The additivity model assumed that field-scale reaction properties in a sediment including surface area, reactive site concentration, and reaction rate can be predicted from field-scale grain-size ...distribution by linearly adding reaction properties estimated in laboratory for individual grain-size fractions. This study evaluated the additivity model in scaling mass transfer-limited, multi-rate uranyl (U(VI)) surface complexation reactions in a contaminated sediment. Experimental data of rate-limited U(VI) desorption in a stirred flow-cell reactor were used to estimate the statistical properties of the rate constants for individual grain-size fractions, which were then used to predict rate-limited U(VI) desorption in the composite sediment. The result indicated that the additivity model with respect to the rate of U(VI) desorption provided a good prediction of U(VI) desorption in the composite sediment. However, the rate constants were not directly scalable using the additivity model. An approximate additivity model for directly scaling rate constants was subsequently proposed and evaluated. The result found that the approximate model provided a good prediction of the experimental results within statistical uncertainty. This study also found that a gravel-size fraction (2 to 8 mm), which is often ignored in modeling U(VI) sorption and desorption, is statistically significant to the U(VI) desorption in the sediment.
•An error-decomposition framework is used to assess the errors in BF-predicted E.•Dominant factors in prediction errors of E are identified for 14 basins over China.•P, PET, R and ΔS contribute ...comparably to prediction errors of E in humid climate.•ΔS dominates prediction errors of E in arid climate.
The Budyko framework (BF) has been used to predict evaporation (E) at annual or monthly time scales, but few studies have analyzed the errors in the predicted E in a systematic manner. This study develops an error-decomposition framework which expresses the errors in the BF-predicted annual and monthly E as a function of (1) the anomalies (i.e. deviations from the long-term mean) of precipitation (P), potential evapotranspiration (PET), runoff (R) and catchment water storage change (ΔS), (2) the (long-term) mean water storage change, and (3) the mean difference between the predicted and actual E. The error variance of BF-predicted E can be decomposed into the variance and covariance terms of P, PET, R and ΔS. The relative contribution of each of these controlling factors to the total error variance of E are evaluated at 14 major river basins in China with the mean annual aridity index ranging between 0.55 and 11.78. It is found that climatic factors (P and PET) and catchment responses (R and ΔS) play different roles in the errors of predicted E among diverse climates of 14 basins. Under the humid (energy-limited) condition, the variance and covariance terms of P, PET, R and ΔS are comparably important in the contribution to the prediction error variance of E. In contrast, under the arid (water-limited) condition the error variance of predicted E is dominated by the magnitude of ΔS anomalies. Results of this study suggest that the incorporation of ΔS into BF can improve the predictability of annual and monthly E more under the arid climates than humid climates.
There are many factors affecting submarine groundwater discharge (SGD). However, systematic study of the influences of these factors is still limited. In this study, numerical modeling is performed ...to quantitatively explore the influences of various factors on SGD in a coastal aquifer. In such locations, tidal and terrestrial hydraulic gradients are the primary forces driving fresh and salt water movement. Unlike steady-state flow, dynamic fresh and salt water mixing at the near-shore seafloor may form an intertidal mixing zone (IMZ) near the surface. By constructing a general SGD model, the effects of various model components such as boundary conditions, model geometry and hydraulic parameters are systematically studied. Several important findings are obtained from the study results: (1) Previous studies have indicated there will be a freshwater discharge tube between the classic transition zone and the IMZ. However, this phenomenon may become unclear with the increase of heterogeneity and anisotropy of the medium’s conductivity field. (2) SGD and IMZ are both more sensitive to the vertical anisotropy ratio of hydraulic conductivity (K
x
/K
z
) than to the horizontal ratio (K
x
/K
y
). (3) Heterogeneity of effective porosity significantly affects SGD and IMZ. (4) Increase of the storage coefficient decreases fresh water discharge but increases mixing salt water discharge and total SGD. The increase will also change the shape of the IMZ. (5) Variation of dispersivities does not affect SGD, but significantly changes the distributions of the IMZ and the whole mixing zone. These findings will be helpful to the sampling design of field studies of SGD and to the application of dynamic SGD models to field sites for model development and calibration.
Abstract Drought is one of the major natural hazards with a possibly devastating impact on the regional environment, agriculture, and water resources. Previous studies have assessed the historic ...changes in meteorological drought over various regional scales but have rarely considered hydrological drought due to limited hydrological observations. Here, we use long-term (1960–2012) hydro-meteorological data to analyze the meteorological and hydrological drought comparatively in the Pearl River basin (PRB) in southern China using the standardized precipitation index (SPI) and the standardized runoff index (SRI). The results indicate a strong positive correlation between the SPI and SRI, and the correlation tends to be stronger at the longer timescale. The SPI is reliable to substitute for the SRI to represent the hydrological drought at the long-term scale (e.g., 12 months or longer). Trend analysis reveals a noticeably wetting trend mainly in the eastern regions and a significant drying trend mainly in the western regions and the downstream area of the PRB. The drought frequency is spatially heterogeneous and varies slightly at the interannual scale. Overall, the drought is dominated by noticeable cycles of shorter periodicity (0.75–1.8 years), and periodic cycles in the meteorological drought are mainly responsible for those in the hydrological drought.
