the adjustment of internal water potential in response to salt-stress. The model is used to interpret a paradox observed in salt-tolerant species where transpiration is maximum at an intermediate value of salinity ( ), and is lower in more fresh ( ) and more saline ( ) conditions. Such non-monotonic transpiration-salt concentration ( T r – C ) patterns can be largely explained by plant osmoregulation, while the peak of transpiration at tends to disappear over longer time scales, when ionic stress appears and morphological adaptations become predominant. Osmoregulation emerges here as a water-saving behavior similar to the strategies that xerophytes use to cope with aridity. The maximum of transpiration at is thus the result of a trade-off between the enhancement of salt-tolerance and optimal carbon assimilation.
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    Electronic ISSN: 1944-7973
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  • 2
    Publication Date: 2018-03-15
    Description: A state-of-the-art network of water quality sensors was established in 2012 to gather year-round high temporal frequency hydrochemical data in streams and rivers throughout the state of New Hampshire. This spatially-extensive network includes eight headwater stream and two main-stem river monitoring sites, spanning a variety of stream orders and land uses. Here we evaluate the performance of nitrate, fluorescent dissolved organic matter (fDOM), and turbidity sensors included in the sensor network. Nitrate sensors were first evaluated in the laboratory for interference by different forms of dissolved organic carbon (DOC), and then for accuracy in the field across a range of hydrochemical conditions. Turbidity sensors were assessed for their effectiveness as a proxy for concentrations of total suspended solids (TSS) and total particulate C and N, and fDOM as a proxy for concentrations of dissolved organic matter. Overall sensor platform performance was also examined by estimating percentage of data loss due to sensor failures or related malfunctions. Although laboratory sensor trials show that DOC can affect optical nitrate measurements, our validations with grab samples showed that the optical nitrate sensors provide a reliable measurement of NO 3 concentrations across a wide range of conditions. Results showed that fDOM is a good proxy for DOC concentration (r 2 =0.82) but is a less effective proxy for dissolved organic nitrogen (r 2 =0.41). Turbidity measurements from sensors correlated well with TSS (r 2 =0.78), PC (r 2 =0.53) and PN (r 2 =0.51).
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  • 3
    Publication Date: 2018-03-15
    Description: Sociohydrological studies use interdisciplinary approaches to explore the complex interactions between physical and social water systems and increase our understanding of emergent and paradoxical system behaviors. The dynamics of community values and social cohesion, however, have received little attention in modeling studies due to quantification challenges. Social structures associated with community-managed irrigation systems around the world, in particular, reflect these communities' experiences with a multitude of natural and social shocks. Using the Valdez acequia (a communally-managed irrigation community in northern New Mexico) as a simulation case study, we evaluate the impact of that community's social structure in governing its responses to water availability stresses posed by climate change. Specifically, a system dynamics model (developed using insights from community stakeholders and multiple disciplines that captures biophysical, socioeconomic, and sociocultural dynamics of acequia systems) was used to generate counterfactual trajectories to explore how the community would behave with streamflow conditions expected under climate change. We found that earlier peak flows, combined with adaptive measures of shifting crop selection, allowed for greater production of higher value crops and fewer people leaving the acequia. The economic benefits were lost, however, if downstream water pressures increased. Even with significant reductions in agricultural profitability, feedbacks associated with community cohesion buffered the community's population and land parcel sizes from more detrimental impacts, indicating the community's resilience under natural and social stresses. Continued exploration of social structures is warranted to better understand these systems' responses to stress and identify possible leverage points for strengthening community resilience.
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  • 4
    Publication Date: 2018-03-15
    Description: The analysis of transit/residence time distributions (TTDs and RTDs) provides important insights into the dynamics of stream-water ages and subsurface mixing. These insights have significant implications for water quality. For a small agricultural catchment in central Germany, we use a 3D fully coupled surface-subsurface hydrological model to simulate water flow and perform particle tracking to determine flow paths and transit times. The TTDs of discharge, RTDs of storage and fractional StorAge Selection (fSAS) functions are computed and analyzed on daily basis for a period of 10 years. Results show strong seasonal fluctuations of the median transit time of discharge and the median residence time, with the former being strongly related to the catchment wetness. Computed fSAS functions suggest systematic shifts of the discharge selection preference over four main periods: In the wet period, the youngest water in storage is preferentially selected, and this preference shifts gradually towards older ages of stored water when the catchment transitions into the drying, dry and wetting periods. These changes are driven by distinct shifts in the dominance of deeper flow paths and fast shallow flow paths. Changes in the shape of the fSAS functions can be captured by changes in the two parameters of the approximating Beta distributions, allowing the generation of continuous fSAS functions representing the general catchment behavior. These results improve our understanding of the seasonal dynamics of TTDs and fSAS functions for a complex real-world catchment and are important for interpreting solute export to the stream in a spatially implicit manner.
