Showing papers on "Water flow published in 2000"

Classification of geomorphological effects downstream of dams

Abstract: The effects of dams on downstream geomorphology are reviewed and a typology is devised, consisting of nine cases. The classification can be seen as a further development of Lane's balance between water discharge, sediment load, grain size, and river slope. Depending on changes in released water flow and changes in released sediment load, relative to the transport capacity of the flow, it is possible to estimate resulting cross-sectional geomorphology. The longitudinal extent of changes and their variability with time, and the tributary response to altered mainstream cross-section changes, are also discussed.

Water uptake by roots: effects of water deficit

Abstract: The variable hydraulic conductivity of roots (Lp(r)) is explained in terms of a composite transport model. It is shown how the complex, composite anatomical structure of roots results in a composite transport of both water and solutes. In the model, the parallel apoplastic and cell-to-cell (symplastic and transcellular) pathways play an important role as well as the different tissues and structures arranged in series within the root cylinder (epidermis, exodermis, cortex, endodermis, stelar parenchyma). The roles of Casparian bands and suberin lamellae in the root's endo- and exodermis are discussed. Depending on the developmental state of these apoplastic barriers, the overall hydraulic resistance of roots is either more evenly distributed across the root cylinder (young unstressed roots) or is concentrated in certain layers (exo- and endodermis in older stressed roots). The reason for the variability of root Lp(r), is that hydraulic forces cause a dominating apoplastic flow of water around protoplasts, even in the endodermis and exodermis. In the absence of transpiration, water flow is osmotic in nature which causes a high resistance as water passes across many membranes on its passage across the root cylinder. The model allows for a high capability of roots to take up water in the presence of high rates of transpiration (high demands for water from the shoot). By contrast, the hydraulic conductance is low, when transpiration is switched off. Overall, this results in a non-linear relationship between water flow and forces (gradients of hydrostatic and osmotic pressure) which is otherwise hard to explain. The model allows for special root characteristics such as a high hydraulic conductivity (water permeability) in the presence of a low permeability of nutrient ions once taken up into the stele by active processes. Low root reflection coefficients are in line with the idea of some apoplastic bypasses for water within the root cylinder. According to the composite transport model, the switch from the hydraulic to the osmotic mode is purely physical. In the presence of heavily suberized roots, the apoplastic component of water flow may be too small. Under these conditions, a regulation of radial water flow by water channels dominates. Since water channels are under metabolic control, this component represents an 'active' element of regulation. Composite transport allows for an optimization of the water balance of the shoot in addition to the well-known phenomena involved in the regulation of water flow (gas exchange) across stomata. The model is employed to explain the responses of plants to water deficit and other stresses. During water deficit, the cohesion-tension mechanism of the ascent of sap in the xylem plays an important role. Results are summarized which prove the validity of the coehesion/tension theory. Effects of the stress hormone abscisic acid (ABA) are presented. They show that there is an apoplastic component of the flow of ABA in the root which contributes to the ABA signal in the xylem. On the other hand, (+)-cis-trans-ABA specifically affects both the cell level (water channel activity) and water flow driven by gradients in osmotic pressure at the root level which is in agreement with the composite transport model. Hydraulic water flow in the presence of gradients in hydrostatic pressure remains unchanged. The results agree with the composite transport model and resemble earlier findings of high salinity obtained for the cell (Lp) and root (Lp(r)) level. They are in line with known effects of nutrient deprivation on root Lp(r )and the diurnal rhythm of root Lp(r )recently found in roots of LOTUS.