Fractional advection–dispersion equations (FADEs) have been widely used in hydrological research to simulate the anomalous solute transport in surface and subsurface water. However, a large gap still ...exists between real‐world application (i.e., being a prediction tool) and theoretical FADEs. To better understand this disparity, the FADEs are firstly reviewed from the perspective of fractional‐in‐time and fractional‐in‐space, as well as the anomalous characteristics described by those functions. Then, challenges for the application of FADEs are summarized, including the theoretical gap of FADEs that needs multidisciplinary efforts to fill, extensive requirements for computation techniques and mathematical knowledge to apply the FADEs, the poor predictability for most parameters in the FADEs, and the limitations for collecting geologic information of flow fields. Then, some suggestions are given for future work, such as developing excellent code sets and mature simulation software with a friendly interface. This kind of work would alleviate the computation workload of hydrologists, especially those without coding expertise. Summarizing and determining the value ranges of the important parameters are needed (e.g., the order of the fractional derivative), through the extensive field, laboratory, and numerical experiments rather than blindly using their mathematical ranges. It is also needed to perform global sensitivity analysis for the FADEs. Meanwhile, comprehensive comparison work is necessary for suggesting a model suitable for a specific problem. Last but not least, further research is clearly needed to establish a link between nonlocal parameters and the heterogeneity property to develop more efficient fractional order partial differential equations.
This article is categorized under:
Science of Water > Methods
Science of Water > Water Quality
A number of factors that can lead to non‐Fickian transport. The hyporheic zones, side pools, vegetation growth, fallen trees, and large stones in a river can trap contaminants and result in a heavy‐tailed breakthrough curve. The high permeability zones in an aquifer can lead to preference flow and to the early arrival of solutes.
The spill pressure of the contaminant source is an important factor affecting the amount, location, form, and behavior of the dense non-aqueous phase liquids (DNAPLs) that plume in a contaminated ...subsurface environment. In this study, perchloroethylene (PCE) infiltration, distribution and, remediation via a surfactant-enhanced aquifer remediation (SEAR) technique for a PCE spill event are simulated to evaluate the effects of the spill pressure of the contaminant source on the DNAPLs’ behavior in two-dimensional homogeneous and heterogeneous aquifers. Five scenarios with different spill pressures of contamination sources are considered to perform the simulations. The results indicate that the spill pressure of the contaminant source has an obvious influence on the distribution of DNAPLs and the associated efficiency of remediation in homogeneous and heterogeneous aquifers. As the spill pressure increases, more and more contaminants come into the aquifer and the spread range of contamination becomes wider and wider. Simultaneously, the remediation efficiency of contamination also decreases from 93.49% to 65.90% as the spill pressure increases from 33.0 kPa to 41.0 kPa for a heterogeneous aquifer with 200 realizations. The simulation results in both homogeneous and heterogeneous aquifers show the same influence of the spill pressure of the contaminant source on PCE behaviors in the two-dimensional model. This study indicates that the consideration of the spill pressure of the contaminant sources (such as underground petrol tanks, underground oil storage, underground pipeline, and landfill leakage) is essential for the disposal of contaminant leakage in the subsurface environment. Otherwise, it is impossible to accurately predict the migration and distribution of DNAPLs and determine the efficient scheme for the removal of contaminant spills in groundwater systems.
Parameter estimation in variable‐density groundwater flow systems is confronted with challenges of strong nonlinearity and heavy computational burden. Relying on a variant of the Henry problem, we ...evaluate the performance of a domain localization scheme of the iterative ensemble Kalman filter in the framework of data assimilation settings for variable‐density groundwater flows in a seawater intrusion scenario. The performance of the approach is compared against (a) the corresponding domain localization scheme of the ensemble Kalman filter in its standard formulation as well as (b) a covariance localization scheme of the latter. The equivalent freshwater head, hf, and salinity, Sa, are set as the target state variables. The randomly heterogeneous field of equivalent freshwater hydraulic conductivity, Kf, is considered as the system parameter field. Density‐independent and density‐driven flow settings are considered to evaluate the assimilation results using various methods and data. When only hf data are assimilated, all tested approaches perform generally well and a localization scheme embedded in the iterative ensemble Kalman filter appears to consistently outperform the domain localized version of the standard ensemble Kalman filter (EnKF) in a density‐driven scenario; Dirichlet boundary conditions tend to show a more pronounced negative effect on estimating Kf for density‐independent than for density‐dependent flow conditions; hf data are more informative in a density‐dependent than in a density‐independent setting. The sole use of Sa information does not yield satisfactory updates of hf for the covariance localization scheme of the standard EnKF, while the sole use of hf does. The domain localization scheme leads to difficulties in the attainment of global filter convergence when only Sa data are used. A covariance localization scheme associated with a standard EnKF can significantly alleviate this issue.
