| 1. |
- Bayer-Raich, Martí, et al.
(författare)
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Breakthrough of attenuating contaminant plumes in pumping wells: Analytical model and implications for integral pumping tests.
- 2009
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Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 45, s. W02413-
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Tidskriftsartikel (refereegranskat)abstract
- Monitoring of contaminant plumes and predicting their future fate are essential for effective management of groundwater contaminants. Solute breakthrough curves from operating pumping wells can provide information on the water quality in relatively large aquifer regions, which may be unavailable to instrumentation and direct measurement for practical and/or economical reasons. Relations between spatially varying aquifer concentrations C0(x, y) initially surrounding a well and temporally varying concentrations of subsequently extracted well water, Cp(t), then need to be quantified. However, limited applicability of analytical expressions and numerical inaccuracies related to solving transport equations for converging flow fields hamper such quantifications even in homogeneous aquifers. We use a stream-tube approach and provide a general problem formulation that accounts for first-order degradation and linear, instantaneous, sorption/retardation in heterogeneous aquifers. An analytical expression is obtained for homogeneous aquifer conditions (in the well vicinity), relating any given initial C0(x, y) function and the subsequent contaminant breakthrough Cp(t) in the well. Results for wide plumes subject to first-order degradation show that concentrations at the extraction well will increase as a function of pumping time. This increase is despite the fact that late-time data reflect longer transport paths (to the well), along which mass is removed through degradation. We also derive unique solutions for the inverse problem, in particular considering how the average contaminant concentration Cav (averaged along a control plane through the well within its capture zone, perpendicular to the mean groundwater flow direction) depends on the measurable Cp(t). The solutions demonstrate that the longer the pumping time, the more sensitive the solutions for Cav become to degradation rate constants, which if needed can be determined in situ using multiple control planes.
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| 2. |
- Blazkova, Sarka, et al.
(författare)
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A limits of acceptability approach to model evaluation and uncertainty estimation in flood frequency estimation by continuous simulation : Skalka catchment, Czech Republic
- 2009
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Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 45, s. W00B16-
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Tidskriftsartikel (refereegranskat)abstract
- In this study continuous simulation flood frequency predictions on the Skalka catchment in the Czech Republic (672 km 2, range of altitudes from 460 to 1041 m above sea level), are compared against summary information of rainfall characteristics, the flow duration curve, and the frequency characteristics of flood discharges and snow water equivalent using the generalized likelihood uncertainty estimation limits of acceptability approach outlined by Beven (2006). Limits of acceptability have been defined, prior to running the Monte Carlo model realizations for subcatchment rainfalls, discharges (using rating data) at 5 sites within the catchment, and snow water equivalent in 13 snow zones, 4 of which have observed data. Flood frequency and flow duration data at the outlet of the whole catchment are not used in the evaluation but are used to test the predictions. In order to get sufficient behavioral models to assess adequately the prediction uncertainty it was necessary to refine the model structure, sample the model space more densely, and, in the end, relax the limits of acceptability to allow for a strong realization effect in predicted flood frequencies. We use a procedure of scoring deviations relative to the limits of acceptability to identify the minimum extension of the limits across all criteria to obtain a sample of 4192 parameter sets that were accepted as potentially useful in prediction. Results show that individual model realizations, with the same parameter values, of similar length to the observations can vary significantly in acceptability. Long-term simulations of 10,000 years for retained models were used to obtain uncertain estimates of the 1000 year peak and associated flood hydrographs required for the assessment of dam safety at the catchment outlet.
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| 3. |
- Bottacin-Busolin, Andrea, 1980-, et al.
(författare)
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Probabilistic description of grain resistance from simultaneous flow field and grain motion measurements
- 2008
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 44:9, s. W09419-
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Tidskriftsartikel (refereegranskat)abstract
- Experiments were carried out using a mobile gravel bed placed in a tilting flume with a modified particle image velocimetry (PIV) system. Individual grain movements were surveyed using data from time series of images. Near-bed velocity flow field measurements were made simultaneously above the same area of the sediment surface by applying cross-correlation techniques to the collected plan view images. Statistics of grain motions were collected through a semiautomatic procedure. Significant changes in the flow field were observed in the proximity of the entrained or deposited particles. A strong correlation is shown between the changes in the local streamwise and lateral velocity and the movement of the grains. The theory of Grass is revisited and developed based on the experimental results. The probability distribution of individual grain resistance has been derived from the statistics of the near-bed velocity field and of the entrainment risk.
