Water Resources Research

Journal: Water Resources Research

Abbreviation

Water Resour. Res.

Publisher

American Geophysical Union

ISSN

0043-1397
1944-7973

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Publications 1 - 10 of 117

Experimental and numerical studies on excess-air formation in quasi-saturated porous media
Klump, Stephan; Cirpka, Olaf A.; Surbeck, Heinz; et al. (2008)
Water Resources Research
The concentrations of conservative groundwater components of atmospheric origin often exceed the concentrations in equilibrium with the atmosphere. This phenomenon, called “excess air”, is caused by gas exchange between entrapped gas and groundwater. We present experimental results from a horizontal quasi-saturated sand column, in which we have analyzed excess-air formation by noble-gas analysis and measurement of the total dissolved gas pressure. The experimental results agree with a numerical model based on kinetic dissolution of spherical gas bubbles. Parameter studies show that the dissolution of entrapped gas is controlled by gas-transfer kinetics only when the contact time between the water and gas phases is relatively short. In natural systems, this could be the case for gas exchange within the hyporheic zone of natural rivers. In the typical situation of excess-air formation upon regional groundwater recharge, it seems acceptable to apply gas-transfer models assuming local equilibrium. Additional parameter studies show that the direction of flow considerably affects the evolution of dissolution fronts. Our experimental and numerical results show that apparently unfractionated excess air is formed in the presence of a progressively dissolving gas phase, that is, the excess-air component has an elemental composition similar to that of free atmospheric air. This observation contradicts the common conceptual model that unfractionated excess air indicates the complete dissolution of entrapped air, as formulated in the unfractionated excess air (UA) model (Heaton and Vogel, 1981) which is also contained as a special case in the closed-system equilibrium (CE) model of Aeschbach-Hertig et al. (2000). We conjecture that the estimated fractionation factor, F, and amount of initial gas, A, determined by fitting the CE model to noble-gas concentrations, are biased when F is zero (apparently unfractionated excess air) and A corresponds to initial gas saturations smaller than one per cent.
Spatially resolved water content profiles from inverted time domain reflectometry signals
Oswald, Benedikt; Benedickter, Hansruedi Rudolf; Bächtold, Werner; et al. (2003)
Water Resources Research
We present a method for extracting spatially resolved water content profiles θ(x) from a two-wire time domain reflectometry (TDR) probe. The profile θ(x) is represented in terms of the dielectric εr(x) and ohmic σ(x) properties in the longitudinal direction of the TDR probe. We solve the inverse problem iteratively by combining a one-dimensional time domain solution of the transmission line equations and a genetic optimization method. The method is capable of finding the global optimum in a complicated error landscape without initial assumptions, except physically reasonable limits. The method utilizes both the position and the magnitude of the TDR signal. We analyze water content profiles from laboratory measurements and demonstrate that the achievable spatial resolution can be made as low as 2 cm and even smaller. The present implementation of the numerical code demonstrates the practical feasibility of spatially resolved water content profiles.
Evaporation suppression from water reservoirs
Assouline, S.; Narkis, K.; Or, Dani (2011)
Water Resources Research
Environmental, political, and economic determinants of water quality monitoring in Europe
Beck, Lucas; Bernauer, Thomas; Kalbhenn, Anna (2010)
Water Resources Research
Effective monitoring is essential for effective pollution control in national and international water systems. To what extent are countries' monitoring choices driven by environmental criteria, as they should be? And to what extent are they also influenced by other factors, such as political and economic conditions? To address these questions, we describe and explain the evolution of one of the most important international environmental monitoring networks in Europe, the one for water quality, in the time period 1965–2004. We develop a geographic information system that contains information on the location of several thousand active monitoring stations in Europe. Using multivariate statistics, we then examine whether and to what extent the spatial and temporal clustering of monitoring intensity is driven by environmental, political, and economic factors. The results show that monitoring intensity is higher in river basins exposed to greater environmental pressure. However, political and economic factors also play a strong role in monitoring decisions: democracy, income, and peer pressure are conducive to monitoring intensity, and monitoring intensity generally increases over time. Moreover, even though monitoring is more intense in international upstream-downstream settings, we observe only a weak bias toward more monitoring downstream of international borders. In contrast, negative effects of European Union (EU) membership and runup to the EU's Water Framework Directive are potential reasons for concern. Our results strongly suggest that international coordination and standardization of water quality monitoring should be intensified. It will be interesting to apply our analytical approach also to other national and international monitoring networks, for instance, the U.S. National Water-Quality Assessment Program or the European Monitoring and Evaluation Program for air pollution.