Stormflow generation in steep forested headwaters: a linked hydrogeomorphic paradigm

Abstract: Headwater catchments are sources of sediments, nutrients, and biota for larger streams, yet the hydrologic pathways that transport these materials remain unclear. Dynamics of stormflow generation related to landform attributes and antecedent rainfall were investigated in a steep forested headwater catchment at Hitachi Ohta Experimental Watershed, Japan. Such headwater catchments are deeply incised: the narrow riparian corridors have limited capacities to store and transmit water to streams. Storm runoff was monitored at several nested scales within the catchment: (1) 2·48 ha first-order drainage (FB); (2) incipient 0·84 ha first-order drainage (FA) comprized of two zero-order basins; (3) 0·25 ha zero-order basin (ZB); and (4) 45 m2 hillslope segment (HS), including subsurface matrix flow (MF) and preferential flow (PF). Results from applied tracer and staining tests as well as observations of piezometric, tensiometric, and subsurface temperature responses were also employed to elucidate hydrologic pathways during storms. During the driest conditions, water yield from FB was only 1%; runoff occurred as saturated overland flow from the small riparian zone and direct channel interception. For slightly wetter conditions, subsurface flow from the soil matrix augmented stormflow. As wetness increased, two significant non-linear hydrologic responses occurred: (1) threshold response in geomorphic hollows (zero-order basins) where runoff initiated after an accumulation of shallow groundwater; and (2) self-organization and expansion of preferential flow pathways, which facilitate subsurface drainage. Stormflow increases observed during periods of increasing antecedent wetness depend upon temporal and spatial linkages and the unique hydrologic behavior of three components: (1) narrow riparian corridors; (2) linear hillslopes; and (3) geomorphic hollows. These linkages form the basis for an emerging hydrogeomorphic concept of stormflow generation for steep forested headwaters. Knowledge of stormflow response is critical to the assessment of management practices in these headwater areas as well as the routing of water and materials to larger stream systems. Copyright © 2000 John Wiley & Sons, Ltd.

Amelioration strategies for saline soils: a review

Abstract: Sodic soils are characterized by the occurrence of excess sodium (Na+) to levels that can adversely affect soil structure and disturb availability of some nutrients to plants. Such changes ultimately affect crop growth and yield. There are large areas of the world that exist under sodic soils and need attention for efficient, inexpensive and environmentally feasible amelioration. Sodic soil amelioration involves increase in calcium (Ca2+) on the cation exchange sites at the expense of Na+. The replaced Na+ together with excess soluble salts, if present, is removed from the root zone through infiltrating water as a result of excessive irrigations. Records nearly a century old reveal the use of water, crop, chemical amendment, electric current, and tillage as amelioration tools for such soils. Among the amelioration strategies, chemical amendments have an extensive usage. Owing to gradual increases in amendment cost in some parts of the world during the last two decades, this amelioration strategy has become cost-intensive, particularly for the subsistence farmers in developing countries. In the meantime, phytoremediation with low initial investment has emerged as a potential substitute of chemical amelioration. Phytoremediation works through plant root action that helps dissolve native soil calcite (CaCO3) of low solubility to supply adequate levels of Ca2+ for an effective Na+−Ca2+ exchange without the application of an amendment. Although significant progress has been achieved in improving amelioration methods, a great deal of work remains to analyse the economics of such methods with focus on (1) the long-term sustainability of the amelioration projects and (2) the consequences of amelioration for the farmer himself, other growers and society as a whole. Computer modelling may help assess economic viability of different soil amelioration methods to extend results broadly to other similar locations. In addition, computer modelling to stimulate movement and reactions of salts in sodic soils has been a potentially useful complement to experimental data. However, such models need evaluation under field conditions. Copyright © 2001 John Wiley & Sons, Ltd.

Modeling Soil–Landscape and Ecosystem Properties Using Terrain Attributes

Abstract: Soil-landscape patterns result from the integration of short- and long-term pedogeomorphic processes. A 2-ha hillslope catena in California shows short-distance variation in A horizon depth from 8 to 80 cm and in soil depth from 8 to >450 cm in convex to concave positions. Similar variations in net primary productivity (NPP) and soil C represent significant information often not captured by soil survey maps. Strong correlations between these measured soil-landscape variables and explanatory digital terrain attributes are used to develop quantitative soil-landscape models. We were able to account for between 52 and 88% of soil property variance using easily computed terrain variables such as slope and flow accumulation. Spatial implementation of the models suggest lateral redistribution processes resulting in differential accumulation of C and soil mass in convergent and divergent landscape positions. The models are explicit and quantitative, which enables their use for testing hypotheses about the spatial distribution of fine-scale landscape and ecosystem processes and for parameterizing spatially distributed hydrological and ecosystem simulation models.

