Spatial Variability Of Flow Research Articles

Effects of increasing complexity in biogeochemistry and hydrology on variability of total phosphorus concentration in models of a low flow subtropical wetland

In south Florida, treatment wetlands known as stormwater treatment areas (STAs) are used to reduce total phosphorus (P) concentrations of agricultural and urban stormwater runoff before discharging into the Everglades Protection Area. The remobilization of P from immobile forms stored in soils and litter has the potential to negatively affect STA treatment efficiency. Phosphorus biogeochemical dynamics in one of the best performing Everglades STA flow-ways, STA-2 FW1, were modeled mechanistically and empirically using diverse streams of data (flow, total P in water, soil, floc, vegetation). We evaluated the effect of P wetland uptake and hydrologic transport on STA-2 FW1's simulated outflow P concentrations by juxtaposing varying complexities of biogeochemistry (1–2 vs 13 wetland P pools) and hydrology (uniform vs. nonuniform flow) models. The uniform flow model considers a constant velocity with each time step while velocity in the nonuniform flow model is a stochastic ensemble mimicking spatial flow variability. When coupled with uniform or nonuniform flow, the variability of water column P concentration in the most complex model was significantly different from the two simple biogeochemistry models, although it was lower compared to data in all models. Regardless of the biogeochemistry model, uniform versus nonuniform transport did not affect simulated outflow P concentration. Significant interaction effects between the choices of biogeochemistry and hydrology models were observed. The most complex model included specific compartments for vegetation, floc and soil and reproduced data trends of more P accumulated in upstream areas that decreases with increasing distance away from inflow. Modeling biotic mechanisms and tracking P in vegetation and sediments, confirms the importance of including biogeochemistry in understanding and predicting outflow P concentrations in STA-2 FW1. This modeling tool could be used to guide future STA data collection needs and inform Everglades STA performance optimization efforts.

Electrical resistivity tomography (ERT) measurements during water flow in a date palm stem segment

Commercial cultivation of date palms has high economic significance, particularly in arid and hyper-arid regions. Achieving commercially viable yield of dates amid water scarcity and enforced restrictions on water use requires optimising the amount and timing of irrigation. To that end, better understanding of the temporal dynamics of the water use of date palm is of great importance. Traditionally, this can be achieved through sap flow estimates obtained using heat dissipation probes. However, such measurements lack the ability to provide information on the spatial distribution of sap flow, which is important considering that date palms transport water in the entire stem cross-section. The aim of this study is to evaluate whether electrical resistivity tomography measurements have the potential to provide information on the spatial variability of flow in date palm stems. For this, flow was induced through a stem segment that was obtained from a felled date palm tree. ERT measurements were continuously obtained throughout a cycle of flow and no-flow periods. The results showed that the mean bulk electrical conductivity varied strongly due to changes in the flow conditions. In addition, it was found that the electrical conductivity of the outflow was much higher than that of the inflow, which indicates the release of stored salt from the stem segment. Analysis of the spatial distribution of the electrical conductivity suggested that flow mainly occurred in a limited part of the cross-sectional area of the stem. Overall, it was concluded that the ERT is a promising tool to investigate the spatial variability of water flow in date palm stems.

Porewater flow patterns in surficial cold seep sediments inferred from conservative tracer profiles and early diagenetic modeling

Porewater flow at cold seeps controls methane oxidation and carbonate precipitation rates, and also plays an important role in gas hydrate dynamics. We constrain the porewater flow rates in the upper sediment of Gulf of Mexico cold seeps by fitting measured 226Ra and Cl− profiles. The approach exploits their natural increase with depth through shallow sediments at non-brine (226Ra) and brine (Cl−) seeps. Vertical flow velocities at non-brine and brine seeps are found to be similar in magnitude, typically on the order of 0.1 m y−1. The accuracy of the flow estimates is assessed and shown to depend substantially on uncertainties in both concentrations and sediment tortuosity. Model simulations further reveal that the natural heterogeneity of sediment permeability can cause considerable lateral variability in vertical flow at the scale of a sediment core, which implies that the coring location and core size can have a substantial effect on the estimated porewater velocities. The formation of gas hydrates and calcium carbonates reduce sediment permeability, and decrease the spatial variability in flow over time, as the formation rates are higher along preferential flow paths. This analysis of the effect of spatiotemporal variability on chemical profiles improves the assessment of uncertainties in the estimated magnitude of flow, and is important for the evaluation of benthic-pelagic coupling at seeps.