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  • 5
    Publication Date: 2018-03-15
    Description: Municipal water providers increasingly respond to drought by implementing outdoor water use restrictions to reduce urban water withdrawals and maintain water availability. However, restricting urban outdoor water use to support watershed-scale drought resilience may generate unanticipated cross-scale interactions, for example, by altering drought response and recovery in urban vegetation or urban streamflow. Despite this, urban water conservation is rarely conceptualized or modeled as endogenous to the water cycle. Here, we investigate cross-scale interactions among urban water conservation and water availability, water use, and sociohydrological response in Austin, TX (USA) during a recent anthropogenic (human-influenced) drought. Multi-scalar statistical analyses demonstrated that outdoor water conservation for reservoir management at the municipal scale produced responses that can cascade both ‘upwards' from the city to the watershed (e.g., decoupling streamflow patterns upstream and downstream of Austin at the watershed scale) and ‘downwards' to exert heterogeneous effects within the city (e.g., redistributing water along a socioeconomic gradient at sub-municipal scales, with effects on terrestrial and aquatic ecosystems). We suggest that adapting to anthropogenic drought through irrigation curtailment requires sustained engagement between hydrology and social sciences to integrate socioeconomic status and political feedbacks within and among irrigator groups into the water cycle. Findings from this cross-disciplinary study highlight the importance of a multi-scalar and spatially-explicit perspectives in urban sociohydrology research to uncover how water conservation as adaptation to anthropogenic drought links hydrological processes with issues of socioeconomic inequality and spatiotemporal scale in the Anthropocene.
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  • 6
    Publication Date: 2018-03-13
    Description: Shallow groundwater interacts strongly with surface water across a quarter of global land area, affecting significantly the terrestrial eco-hydrology and biogeochemistry. We examined groundwater behavior subjected to unimodal impulse and irregular surface water fluctuations, combining physical experiments, numerical simulations and functional data analysis. Both the experiments and numerical simulations demonstrated a damped and delayed response of groundwater table to surface water fluctuations. To quantify this hysteretic shallow groundwater behavior, we developed a regression model with the Gamma distribution functions adopted to account for the dependence of groundwater behaviour on antecedent surface water conditions. The regression model fits and predicts well the groundwater table oscillations resulting from propagation of irregular surface water fluctuations in both laboratory- and large-scale aquifers. The coefficients of the Gamma distribution function vary spatially, reflecting the hysteresis effect associated with increased amplitude damping and delay as the fluctuation propagates. The regression model, in a relatively simple functional form, has demonstrated its capacity of reproducing high-order non-linear effects that underpin the surface water and groundwater interactions. The finding has important implications for understanding and predicting shallow groundwater behavior and associated biogeochemical processes, and will contribute broadly to studies of groundwater-dependent ecology and biogeochemistry.
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  • 7
    Publication Date: 2018-03-13
    Description: Biofilms are ubiquitous bacterial communities that grow in various porous media including soils, trickling and sand filters. In these environments, they play a central role in services ranging from degradation of pollutants to water purification. Biofilms dynamically change the pore structure of the medium through selective clogging of pores, a process known as bioclogging. This affects how solutes are transported and spread through the porous matrix, but the temporal changes to transport behavior during bioclogging are not well understood. To address this uncertainty, we experimentally study the hydrodynamic changes of a transparent 3D porous medium as it experiences progressive bioclogging. Statistical analyses of the system's hydrodynamics at four time points of bioclogging (0, 24, 36 and 48 hrs in the exponential growth phase) reveals exponential increases in both average and variance of the flow velocity, as well as its correlation length. Measurements for spreading, as mean-square displacements, are found to be non-Fickian and more intensely superdiffusive with progressive bioclogging, indicating the formation of preferential flow pathways and stagnation zones. A gamma distribution describes well the Lagrangian velocity distributions and provides parameters that quantify changes to the flow, which evolves from a parallel pore arrangement under unclogged conditions, toward a more serial arrangement with increasing clogging. Exponentially evolving hydrodynamic metrics agree with an exponential bacterial growth phase, and are used to parameterize a correlated continuous time random walk model with a stochastic velocity relaxation. The model accurately reproduces transport observations and can be used to resolve transport behavior at intermediate time points within the exponential growth phase considered.