Water uptake by plant roots: An integration of views

Abstract: A COMPOSITE TRANSPORT MODEL is presented which explains the variability in the ability of roots to take up water and responses of water uptake to different factors The model is based on detailed measurements of 'root hydraulics' both at the level of excised roots (root hydraulic conductivity, Lpr) and root cells (membrane level; cell Lp) using pressure probes and other techniques The composite transport model integrates apoplastic and cellular components of radial water flow across the root cylinder It explains why the hydraulic conductivity of roots changes in response to the nature (osmotic vs hydraulic) and intensity of water flow The model provides an explanation of the adaptation of plants to conditions of drought and other stresses by allowing for a `coarse regulation of water uptake' according to the demands from the shoot which is favorable to the plant Coarse regulation is physical in nature, but strongly depends on root anatomy, eg on the existence of apoplastic barriers in the exo- and endodermis Composite transport is based on the composite structure of roots A `fine regulation' results from the activity of water channels (aquaporins) in root cell membranes which is assumed to be under metabolic and other control

The Diabatic Deacon Cell

Abstract: Southern Ocean air–sea fluxes from the COADS dataset are examined for compatibility between buoyancy gain and northward Ekman transport. An analysis in density classes points to an upwelling of Upper Circumpolar Deep Water and subsequent buoyancy gain from air–sea exchange as water flows northward in the Ekman layer. The Upper Circumpolar Deep Water layer is too shallow to admit southward geostrophic flow along topography, and an eddy mass flux mechanism for southward transport in this layer to replenish the upwelling is advanced, based on observed large-scale potential vorticity gradients. Estimates of the strength and vertical structure of the meridional flow are given using repeated hydrographic sections and a new climatology.

SHETRAN: Distributed River Basin Flow and Transport Modeling System

Abstract: Physically based spatially distributed (PBSD) river basin models have been available for over 10 years. One of their strengths lies in the way the surface and subsurface are represented as coupled parts of a whole, giving ground-water flows that are controlled by such factors as realistic surface saturation and infiltration, and surface conditions that are controlled by realistic groundwater levels, discharges, and so forth. PBSD sed- iment and solute transport models can be integrated into PBSD river basin modeling systems, and the integrated systems are powerful tools for studying the environmental impacts associated with land erosion, pollution, and the effects of changes in land use and climate, and also in studying surface-water and ground-water resources and their management. SHETRAN takes PBSD river basin modeling a step further, in that multifraction sediment transport and multiple, reactive solute transport are handled within a single system, fully coupled to water flow, and the subsurface is modeled as a fully 3D variably saturated heterogeneous medium. SHETRAN therefore has a substantial capability for addressing environmental and water resources problems that span the traditional disciplines of river basin and ground-water modeling.

Simulation of daily and monthly stream discharge from small watersheds using the swat model

Abstract: The Soil and Water Assessment Tool (SWAT) was evaluated and parameter sensitivities were determined while modeling daily streamflows in a small central Kentucky watershed over a two-year period. Streamflow data from 1996 were used to calibrate the model and streamflow data from 1995 were used for evaluation. The model adequately predicted the trends in daily streamflow during this period although Nash-Sutcliffe R 2 values were –0.04 and 0.19 for 1995 and 1996, respectively. The model poorly predicted the timing of some peak flow values and recession rates during the last half of 1995. Excluding daily peak flow values from August to December improved the daily R 2 to 0.15, which was similar to the 1996 daily R 2 value. The Nash-Sutcliffe R 2 for monthly total flows were 0.58 for 1995 and 0.89 for 1996 which were similar to values found in the literature. Since very little information was available on the sensitivity of the SWAT model to various inputs, a sensitivity analysis/calibration procedure was designed to evaluate parameters that were thought to influence stream discharge predictions. These parameters included, drainage area, slope length, channel length, saturated hydraulic conductivity, and available water capacity. Minimization of the average absolute deviation between observed and simulated streamflows identified optimum values/ranges for each parameter. Saturated hydraulic conductivity, alpha baseflow factor, drainage area, channel length, and channel width were the most sensitive parameters in modeling the karst influenced watershed. The sensitivity analysis process confirmed die trace studies in the karst watershed that a much larger area contributes to streamflow than can be described by the topographic boundaries. Overall, the results indicate that the SWAT model can be an effective tool for describing monthly runoff from small watersheds in central Kentucky that have developed on karst hydrology however calibration data are necessary to account for solution channels draining into or out of the topographic watershed.