Subtidal circulation in a microtidal Mediterranean bay

We examine the role of different forcings on the subtidal circulation in a microtidal bay with freshwater inputs in the NW Mediterranean Sea: Alfacs Bay. Observations of subtidal flow in summer 2013 and winter 2014 reveal a two-layered, vertically sheared circulation. During the summer, there is a significant positive correlation between surface currents and winds along the main axis of the bay, while a negative correlation is observed between wind and the bottom layers. During the winter, the cross-shore response is correlated with the most energetic winds, showing a two-layered vertical structure inside the bay and a nearly depth-independent water motion caused by high wind speeds at the bay mouth. The vertical structure of the velocities, as determined through empirical orthogonal function analysis, confirms that surface layers are affected by winds and bottom currents correlated negatively with winds as a response of the wind set-up. Seasonal mean circulation reveals gravitational exchange at the bay mouth during the summer. However, mean circulation is unclear in the inner bay and close to the drainage channels. Observed flow patterns are supported by modelling results that confirm the persistence of averaged current in the low-frequency dynamics. Re-circulation areas in the inner bay indicate the rich spatial variability in flow at low-frequency time scales.

Impacts of Realistic Urban Heating, Part I: Spatial Variability of Mean Flow, Turbulent Exchange and Pollutant Dispersion

As urbanization progresses, more realistic methods are required to analyze the urban microclimate. However, given the complexity and computational cost of numerical models, the effects of realistic representations should be evaluated to identify the level of detail required for an accurate analysis. We consider the realistic representation of surface heating in an idealized three-dimensional urban configuration, and evaluate the spatial variability of flow statistics (mean flow and turbulent fluxes) in urban streets. Large-eddy simulations coupled with an urban energy balance model are employed, and the heating distribution of urban surfaces is parametrized using sets of horizontal and vertical Richardson numbers, characterizing thermal stratification and heating orientation with respect to the wind direction. For all studied conditions, the thermal field is strongly affected by the orientation of heating with respect to the airflow. The modification of airflow by the horizontal heating is also pronounced for strongly unstable conditions. The formation of the canyon vortices is affected by the three-dimensional heating distribution in both spanwise and streamwise street canyons, such that the secondary vortex is seen adjacent to the windward wall. For the dispersion field, however, the overall heating of urban surfaces, and more importantly, the vertical temperature gradient, dominate the distribution of concentration and the removal of pollutants from the building canyon. Accordingly, the spatial variability of concentration is not significantly affected by the detailed heating distribution. The analysis is extended to assess the effects of three-dimensional surface heating on turbulent transfer. Quadrant analysis reveals that the differential heating also affects the dominance of ejection and sweep events and the efficiency of turbulent transfer (exuberance) within the street canyon and at the roof level, while the vertical variation of these parameters is less dependent on the detailed heating of urban facets.

A probabilistic formulation of bed load transport to include spatial variability of flow and surface grain size distributions

AbstractBed load fluxes are typically calculated as a function of the reach averaged boundary shear stress and a representative bed grain size distribution. In steep, rough channels, heterogeneous bed surface texture and macro‐roughness elements cause significant local deviations from the mean shear stress but this variability is often omitted in bed load calculations. Here we present a probabilistic bed load transport formulation that explicitly includes local variations in the flow field and grain size distribution. The model is then tested in a 10% gradient stream, to evaluate its predictive capability and to explore relations between surface grain size sorting and boundary shear stress. The boundary shear stress field, calculated using a quasi‐3D hydraulic model, displayed substantial variability between patch classes, but the patch mean dimensionless shear stress varied inversely with patch median grain size. We developed an empirical relation between the applied shear stress on each patch class and the reach averaged shear stress and median grain size. Predicted sediment volumes using this relation in our bed load equation were as accurate as those using complete shear stress distributions and more accurate than current bed load transport equations. Our results suggest that when spatially variable grain size distributions (e.g., patches of sediment) are present they must be explicitly included in bed load transport calculations. Spatial variability in shear stress was relatively more important than grain size variations for sediment transport predictions.