Key Points
We study localized forms of iterative ensemble Kalman filter for data assimilation in a density‐driven flow
Equivalent freshwater head has higher data worth than salinity for estimating both equivalent freshwater head and salinity
A local analysis scheme is not necessarily associated with a guaranteed filter convergence when only salinity data are used
The semiarid Mu Us Sandy Land (MUSL) was selected for this study. It is in the farming‐pastoral ecotone of north China and functions as an eco-environmental barrier.
Afforestation can mitigate ...desertification and soil erosion by improving hydrologic condition, which is particularly true for semiarid and arid regions. However, little is known about the quantitative response of hydrologic improvement to afforestation level that can be measured by leaf area index (LAI). The objective was to setup and use a physically-based model to quantitatively assess the dynamics of water fluxes from Salix psammophila afforestation in the MUSL.
Across the assessment period of 28 April to 3 October 2016, the total transpiration was determined to be about 294.4 mm. As LAI increased, while the transpiration tended to increase and the evaporation to decrease, the evapotranspiration tended to increase with increase of LAI until LAI =2.0 and then plateaued for LAI >2.0. On the other hand, the recharge rate tended to decrease with increase of LAI until LAI =2.0 and then plateaued for LAI >2.0. Overall, the impacts of Salix psammophila afforestation on soil-water replenishment and groundwater recharge would plateau for LAI >2.0, mandating a good balance between solving large-scope eco-environmental problems by Salix psammophila afforestation and sustaining water resources in the long run.
Display omitted
•The HYDRUS-1D performed fairly in simulating fluxes modulated by Salix psammophila.•The water vapor was significant on fluxes but negligible on bottom drainage.•The vapor flux near the soil surface might contribute about 26% to the total flux.•As LAI increased, drainage tended to decrease, whereas ETa showed slight changes.
Characterizing the groundwater flow pattern in a landslide would help to establish a monitor-warning system to predict the movement of a highly concealed and extremely hazard translational landslide. ...The complex fracture network in this type of landslide affects the rainfall infiltration process and the groundwater flow. In this paper, multi-tracer tests, injection tests and electrical resistivity tomography were used to investigate the hydrogeological characteristics and groundwater dynamics of the Kualiangzi translational rock landslide, which is located in the northeast of the Sichuan Basin, China. The study results indicate that there are two kinds of groundwater flow mode, in the landslide, the concentrated mode and the dispersed mode. Tracer and injection test results indicate that the groundwater flow in the landslide is mainly controlled by a vertical preferential flow pathway (concentrated mode), of which the direction is approximately perpendicular to the sliding direction of the landslide. The main runoff direction in the middle of this landslide is southwest according to electrical resistivity tomography. The hydraulic conductivity in the preferential direction is more than 3750 times larger than that perpendicular to the direction. The groundwater flow along the weathered sandstone and mudstone media in the vicinity of the slip surface is very slow (dispersed mode). The results reveal the existence of a preferential flow pathway. The hydrogeological conceptual model is considered as an unbounded domain with an anisotropic medium in the whole area. This hydrogeological model clearly describes the hydrological conditions, and can help establish a monitor-warning system in a translational landslide.
Objective evaluation of the relationships among different types of droughts remains a challenging task due to the combined impacts of climate change and land surface modification caused by human ...activities. Based on the Standardized Precipitation Index (SPI) and Standardized Runoff Index (SRI) at the 3- and 6-month timescales, this study presents a systematic analysis of the relationships between the severity (
S
) and duration (
D
) of meteorological (MD) and hydrological droughts (HD) in the three catchments of the upper Huai River Basin in China. The relation between SPI and SRI is explored by the maximal information coefficient and the mutual entropy. The spatial propagation mechanism of MD is identified by the centroid trajectory, and the response of HD to MD is quantified by the model averaging method. The results indicate a drying (wetting) trend in the upstream (downstream) area, while the centroid trajectory of MD is found in the midstream area, but not associated with the large (or small)
S
and
D
simultaneously. There is a strong correlation (determination coefficient > 0.55) between SPI and SRI in all three subareas, particularly at the 6-month timescale. The increasing influences of human activities (e.g., regulation of water conservancy facilities) from upstream to downstream lead to a weaker correlation between SPI and SRI as well as a decreasing threshold of
D
for MD to trigger HD in downstream. By contrast, the drier climatic conditions are the main reason for the increasing threshold of
S
for MD to trigger HD from upstream to downstream.
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