Field measurement of soil surface hydraulic properties by disc and ring infiltrometers: A review and recent developments

Abstract: Soil management influences physical properties and mainly the soil hydraulic functions. Their measurement becomes one of the research preferences in this branch of applied soil science. Tension disc and pressure ring infiltrometers have become very popular devices for the in situ estimates of soil surface hydraulic properties. Their use for measuring solute–water transfer parameters of soils is now well established too. A number of publications testify that both devices have been extensively used all around the world for different purposes. In this review, a short introduction is devoted to the background theory and some examples are given to show how the theory can be used to determine hydraulic conductivity and sorptivity from measured cumulative infiltration. The methods of analysis of cumulative infiltration are based either on quasi-analytical solutions of the flow equation for homogeneous soil profile or on inverse parameter estimation techniques from the numerical solution of flow equation whether the soil profile is homogeneous or not. The disc infiltrometer has also been shown as a suitable device for inferring parameters describing the water-borne transport of chemicals through near saturated soils. Associated with conservative tracers, it has been recognized as a promising tool for the determination of both hydraulic and solute transport properties as well as for other parameters such as mobile/immobile water content fraction or exchange coefficient. An emphasis is put here on some published studies performed in different soils and environmental conditions focusing on heterogeneous soil profiles (crusted soils) or structured cultivated soils (aggregated soils), either when local water transport process is studied or when field spatial variability is investigated. Some new research studies such as water–solute transfer in structured or swelling–shrinking soils and multi-interactive solute transport are emerging. A number of challenges still remain unresolved for both theory and practice for tension and pressure infiltrometers. They include questions on how to consider and characterize saturated–unsaturated preferential flow or preferential transport process (including hydrodynamic instabilities) induced by biological activity (e.g. capillary macropores, earthworm holes or root channels) by specific pedagogical conditions (e.g. cracking, crusting) and by soil management practices (i.e. conservation tillage).

Semiarid Crop Production from a Hydrological Perspective: Gap between Potential and Actual Yields

Abstract: Rapid population growth in the dry climate regions, arable land scarcity, and irrigation expansion limitations direct our interest to possibilities of yield increase in rainfed agriculture Literature, however, indicates large differences between actual and potential yields, and between yields on farmers’ fields and research stations This article focuses on the determinants of these yield gaps and the windows of opportunity for yield increase on the farmer's field together with the agricultural challenges involved The study links the conventional approach to estimate crop water requirements and dry spell effects on biomass production to a conceptual Green Water Crop Model This model addresses the effects on crop yields of the sequential diversions of infiltrating rainfall (rainwater partitioning into runoff, plant available soil water, and deep percolation) and of different relations between nonproductive evaporation flow and productive transpiration flow, defined together as green water flow Also, th

Physically-Based River Basin Modelling within a GIS: the LISFLOOD Model.