Spatial Variability of the Flow and Turbulence Within a Model Canopy

The spatial variability of the mean flow and turbulence in and above a model canopy is investigated using three-dimensional laser Doppler velocimetry. The mean flow and turbulence are shown to be highly variable in space within the canopy but rapidly converge above the canopy. The coherent variations in the mean flow generate dispersive fluxes contributing almost a fifth to the total flux of momentum, and a greater contribution to the divergence of the flux, within the canopy. The higher-order turbulent statistics are more variable than the mean flow and often strongly correlated in space to variations in the mean flow. The implications of this microscale spatial variability for both field experiments and other laboratory experiments into canopy flow are discussed.

Using flow signatures and catchment similarities to evaluate the E-HYPE multi-basin model across Europe

Open data make it possible to set up multi-basin models for large domains across environmental, climate and administrative boundaries. This study presents new methods for evaluating a number of aspects of multi-basin model performance, while exploring the performance of the E-HYPE_v2.1 model for several evaluation criteria in 181 independent river gauges across the European continent. Embedded model assumptions on dominant flow generating mechanisms are analysed by correlating physiographical characteristics to the flow regime. The results indicate that the model captures the spatial variability of flow and is therefore suitable for predictions in ungauged basins. The model shows good performance of long-term means and seasonality, while short-term daily variability is less well represented, especially for Mediterranean and mountainous areas. Major identified shortcomings refer to the resolution of precipitation patterns, aquifer exchanges, water extractions and regulation. This will guide the work with the next model version for which improvements in input data, processes and calibration have been identified to potentially contribute most to improved model performance.EDITOR D. Koutosyiannis ASSOCIATE EDITOR A. Efstratiadis

Spatial variability of flow over a river-influenced inner shelf in coastal Alabama during spring

Spring-time water column velocity data in 2011 and density data from a series of spring-time hydrographic surveys from 2008 to 2011 were used to examine the spatial variability of the circulation over the inner shelf of the Mississippi Bight off Mobile Bay. Spring-time depth-averaged currents were eastward at all sites, but the vertical profiles were different. East of Mobile Bay the along-shelf flow was eastward, with an offshore component at the surface and an onshore component at depth, indicative of upwelling circulation. West of Mobile Bay the along-shelf flow was also eastward, with a characteristic region of negative vertical shear in the upper layer of the water column. The deeper site had an across-shelf flow structure similar to the east sites, while the shallower site exhibited onshore flow throughout the water column. These spatial differences are attributed, in part, to the seasonally averaged effects of local wind forcing and discharge. In terms of wind forcing, the depth-averaged along-shelf current responded to along-shelf wind asymmetrically in favor of upwelling (more transport in upwelling than during downwelling). Thus, weak seasonal downwelling favorable wind conditions did not inhibit the velocity profiles from having upwelling circulation. West of Mobile Bay, negative vertical shears in the upper portion of the velocity profiles were consistent with the influence of freshwater discharge. This freshwater influence is supported with available chlorophyll-a data (as a freshwater proxy), which showed an enhanced freshwater influence west of Mobile Bay. In addition, across-shelf density data showed a shallow lens of freshwater west of Mobile Bay. These findings have implications for understanding the transport of river-derived nutrients on the Mississippi–Alabama shelf.