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| 4. |
- Broxton, P, et al.
(författare)
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On the role of aspect to quantify water transit times in small mountainous catchments
- 2009
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Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 45, s. W08427-W08427
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Tidskriftsartikel (refereegranskat)abstract
- In the current empirical study, we provide evidence about how the hydrologic responses of small mountain catchments are related to aspect (slope direction; exposure) at Redondo Peak, located in the Valles Caldera, New Mexico, USA. Specifically, we test the hypothesis that the transit time of water is related to the catchment aspect. Aspect is an easily measurable and transferable topographic characteristic that is related to the amount of direct solar radiation a particular catchment receives, and therefore, different catchments with different aspects have different rates of snow ablation, evapotranspiration, and water cycling in general. Transit times, which describe the time between when water enters the catchment as precipitation and when it leaves as stream flow, captures many hydrologic features such as flowpath variability and the combined effects of water storage and water fluxes. We have designed an experiment that involves field data collection, isotopic analysis of stream and precipitation samples, and the estimation of transit times using lumped-parameter convolution for 15 sites in small (1-15 km2) catchments that drain different aspects of Redondo Peak. Our data suggests that isotopic variability and estimated transit times are both related to aspect. Other potential relationships between topographical features (such as flowpath length, slope gradient, and elevation) and isotopic measurements of streamwater suggest that landscape and hydrological features are interconnected at Redondo Peak, but these links are not conclusive, suggesting that these topographic indicators do not fully explain the variability of water cycling in these small mountain catchments.
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| 5. |
- Buytaert, Wouter, et al.
(författare)
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Regionalization as a learning process
- 2009
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Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 45, s. W11419-
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Tidskriftsartikel (refereegranskat)abstract
- This paper deals with the uncertainty involved in geographical migration of hydrological model structures, commonly known as regionalization. Regionalization relies on the hypothesis that calibrated parameter sets from a donor catchment can be useful to predict discharge of an ungauged catchment. However, since every catchment is unique, model parameters need to be adapted for differences between a calibration and a prediction catchment, either by transformation or further selection. This process is inherently uncertain. Model parameters, and therefore the required changes, do not exactly represent quantities that we can measure or calculate. This paper outlines an approach to learn about how model parameters should be transformed between a gauged and an ungauged catchment. The approach consists of an iterative process, in which a model structure is applied successively to gauged catchments. After each step, parameter behavior is evaluated as a function of catchment properties and intercatchment similarities. The method is illustrated with an application of a customized version of TOPMODEL to a set of catchments in the Ecuadorian Andes. First, parameter sets are generated for a donor catchment. This model ensemble is then used to predict the discharge of the other catchments, after applying a stochastic parameter transformation to account for the uncertainty in the model migration. The parameter transformation is then evaluated and improved before further application. The case study shows that accurate predictions can be made for predicted basins. At the same time, knowledge is gained about model behavior and potential model limitations.
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| 6. |
- Cvetkovic, Vladimir, et al.
(författare)
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Sorbing tracer experiments in a crystalline rock fracture at Aspo (Sweden) : 2. Transport model and effective parameter estimation
- 2007
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 43:11
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Tidskriftsartikel (refereegranskat)abstract
- [1] Transport and retention of sorbing tracers in a single, altered crystalline rock fracture on a 5 m scale is investigated. We evaluate the results of a comprehensive field study ( referred to as Tracer Retention Understanding Experiments, first phase ( TRUE- 1)), at a 400 m depth of the Aspo Hard Rock Laboratory ( Sweden). A total of 16 breakthrough curves are analyzed, from three test configurations using six radioactive tracers with a broad range of sorption properties. A transport- retention model is proposed, and its applicability is assessed based on available data. We find that the conventional model with an asymptotic power law slope of - 3/ 2 ( one- dimensional diffusion into an unlimited rock matrix) is a reasonable approximation for the conditions of the TRUE- 1 tests. Retention in the altered rock of the rim zone appears to be significantly stronger than implied by retention properties inferred from generic ( unaltered) rock samples. The effective physical parameters which control retention ( matrix porosity and retention aperture) are comparable for all three test configurations. The most plausible in situ ( rim zone) porosity is in the range 1% - 2%, which constrains the effective retention aperture to the range 0.2 - 0.7 mm. For all sorbing tracers the estimated in situ sorption coefficient appears to be larger by at least a factor of 10, compared to the value inferred from through- diffusion tests using unaltered rock samples.