Abstract: Although many geographical information systems (GISs) are very advanced in data processing and display, current GIS are not capable of physically based modelling. Especially, simulating transport of water and pollutants through landscapes is a problem in a GIS environment. A number of specific routing methods are needed in a GIS for hydrologic modelling, amongst these are the numerical solutions of the Saint-Venant equations, such as the kinematic wave approximation for transport of surface water in a landscape. The PCRaster Spatial Modelling language is a GIS capable of dynamic modelling. It has been extended recently with a kinematic wave approximation simulation tool to allow for physically based water flow modelling. The LISFLOOD model is an example of a physically based model written using the PCRaster GIS environment. The LISFLOOD model simulates river discharge in a drainage basin as a function of spatial data on topography, soils and land cover. Although hydrological modelling capabilities have largely increased, there is still a need for development of other routing methods, such as a diffusion wave. Copyright © 2000 John Wiley & Sons, Ltd.

Hydrodynamic stimuli and the fish lateral line.

Abstract: Sensory systems need to distinguish biologically relevant stimuli from background noise. Here we investigate how the lateral-line mechanosensory system of the fish senses minute water motions1 in the vicinity while exposed to running water. We find that one class of receptor in the lateral line, the canal neuromasts, can respond to hydrodynamic stimuli even in the presence of unidirectional water flow, whereas superficial neuromasts, which predominate in still-water fish, cannot.

Conservation tillage and macropore factors that affect water movement and the fate of chemicals

Abstract: A thorough understanding of how conservation tillage influences water quality is predicated on knowledge of how tillage affects water movement. This paper summarizes the effects of conservation tillage on water movement and quality mainly based on long-term experiments on Luvisols at the North Appalachian Experimental Watershed near Coshocton, OH, USA. Conservation tillage can have a much larger effect on how water moves through the soil than it does on the total amount percolating to groundwater. Soil macroporosity and the proportion of rainfall moving through preferential flow paths often increase with the adoption of conservation tillage and can contribute to a reduction in surface runoff. In some medium- and fine-textured soils most of the water that moves to the subsoil during the growing season (May-October) is probably transmitted by macropores. If a heavy, intense storm occurs shortly after surface application of an agricultural chemical to soils with well-developed macroporosity, the water transmitted to the subsoil by the macropores may contain significant amounts of applied chemical, up to a few per cent, regardless of the affinity of the chemical for the soil. This amount can be reduced by an order of magnitude or more with the passage of time or if light rainstorms precede the first major leaching event. Because of movement into the soil matrix and sorption, solutes normally strongly adsorbed by the soil should only be subject to leaching in macropores in the first few storms after application. Even under extreme conditions, it is unlikely that the amount of additional adsorbed solute transported to groundwater will exceed a few per cent of the application when conservation tillage is used instead of conventional tillage. In the case of non-adsorbed solutes, such as nitrate, movement into the soil matrix will not preclude further leaching. Therefore, when recharge occurs during the dormant season thorough flushing of the soil, whether macropores are present or not, can move the remaining solutes to groundwater. Thus, the net effect of tillage treatment on leaching of non-adsorbed solutes should be minimal.

Export of dissolved organic carbon and nitrogen from Gleysol dominated catchments – the significance of water flow paths

Abstract: In this study, we estimated whether changes in hydrological pathwaysduring storms could explain the large temporal variations of dissolvedorganic carbon (DOC) and nitrogen (DON) in the runoff of threecatchments: a forest and a grassland sub-catchment of 1600m2 delineated by trenches, and a headwater catchment of 0.7km2. The average annual DOC export from the sub-catchments was 185 kg DOCha−1 y−1 for the forest, 108 kg DOCha−1 y−1 for the grassland and 84 kgDOC ha−1 y−1 for the headwatercatchment. DON was the major form of the dissolved N in soil and streamwater. DON export from all catchments was approximately 6 kg Nha−1 y−1, which corresponded to 60% ofthe total N export and to 50% of the ambient wet N deposition. DOC andDON concentrations in weekly samples of stream water were positivelycorrelated with discharge. During individual storms, concentrations andproperties of DOC and DON changed drastically. In all catchments, DOCconcentrations increased by 6 to 7 mg DOC l−1 comparedto base flow, with the largest relative increment in the headwatercatchment (+350%). Concentrations of DON, hydrolysable amino acids, andphenolics showed comparable increases, whereas the proportion ofcarbohydrates in DOC decreased at peak flow. Prediction of DOC and DONconcentrations by an end-member mixing analysis (EMMA) on the base ofinorganic water chemistry showed that changes in water flow pathslargely explained these temporal variability. According to the EMMA, thecontribution of throughfall to the runoff peaked in the initial phase ofthe storm, while water from the subsoil dominated during base flow only.EMMA indicated that the contribution of the DOC and DON-rich topsoil washighest in the later stages of the storm, which explained the highestDOC and DON concentrations as the hydrograph receded. Discrepanciesbetween observed and predicted concentrations were largest for thereactive DOC compounds such as carbohydrates and phenolics. Theyoccurred at base flow and in the initial phase of storms. This suggeststhat other mechanisms such as in-stream processes or a time-variantrelease of DOC also played an important role.