Temporal dependence structure in weights in a multiplicative cascade model for precipitation
Paschalis, Athanasios; Molnar, Peter; Burlando, Paolo (2012)
Water Resources Research
We investigate the ability of the multiplicative random cascade model to accurately simulate temporal precipitation. Specifically, we explore the effect of the dependence structure in cascade weights due to clustering and within-storm variability on the temporal correlation in simulated precipitation, and we compare the results with data at 69 stations with 10 min precipitation records in Switzerland. Correlation is quantified with the oscillation coefficient, which is a measure of patterns of fluctuations in data. Simulation results show that the assumption of temporal independence in cascade weights is generally not supported by observations of both rainfall and snowfall, which show generally higher correlation (lower fluctuations) at the hourly time resolution. Seasonal signatures are also apparent, with higher correlation in the cold season with dominant stratiform precipitation than in the warm season with convective precipitation. Measurement artifacts caused by the tipping bucket mechanism at high resolutions (10 min) are shown to play a significant role in the estimation of the correlation structure in cascade weights because of the quantization of precipitation intensity by the tip volume and sampling time resolution of the gauge. These effects are smoothed out at resolutions above 1 h when the oscillation coefficients become independent of resolution. Such measurement artifacts may have an important effect on the estimated scaling and correlation behavior in precipitation at high temporal resolutions.
The value of glacier mass balance, satellite snow cover images, and hourly discharge for improving the performance of a physically based distributed hydrological model
Finger, David; Pellicciotti, Francesca; Konz, Markus; et al. (2011)
Water Resources Research
Physically based hydrological models describe natural processes more accurately than conceptual models but require extensive data sets to produce accurate results. To identify the value of different data sets for improving the performance of the distributed hydrological model TOPKAPI we combine a multivariable validation technique with Monte Carlo simulations. The study is carried out in the snow and ice-dominated Rhonegletscher basin, as these types of mountainous basins are generally the most critical with respect to data availability and sensitivity to climate fluctuations. Each observational data set is used individually and in combination with the other data sets to determine a subset of best parameter combinations out of 10,000 Monte Carlos runs performed with randomly generated parameter sets. We validate model results against discharge, glacier mass balance, and satellite snow cover images for a 14 year time period (1994–2007). While the use of all data sets combined provides the best overall model performance (defined by the concurrent best agreement of simulated discharge, snow cover and mass balance with their respective measurements), the use of one or two variables for constraining the model results in poorer performance. Using only one data set for constraining the model glacier mass balance proved to be the most efficient observation leading to the best overall model performance. Our main result is that a combination of discharge and satellite snow cover images is best for improving model performance, since the volumetric information of discharge data and the spatial information of snow cover images are complementary.
Climate and vegetation controls on the surface water balance: Synthesis of evapotranspiration measured across a global network of flux towers
Williams, Christopher A.; Reichstein, Markus; Buchmann, Nina; et al. (2012)
Water Resources Research
The Budyko framework elegantly reduces the complex spatial patterns of actual evapotranspiration and runoff to a general function of two variables: mean annual precipitation (MAP) and net radiation. While the methodology has first-order skill, departures from a globally averaged curve can be significant and may be usefully attributed to additional controls such as vegetation type. This paper explores the magnitude of such departures as detected from flux tower measurements of ecosystem-scale evapotranspiration, and investigates their attribution to site characteristics (biome, seasonal rainfall distribution, and frozen precipitation). The global synthesis (based on 167 sites with 764 tower-years) shows smooth transition from water-limited to energy-limited control, broadly consistent with catchment-scale relations and explaining 62% of the across site variation in evaporative index (the fraction of MAP consumed by evapotranspiration). Climate and vegetation types act as additional controls, combining to explain an additional 13% of the variation in evaporative index. Warm temperate winter wet sites (Mediterranean) exhibit a reduced evaporative index, 9% lower than the average value expected based on dryness index, implying elevated runoff. Seasonal hydrologic surplus explains a small but significant fraction of variance in departures of evaporative index from that expected for a given dryness index. Surprisingly, grasslands on average have a higher evaporative index than forested landscapes, with 9% more annual precipitation consumed by annual evapotranspiration compared to forests. In sum, the simple framework of supply- or demand-limited evapotranspiration is supported by global FLUXNET observations but climate type and vegetation type are seen to exert sizeable additional controls.