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| 7. |
- Cvetkovic, Vladimir, et al.
(författare)
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Sorbing tracer experiments in a crystalline rock fracture at Aspo (Sweden) : 3. Effect of microscale heterogeneity
- 2008
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 44:12
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Tidskriftsartikel (refereegranskat)abstract
- We study the impact of microscale (10(-3) -10(-2) m) heterogeneity in material and structural properties on sorbing tracer transport in a single crystalline fracture of the TRUE-1 tests (mesoscale, 5 m) at Aspo (Sweden). The analysis is based on the microscale characterization results as presented in part 1 of this series. Our main objective in this last part of the series is to provide an independent interpretation (or "prediction'') of the effective parameters as estimated from calibration in part 2 by combining the data presented in part 1 with analytical and numerical transport modeling. We show here that the independent information from microscopic characterization can be used for "predicting'' the effective diffusion time t(d) reasonably well; a discrepancy is to be expected given the uncertainties of microscale retention properties, in particular of the sorption coefficient.
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| 8. |
- Destouni, Georgia, et al.
(författare)
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Quantifying hydrological and tidal influences on groundwater discharges into coastal waters
- 2005
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Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 41:12, s. W12427-
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Tidskriftsartikel (refereegranskat)abstract
- [1] In coastal aquifers the dynamic mixing zone between intruding seawater and fresh groundwater constitutes a zone of salinity transition that may supply brackish groundwater along with chemical tracers and nutrients to coastal waters. Tidal influence has been proposed as a possible mechanism for enhancement of recirculated seawater, total submarine groundwater discharge (SGD), and associated tracer loading through salinity transition zones into coastal waters. We show that tidal oscillation may, for relatively low SGD cases, considerably increase the average recirculated seawater component of total SGD relative to nontidal conditions. High SGD cases, however, are dominated by and require large fresh groundwater flow components also under tidal conditions; this result is obtained from a wide range of different groundwater simulation scenarios and is supported by direct comparison with field data from different reported high-SGD sites in the world. For cases with hydrologically limited fresh groundwater flow directly into the sea we propose that observed excessive coastal loading of groundwater-derived tracers may be the result of large groundwater flow and transport into unmonitored coastal stream reaches, in addition to SGD.
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| 9. |
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| 10. |
- Fagerlund, Fritjof, et al.
(författare)
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Modeling of nonaqueous phase liquid (NAPL) migration in heterogeneous saturated media : Effects of hysteresis and fluid immobility in constitutive relations
- 2008
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Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 44:3
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Tidskriftsartikel (refereegranskat)abstract
- The confidence in model predictions for nonaqueous phase liquid (NAPL) transport in stochastically heterogeneous systems is limited. The fundamental approaches as well as the constitutive models have not been sufficiently validated, mainly because of the lack of appropriate experimental data. Recently, Fagerlund et al. (2007a, 2007b) presented a set of well-controlled laboratory data that are used here (1) to analyze the overall performance of the continuum-based approach for predicting two-phase NAPL-water flow in stochastically heterogeneous media and (2) to compare the predictions from different constitutive models. The five models tested were the nonhysteretic Brooks-Corey-Burdine (BCB) and van Genuchten-Mualem (VGM) models, the hysteretic versions of these models (HBCB and HVGM), and the HVGBCB model, a model combining the hysteretic van Genuchten (HVG) P c -S relation and the hysteretic Brooks-Corey-Burdine k r -S relation. Two cases of NAPL migration were considered: a layered system of two homogeneous sands separated by a dipping interface and a system where one of the layers was stochastically heterogeneous. The results showed that the best models could indeed capture the main characteristics of the spreading and immobilization well, demonstrating the validity of the continuum-based approach for this level of stochastic heterogeneity. Implementation of hysteresis was necessary for correct prediction of the observed speed of NAPL migration as well as the amount of immobilized NAPL. The three hysteretic models were similar in their overall prediction error-based performance. The HVGM model produced less overestimation of NAPL saturations but instead underestimated the entrapment at capillary barriers in comparison to the HBCB and HVGBCB models. The HVGM model also overestimated the speed of NAPL migration, which is attributed to its closed-form k r -S function, for which the VG parameter m has to be fitted under the constraint m = 1 − 1/n. The HVG (and VG) P c -S function, in contrast, used a different set of VG parameters produced with no constraint on m, which better represented the pore size distributions of the sands. A relation for partial nonwetting phase immobility during drainage is also presented.
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