The vulnerability of wetlands to climate change: A hydrologic landscape perspective

Abstract: The vulnerability of wetlands to changes in climate depends on their position within hydrologic landscapes. Hydrologic landscapes are defined by the flow characteristics of ground water and surface water and by the interaction of atmospheric water, surface water, and ground water for any given locality or region. Six general hydrologic landscapes are defined; mountainous, plateau and high plain, broad basins of interior drainage, riverine, flat coastal, and hummocky glacial and dune. Assessment of these landscapes indicate that the vulnerability of all wetlands to climate change fall between two extremes: those dependent primarily on precipitation for their water supply are highly vulnerable, and those dependent primarily on discharge from regional ground water flow systems are the least vulnerable, because of the great buffering capacity of large ground water flow systems to climate change.

Hospital effluents as a source of gadolinium in the aquatic environment

Abstract: Total annual Gd emission of a hospital offering a maximum spectrum medical services using Gd complexes in magnetic resonance imaging was computed and independently measured by ICP/MS. The Gd emission was between 2.1 and 4.2 kg per year, yielding a theoretical concentration of 8.5−30.1 μg per L in the hospital's effluent. Gd concentrations measured on different days were below detection limit (1 μg per L) and 55 μg per L, and annual average concentrations were between 10.5 and 20.5 μg per L as calculated from analytical results, water flow, and total water consumption. The concentrations in the influent of the municipal sewage treatment plant (STP) receiving the effluent were always below detection limit indicating that there was no other major discharge of Gd. Based on consumption data, total Gd input by German hospitals is estimated to be roughly 132 kg per year. An elevation of the natural concentration of Gd in German surface waters by 0.003−0.004 μg per L will result from this amount, if there is no e...

An assessment of ecosystem services: water flow regulation and hydroelectric power production

Abstract: Forest ecosystems in the watersheds of the Yangtze river regulate water flow in the rivers. The value of water flow regulation by ecosystems is usually not realized in situ but may transfer spatially through rivers to another spot out of watersheds where conditions are suitable to realize it. To take into account the transfer of value of biological resources spatially, we developed a process-based simulation model to estimate the capacity of water flow regulation by terrestrial ecosystems, taking into account such major processes as canopy interception, litter absorption, and soil/ground water conservation. In this study we combined models and a GIS-embodied spatial database to assess the capacity and benefits of water flow regulation by ecosystems in Xingshan County, Hubei Province, China. The capacity of water flow regulation differs substantially among the 90 types of vegetation–soil–slope complexes in the watersheds. The simulation model estimates that in a wet season the watershed can retain ∼868.07 ...

Lentic macroinvertebrate assemblage structure along gradients in spatial heterogeneity, habitat size and water chemistry.