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  • 8
    Publication Date: 2018-03-13
    Description: As the costs and regulatory barriers to new water supply development continue to rise, drought management strategies have begun to rely more heavily on temporary conservation measures. While these measures are effective, they often lead to intermittent and unpredictable reductions in revenues that are financially disruptive to water utilities, raising concerns over lower credit ratings and higher rates of borrowing for this capital intensive sector. Consequently, there is growing interest in financial risk management strategies that reduce utility vulnerabilities. This research explores the development of financial index insurance designed to compensate a utility for drought related losses. The focus is on analyzing candidate hydrologic indices that have the potential to be used by utilities across the U.S., increasing the potential for risk pooling, which would offer the possibility of both lower risk management costs and more widespread implementation. This work first analyzes drought-related financial risks for 315 publicly operated water utilities across the country and examines the effectiveness of financial contracts based on several indices both in terms of their correlation with utility revenues and their spatial autocorrelation across locations. Hydrologic-based index insurance contracts are then developed and tested over a 120-year period. Results indicate that risk pooling, even under conditions in which droughts are subject to some level of spatial autocorrelation, has the potential to significantly reduce the cost of managing financial risk.
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  • 9
    Publication Date: 2018-03-13
    Description: Tracer approaches to estimate both porewater exchange (the cycling of water between surface water and sediments, with zero net water flux) and groundwater inflow (the net flow of terrestrially-derived groundwater into surface water) are commonly based on solute mass balances. However, this requires appropriate characterisation of tracer endmember concentrations in exchanging or discharging water. Where either porewater exchange or groundwater inflow to surface water occur in isolation, then the water flux is easily estimated from the net tracer flux if the endmember is appropriately chosen. However, in most natural systems porewater exchange and groundwater inflow will occur concurrently. Our analysis shows that if groundwater inflow ( Q g ) and porewater exchange ( Q p ) mix completely before discharging to surface water, then the combined water flux ( Q g  +  Q p ) can be approximated by dividing the combined tracer flux by the difference between the porewater and surface water concentrations, ( c p – c ). If Q g and Q p do not mix prior to discharge, then ( Q g  +  Q p ) can only be constrained by minimum and maximum values. The minimum value is obtained by dividing the net tracer flux by the groundwater concentration, and the maximum is obtained by dividing by ( c p – c ). Dividing by the groundwater concentration gives a maximum value for Q g . If porewater exchange and groundwater outflow occur concurrently, then dividing the net tracer flux by ( c p – c ) will provide a minimum value for Q p . Use of multiple tracers, and spatial and temporal replication should provide a more complete picture of exchange processes and the extent of subsurface mixing.
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  • 10
    Publication Date: 2018-03-13
    Description: While it is known that farmers adopt different decision making behaviors to cope with stresses, it remains challenging to capture this diversity in formal model frameworks that are used to advance theory and inform policy. Guided by cognitive theory and the theory of bounded rationality, this research develops a novel, socio-hydrological model framework that can explore how a farmer's perception of water availability impacts crop choice and water allocation. The model is informed by a rich empirical data set at the household level collected during 2013 in Kenya's Upper Ewaso Ng'iro basin that shows that the crop type cultivated is correlated with water availability. The model is able to simulate this pattern and shows that near-optimal or ‘satisficing' crop patterns can emerge also when farmers were to make use of simple decision rules and have diverse perceptions on water availability. By focusing on farmer decision-making it also captures the rebound effect, i.e., as additional water becomes available through the improvement of crop efficiencies it will be reallocated on the farm instead of flowing downstream, as a farmer will adjust his (her) water allocation and crop pattern to the new water conditions. This study is valuable as it is consistent with the theory of bounded rationality, and thus offers an alternative, descriptive model in addition to normative models. The framework can be used to understand the potential impact of climate change on the socio-hydrological system, to simulate and test various assumptions regarding farmer behavior and to evaluate policy interventions.
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