Small-scale spatial variability of turbulence statistics, (co)spectra and turbulent kinetic energy measured over a regular array of cube roughness

The spatial variability of flow and turbulence properties above an outdoor scale model of regular cube roughness under neutral stability is investigated using eight simultaneously employed sonic anemometers which are characterized by an extra short transducer span (0.05 m) and high sampling rate (50 Hz). Measurements are conducted in a layer between the top of the cubes with a plan area density of 0.25 and two times the height of the roughness, \(H\). Large spatial variability (horizontal and vertical) is observed at the two lowest measurement levels (\(z=H\) and \(1.25H\)) for all variables considered which include mean wind, Reynolds stress, integral statistics, (co)spectra, budget of turbulent kinetic energy and a spatial quadrant analysis of the momentum transfer. The spatial inhomogeneity almost disappears at \(z = 1.5H\) where normalized variables attain homogeneous surface layer values for neutral stratification. The present results support a new conceptual framework to explain the turbulence behavior just above the canopy consisting of two sublayers which develop in response to the strongly varying surface. Although an individual measurement location below \(z = 1.5H\) is unable to produce representative turbulence statistics above the present bluff body roughness, averaging across a sufficient number of measurement locations to achieve complete spatial sampling of all surface characteristics (canyon spaces, rooftops, etc.), produces representative statistics even at heights which are probably still within the roughness sublayer with values close to the respective inertial subrange predictions.

Observations of spatial flow patterns at the coral colony scale on a shallow reef flat

Although small‐scale spatial flow variability can affect both larger‐scale circulation patterns and biological processes on coral reefs, there are few direct measurements of spatial flow patterns across horizontal scales <100 m. Here flow patterns on a shallow reef flat were measured at scales from a single colony to several adjacent colonies using an array of acoustic Doppler velocimeters on a diver‐operated traverse. We observed recirculation zones immediately behind colonies, reduced currents and elevated dissipation rates in turbulent wakes up to 2 colony diameters downstream and enhanced Reynolds stresses in shear layers around wake peripheries. Flow acceleration zones were observed above and between colonies. Coherent flow structures varied with incident flow speeds; recirculation zones were stronger and wakes were more turbulent in faster flows. Low‐frequency (<0.03 Hz) flow variations, for which water excursions were large compared with the colony diameters (Keulegan‐Carpenter number, KC >1), had similar spatial patterns to wakes, while higher‐frequency variations (0.05–0.1 Hz, KC < 1) had no observable spatial structure. On the reef flat, both drag and inertial forces exerted by coral colonies could have significant effects on flow, but within different frequency ranges; drag dominates for low‐frequency flow variations and inertial forces dominate for higher‐frequency variations, including the wave band. Our scaling analyses suggest that spatial flow patterns at colony and patch scales could have important implications for both physical and biological processes at larger reef scales through their effects on forces exerted on the flow, turbulent mixing, and dispersion.

Three‐dimensional flow in centered pool‐riffle sequences

Key Points Detailed 3D velocity measurements on pools/riffles show stage‐dependent patterns Stage‐dependent patterns include jets, convergence/divergence and rotating cells Stage‐dependent patterns affect spatial flow variability and self‐maintenance

Extreme channelization of fluid and the problem of element mobility during Barrovian metamorphism