Abstract: Littoral zones of small water bodies are spatially heterogeneous habitats, harbouring diverse biotic communities. Despite this apparent heterogeneity, many studies have stressed the importance of water chemistry in determining the structure of littoral macroinvertebrate assemblages. The purpose of this study was to consider the relative importance of several spatial and water chemistry variables in explaining the patterns in the structure of macroinvertebrate assemblages in 21 lentic water bodies in northeastern Finland. Water bodies were selected to represent various habitat conditions ranging from small permanent bog ponds to small forest lakes. According to canonical correspondence analysis (CCA), the most important environmental factors related to assemblage composition were water body area, moss cover, total nitrogen and water hardness. In general, species composition in small bog ponds tended to differ from that in larger lakes with forested shoreline. Total species richness was best explained by a composite variable (PCA) describing physical habitat heterogeneity, species richness being lowest in small bog lakes with simple bottom structure and low amount of aquatic plants. Species numbers in dominant functional feeding groups were related to different environmental factors. Shredder species richness was best explained by a regression model incorporating total nitrogen and the amount of organic matter, both of which were negatively related to the number of shredder species. The number of gatherer species increased with mean substratum particle size. Scraper species richness was negatively affected by the abundance of detritus and positively affected by depth, and a model including both variables explained most of the variation. Variation in the number of predatory species was best explained by a regression model including moss cover and lake area.

The role of the North Aegean in triggering the recent Eastern Mediterranean climatic changes

Abstract: Drastic changes have occurred in the vertical structure of the deep waters of the eastern Mediterranean in the early 1990s, as dense water of Aegean origin has displaced lighter waters of Adriatic origin at the bottom of the deep basins. This work suggests that the initiation of this process took place in the North Aegean in the winter of 1986/1987 and was intensified by another formation event in 1992/1993. The available observations from the North Aegean support such a scenario. Furthermore, we propose that the outflow of Black Sea waters into the Aegean through the Dardanelles could act as an insulator of the deeper layer from the atmosphere, thus absorbing a large part of the heat and salt exchange; despite this fact, the existence of the densest bottom water of the Mediterranean in the North Aegean, and the continuation of density increase for a large period of time, suggests that it is a region of formation, thus that the insulation layer may at times be penetrated. We suggest that reduced Black Sea outflow into the North Aegean could facilitate dense water formation during the passage of cold atmospheric fronts in the winter.

3H and 3He

Abstract: Tritium (3H) is the only radioactive isotope of hydrogen, and has a half-life of 12.43 years (Unterweger et al., 1980). Large quantities of tritium were introduced into the hydrological cycle by atmospheric thermonuclear testing in the 1950s and 1960s, providing a useful environmental tracer for water originating from this period. Tritium decays by beta-emission to 3He, the rare, stable isotope of helium. Under favourable conditions, measurements of both 3H and 3He in groundwater allow the reconstruction of tritium concentrations in precipitation and the determination of water flow paths. Ratios of 3H to 3He can be applied to quantify the extent of radioactive decay, and hence determine subsurface water residence times up to 40 years.

Field-scale water flow and solute transport : SWAP model concepts, parameter estimation and case studies = [Waterstroming en transport van opgeloste stoffen op veldschaal]