G-RUN ENSEMBLE: A Multi-Forcing Observation-Based Global Runoff Reanalysis
Ghiggi, Gionata; Humphrey, Vincent; Seneviratne, Sonia I.; et al. (2021)
Water Resources Research
River discharge is an Essential Climate Variable (ECV) and is one of the best monitored components of the terrestrial water cycle. Nonetheless, gauging stations are distributed unevenly around the world, leaving many white spaces on global freshwater resources maps. Here, we use a machine learning algorithm and historical weather data to upscale sparse in situ river discharge measurements. We provide a global reanalysis of monthly runoff rates for periods covering decades to the past century at a resolution of 0.5° (about 55 km), and with up to 525 ensemble members based on 21 different atmospheric forcing data sets. This global runoff reconstruction, named Global RUNoff ENSEMBLE (G-RUN ENSEMBLE), is evaluated using independent observations from large river basins and benchmarked against other publicly available runoff data sets over the period 1981–2010. The accuracy of the data set is evaluated on observed river flow from basins not used for model calibration and is found to compare favorably against state-of-the-art global hydrological model simulations. The G-RUN ENSEMBLE estimates the global mean runoff volume to range between 3.2 × 104 and 3.8 × 104 km3 yr−1. This publicly available data set (https://doi.org/10.6084/m9.figshare.12794075) has a wide range of applications, including regional water resources assessments, climate change attribution studies, hydro-climatic process studies as well as the evaluation, calibration and refinement of global hydrological models.
Critical evaluation of enhancement factors for vapor transport through unsaturated porous media
Shokri, N.; Lehmann, Peter; Or, Dani (2009)
Water Resources Research
Previous experiments have demonstrated “enhanced” water vapor fluxes from partially saturated porous media relative to fluxes predicted based on Fick's law. This led to various mechanistic and phenomenological enhancement factors to reconcile discrepancies between experiments and predictions. In the present study, analyses of abrupt transitions from liquid- to diffusion-controlled mass transport during evaporation from porous media is used to offer new insights regarding key assumptions and adequacy of experimental setups used to develop vapor transport enhancement factors. We reevaluate these concepts considering the role of capillary-induced liquid flow through the unsaturated zone in supplying the total fluxes. The extent and existence of hydraulic connections through the unsaturated zone has been predicted theoretically and demonstrated experimentally using high-resolution X-ray tomography of sand columns during evaporation. Extending these results to the late stages of evaporation from porous media shows simultaneous capillary induced liquid flow and vapor diffusion supplying the total vapor fluxes as validated by laboratory evaporation experiments. Additionally, analysis of evaporation from porous media provides well defined conditions for testing vapor transport based solely on Fick's law. New insights regarding transport mechanisms controlling evaporation rates from porous media emphasize the extended role of capillary flows beyond previously assumed models and may reconcile observations with prediction without invoking unobservable local thermal gradients and similar enhancement factors. We illustrate that considering the coupling between capillary flow and vapor diffusion predicted by Fick's law of diffusion without any enhancement factors is the essential key to estimate the total vapor flux.
What determines drying rates at the onset of diffusion controlled stage-2 evaporation from porous media?
Shokri, N.; Or, Dani (2011)
Water Resources Research
Early stages of evaporation from porous media are marked by relatively high evaporation rates supplied by capillary liquid flow from a receding drying front to vaporization surface. At a characteristic drying front depth, hydraulic continuity to the surface is disrupted marking the onset of stage-2 evaporation where a lower evaporative flux is supported by vapor diffusion. Observations suggest that in some cases the transition is accompanied by a jump in the vaporization plane from the surface to a certain depth below. The resulting range of evaporation rates at the onset of stage-2 is relatively narrow (0.5–2.5 mm d⁻¹). The objective is to estimate the depth of the vaporization plane that defines vapor diffusion length at the onset of stage-2. The working hypothesis is that the jump length is determined by a characteristic length of connected clusters at the secondary drying front that obeys a power law with the system's Bond number. We conducted evaporation experiments using sands and glass beads of different particle size distributions and extracted experimental data from the literature for model comparison. Results indicate the jump length at the end of stage-1 was affected primarily by porous media properties and less so by boundary conditions. Results show power law relationships between the length of the vaporization plane jump and Bond number with an exponent of −0.48 in good agreement with the percolation theory theoretical exponent of −0.47. The results explain the origins of a relatively narrow range of evaporation rates at the onset of stage-2, and provide a means for estimating these rates.

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