Geochemical profiles across chemically altered zones (selvages) surrounding amphibolite facies quartz+kyanite veins were investigated to determine major, minor, and trace element mass transfer and volume strain due to channelized fluid flow. The three profiles are perpendicular to two veins cutting the Wepawaug Schist, Connecticut, U.S.A., which underwent Barrovian-style metamorphism during the Acadian orogeny. Selvages are highly aluminous with considerably more kyanite, staurolite, and garnet, and less quartz, plagioclase, and mica, than surrounding wallrocks. Kyanite crystals increase in size toward veins, and reach several centimeters in length within veins. Mass balance analysis indicates 50% silica loss and 34% volume loss from selvages, on average. Silica, transferred locally from selvages to veins, accounts for 40 to 80% of the vein silica; the remainder must have been precipitated from fluids that flowed through the veins during regional devolatilization. Fluid flow also transported elements into the rock mass, which were concentrated in the selvages. Gains of Fe, Mn, Y, and HREE were due to the growth of selvage garnet; Fe, Zn, and Li were sequestered into staurolite. Kyanite, staurolite, and garnet growth resulted in Al mass gains. Destruction of mica (particularly muscovite) and of plagioclase in the selvages resulted in losses of K, Na, Ba, Pb, Sn, and volatiles and losses of Na, Sr, and Eu, respectively. Thin Na and Sr enrichment zones associated with increased modal plagioclase are found along the margins between selvages and less altered wallrock and may represent either chemical self-organization produced during diffusive mass transfer and reaction, or are relics from a possible earlier period of Na enrichment in the selvages. Simple, two-dimensional numerical modeling of flow in a fractured porous medium indicates that fluxes vary significantly over short distances (<1 m) adjacent to veins. Fluids, channelized into the high-permeability fracture conduits, carry the bulk of the fluid flow; mass transfer to and from selvages adjacent to the conduits occurred by some combination of diffusion and flow. In contrast, areas distal to the conduits are impoverished in fluid and undergo much more limited infiltration. As a consequence, different workers can come to vastly different conclusions about the magnitude of fluid fluxes and element transfer depending on the part of a fractured outcrop studied. This extreme spatial variability due to channelization can largely explain contrasting views that have arisen in the literature regarding the nature and intensity of non-volatile element mass transfer during Barrovian metamorphism. Determining “average” time-integrated fluid fluxes and levels of element transport across outcrops remains as important research challenges due to the spatial variability of flow.

Coupled 1D–3D hydrodynamic modelling, with application to the Pearl River Delta

Within the hydrodynamic modelling community, it is common practice to apply different modelling systems for coastal waters and river systems. Whereas for coastal waters 3D finite difference or finite element grids are commonly used, river systems are generally modelled using 1D networks. Each of these systems is tailored towards specific applications. Three-dimensional coastal water models are designed to model the horizontal and vertical variability in coastal waters and are less well suited for representing the complex geometry and cross-sectional areas of river networks. On the other hand, 1D river network models are designed to accurately represent complex river network geometries and complex structures like weirs, barrages and dams. A disadvantage, however, is that they are unable to resolve complex spatial flow variability. In real life, however, coastal oceans and rivers interact. In deltaic estuaries, both tidal intrusion of seawater into the upstream river network and river discharge into open waters play a role. This is frequently approached by modelling the systems independently, with off-line coupling of the lateral boundary forcing. This implies that the river and the coastal model run sequentially, providing lateral discharge (1D) and water level (3D) forcing to each other without the possibility of direct feedback or interaction between these processes. An additional disadvantage is that due to the time aggregation usually applied to exchanged quantities, mass conservation is difficult to ensure. In this paper, we propose an approach that couples a 3D hydrodynamic modelling system for coastal waters (Delft3D) with a 1D modelling system for river hydraulics (SOBEK) online. This implies that contrary to off-line coupling, the hydrodynamic quantities are exchanged between the 1D and 3D domains during runtime to resolve the real-time exchange and interaction between the coastal waters and river network. This allows for accurate and mass conserving modelling of complex coastal waters and river network systems, whilst the advantages of both systems are maintained and used in an optimal and computationally efficient way. The coupled 1D–3D system is used to model the flows in the Pearl River Delta (Guangdong, China), which are determined by the interaction of the upstream network of the Pearl River and the open waters of the South China Sea. The highly complex upstream river network is modelled in 1D, simulating river discharges for the dry and wet monsoon periods. The 3D coastal model simulates the flow due to the external (ocean) periodic tidal forcing, the salinity distribution for both dry and wet seasons, as well as residual water levels (sea level anomalies) originating from the South China Sea. The model is calibrated and its performance extensively assessed against field measurements, resulting in a mean root mean square (RMS) error of below 6% for water levels over the entire Pearl River Delta. The model also represents both the discharge distribution over the river network and salinity transport processes with good accuracy, resolving the discharge distribution over the main branches of the river network within 5% of reported annual mean values and RMS errors for salinity in the range of 2 ppt (dry season) to 5 ppt (wet season).

Size sorting in marine muds: Processes, pitfalls, and prospects for paleoflow‐speed proxies

The basis for, and use of, fine grain size parameters for inference of paleoflow speeds is reviewed here. The basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field, experimental data on suspended sediment transport and deposition, and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed. In the deep sea, sorting events occur under resuspension/deposition events in benthic storms. At flow speeds below 10–15 cm s−1, size in the noncohesive “sortable silt” (10–63μm) range is controlled by selective deposition, whereas above that range, removal of finer material by winnowing also plays a role. The best particle size instruments to measure a flow speed–related grain size employ the settling velocity method, while laser diffraction sizers can yield misleading results because of particle shape effects. Potential problems, including source effects, downslope supply on continental margins, spatial variability of flow over bedforms, and influence of ice‐rafted detritus, are examined. A number of studies using the sortable silt flow speed proxy are reviewed, and inverse modeling of grain size distributions is examined. Outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed.

Testing bedload transport formulae using morphologic transport estimates and field data: lower Fraser River, British Columbia

AbstractMorphologic transport estimates available for a 65‐km stretch of Fraser River over the period 1952–1999 provide a unique opportunity to evaluate the performance of bedload transport formulae for a large river over decadal time scales. Formulae tested in this paper include the original and rational versions of the Bagnold formula, the Meyer‐Peter and Muller formula and a stream power correlation. The generalized approach adopted herein does not account for spatial variability in flow, bed structure and channel morphology. However, river managers and engineers, as well as those studying rivers within the context of long‐term landscape change, may find this approach satisfactory as it has minimal data requirements and provides a level of process specification that may be commensurable with longer time scales. Hydraulic geometry equations for width and depth are defined using morphologic maps based on aerial photography and bathymetric survey data. Comparison of transport predictions with bedload transport measurements completed at Mission indicates that the original Bagnold formula most closely approximates the main trends in the field data. Sensitivity analyses are conducted to evaluate the impact of inaccuracies in input variables width, depth, slope and grain size on transport predictions. The formulae differ in their sensitivity to input variables and between reaches. Average annual bedload transport predictions for the four formulae show that they vary between each other as well as from the morphologic transport estimates. The original Bagnold and Meyer‐Peter and Muller formulae provide the best transport predictions, although the former underestimates while the latter overestimates transport rates. Based on our findings, an error margin of up to an order of magnitude can be expected when adopting generalized approaches for the prediction of bedload transport. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Can Satellite Sampling of Offshore Wind Speeds Realistically Represent Wind Speed Distributions? Part II: Quantifying Uncertainties Associated with Distribution Fitting Methods

Abstract Remote sensing tools represent an attractive proposition for measuring wind speeds over the oceans because, in principle, they also offer a mechanism for determining the spatial variability of flow. Presented here is the continuation of research focused on the uncertainties and biases currently present in these data and quantification of the number of independent observations (scenes) required to characterize various parameters of the probability distribution of wind speeds. Theoretical and empirical estimates are derived of the critical number of independent observations (wind speeds derived from analysis of remotely sensed scenes) required to obtain probability distribution parameters with an uncertainty of ±10% and a confidence level of 90% under the assumption of independent samples, and it is found that approximately 250 independent observations are required to fit the Weibull distribution parameters. Also presented is an evaluation of Weibull fitting methods and determination of the fitting method based on the first and third moments to exhibit the “best” performance for pure Weibull distributions. Further examined is the ability to generalize parameter uncertainty bounds presented previously by Barthelmie and Pryor for distribution parameter estimates from sparse datasets; these were found to be robust and hence generally applicable to remotely sensed wind speed data series.

Stochastic groundwater quality management: Role of spatial variability and conditioning

In this paper a methodology for the stochastic management of groundwater quality problems is presented. A stochastic algorithm to solve the coupled flow and mass transport inverse problem is combined with a stochastic management approach to design a reliable pump‐and‐treat scheme. Three main differences exist between this research and previously published research in the field of groundwater management. The first is the method of parameterization, where we have explicitly considered spatial variability of flow and mass transport parameters in our method. Second, we have used adjoint and/or sensitivity equations to calculate first and second derivatives of state variables with respect to hydraulic parameters. Finally, the third point is the nonlinear multiobjective formulation we adopt which enables us to explore the whole trade‐off curve between reliability and cost of remediation. A number of key results are listed here. The stochastic inverse algorithm used here, the representers method, makes it possible to estimate heterogeneity as reflected by the measurements of parameters and state variables. Meanwhile, using the adjoint operator to accurately estimate sensitivity of state variables with respect to the parameters has facilitated the use of the head and concentration measurements in the inverse algorithm. Moreover, stochastic management, which is conditioned on head and concentration measurements simultaneously are shown to provide more realistic and reliable remediation schemes. Finally, with the aid of the multiobjective formulation of the management problem and analysis of its results, it is possible to identify a significant remediation scheme. Such a scheme has the advantage of producing maximum possible reliability with the least possible increase in the total cost of remediation.

Susceptibility of a sandy loam soil to preferential flow as affected by tillage

Flow patterns for water and solutes in structured soils are often heterogeneous. Understanding the spatial variability of flow is essential in solute transport studies and for management of chemical movement. We hypothesized the occurrence of effects of alternative tillage operations for winter wheat on flow patterns and that such effects could be revealed by dye tracing. Tracer studies using the anionic dye Brilliant Blue FCF were conducted on a sandy loam soil (Agrudalf) subjected to four different tillage treatments: (T1) — harrowing two times with a springtine harrow, drilling; (T2) — direct drilling; (T3) — ploughing, light subsurface compaction, one harrowing with a PTO-driven rotary harrow, drilling; (T4) — ploughing, one harrowing with a springtine harrow, drilling. Studies were conducted (i) in the autumn after plant emergence 4 weeks after tillage and planting, and (ii) in the spring 7 months after tillage and planting. Twenty-five millimetres of water containing 4.0 g l −1 of the dye was applied uniformly to the soil surface within 1 h. Plots were excavated 1 day after dye application. Stained flow patterns on 22 vertical 1.00 m 2 soil profiles from each treatment were photographed and subjected to image analysis. Deeply penetrating flow paths were found for all treatments both in the autumn and in the spring. The number of individually bounded, stained flow pathways per metre profile length averaged over depth in the 30–100 cm soil layer (NP 30–100) and the degree of coverage of profile faces with dye in the 0–20 cm layer (DC 0–20) were significantly affected by tillage treatment, both in the spring and in the autumn ( P<0.001). Averaged for the 22 profiles per treatment, NP 30–100 equalled to 2.0, 3.5, 0.7, and 1.8 m −1 for T1, T2, T3 and T4, respectively, while DC 0–20 equalled to 57, 44, 64 and 65%, respectively. Horizon boundaries and other observable structural features related to soil tillage and structural development appeared to be very important for the initiation of preferential flow.

Colloid-facilitated tracer transport by steady random ground-water flow

We study the transport of reactive solute in a three-phase system (water–solid matrix-colloids) in natural porous media. Semianalytical (integral) solutions are derived for the first time, which can be used for computing expected concentration, mass flux, or discharge for the dissolved as well as for colloid-bounded tracer. The results are based on a few simplifying assumptions: advection-dominated transport, linear mass transfer reactions, and steady-state colloidal concentration. Derived semianalytical expressions capture the main features of colloid-facilitated transport (the reversible-equilibrium and irreversible-kinetic sorption of tracers on colloids), and are applicable for the general class of linear sorption processes on the porous matrix. Derived solutions account for spatial variability of flow and sorption parameters, which is relevant for field-scale applications. We apply the theoretical results to the transport of neptunium and plutonium, using flow and transport data from the alluvial aquifer near Yucca Mountain, Nevada. Based on the zeroth and first temporal moment, dimensionless indicators are proposed for assessing the potential impact of colloid-facilitated tracer transport in aquifers. Generic sensitivity curves show the importance of tracer-colloid kinetic rates. Even very low irreversible rates (which will generally be difficult to determine in the laboratory) may yield observable effects for sufficiently long transport times. The obtained results can be used for assessing the significance of colloid-facilitated tracer transport under field conditions, as well as for setting further constraints on relevant parameters which need to be estimated in the field.