Abstract: Water flow and solute transport in top soils are important elements in many environmental studies. The agro- and ecohydrological model SWAP (Soil-Water-Plant-Atmosphere) has been developed to simulate simultaneously water flow, solute transport, heat flow and crop growth at field scale level. The main features and theoretical concepts of SWAP are described. A serious limitation of many model applications is the availability of accurate input parameters. With the rapid increase of processor calculation speed and development of effective optimization algorithms, the optimization of input parameters by inverse modeling has become an attractive option. Typical and verifiable examples of the inverse modeling technique, are the laboratory One-step and Multi-step outflow experiments, which are used to determine the soil hydraulic functions. It is shown that in the One-step method the cumulative outflow data with time are insufficient to derive unique parameter estimates, and should be supplemented with retention data. In Multi-step experiments, where the air pressure is increased in several steps rather than one large step, the cumulative outflow data proved to be sufficient to derive unique and reliable soil hydraulic parameters. The accuracy of field scale model predictions will increase if the inverse problem is also applied to measurements at field scale level. A prior method is presented to select the type of measurements and timing of observations that result in well-posed inverse problems. This method was applied to a theoretical field experiment at a drained loamy soil, cultivated with maize. SWAP in combination with the optimization shell PEST (Parameter ESTimation) were used to perform the prior inverse analysis. At least four selected parameters could be optimized uniquely, using ordinary, generated observations augmented with a random observation error. The prior analysis showed that inclusion of a crop, a tracer as well as measurements at periods with extreme and rapidly changing hydrological conditions improved the accuracy of the optimized parameters significantly. An accurate and efficient numerical solution of Richards' water flow equation is presented. Special attention is given to proper selection of the top boundary condition during the iterative solution of Richards' equation. The stability of the scheme is shown for extreme events of infiltration, soil evaporation and rapidly fluctuating, shallow groundwater levels in two strongly non-linear soils. It is shown that in case of nodal distances of 1 cm and arithmetic spatial averages of the hydraulic conductivity, errors due to numerical discretization are small compared to errors due to hysteresis and horizontal spatial variability of the soil hydraulic functions. Concepts for hysteresis and mobile-immobile regions due to water repellency have been incorporated in SWAP. Both concepts were applied to data sets from two locations with hysteretic and water repellent soils. In general hysteresis retards soil water movement, while preferential flow enhances soil water movement. Application of the hysteresis and mobile-immobile concept improved the correspondence between measured and simulated water and bromide contents. Also an extended model concept for water and solute movement in cracked clay soils is discussed and applied to a field experiment. Inclusion of this concept in SWAP improved considerably the simulation of soil water contents and bromide leaching to the groundwater. The bromide amounts leached were especially sensitive to the saturated hydraulic conductivity of the top layer, the solute transfer from the soil matrix to crack water flow and the mean residence time of rapid drainage. We may expect that in the coming years SWAP will be useful to explore new flow and transport concepts for agro- and ecohydrology, to analyse laboratory and field experiments, to select viable management options, to perform regional studies employing geographical information systems, and to illustrate transport processes for education and extension. Additional index words : clay cracks, crop growth, heat flow, hysteresis, inverse modeling, outflow experiments, Richards' equation, salinization, transpiration, unsaturated zone, water repellency

Osmotic water permeabilities of cultured, well-differentiated normal and cystic fibrosis airway epithelia.

Abstract: Current hypotheses describing the function of normal airway surface liquid (ASL) in lung defense are divergent. One theory predicts that normal airways regulate ASL volume by modulating the flow of isosmotic fluid across the epithelium, whereas an alternative theory predicts that ASL is normally hyposmotic. These hypotheses predict different values for the osmotic water permeability (P(f)) of airway epithelia. We measured P(f) of cultures of normal and cystic fibrosis (CF) airway epithelia that, like the native tissue, contain columnar cells facing the lumen and basal cells that face a basement membrane. Xz laser scanning confocal microscopy recorded changes in epithelial height and transepithelial volume flow in response to anisosmotic challenges. With luminal hyperosmotic challenges, transepithelial and apical membrane P(f) are relatively high for both normal and CF airway epithelia, consistent with an isosmotic ASL. Simultaneous measurements of epithelial cell volume and transepithelial water flow revealed that airway columnar epithelial cells behave as osmometers whose volume is controlled by luminal osmolality. Basal cell volume did not change in these experiments. When the serosal side of the epithelium was challenged with hyperosmotic solutions, the basal cells shrank, whereas the lumen-facing columnar cells did not. We conclude that (a) normal and CF airway epithelia have relatively high water permeabilities, consistent with the isosmotic ASL theory, and the capacity to restore water on airway surfaces lost by evaporation, and (b) the columnar cell basolateral membrane and tight junctions limit transepithelial water flow in this tissue.

Water Quality Degradation Effects on Freshwater Availability: Impacts of Human Activities

Abstract: The quality of freshwater at any point on the landscape reflects the combined effects of many processes along water pathways. Human activities on all spatial scales affect both water quality and quantity. Alteration of the landscape and associated vegetation has not only changed the water balance, but typically has altered processes that control water quality. Effects of human activities on a small scale are relevant to an entire drainage basin. Furthermore, local, regional, and global differences in climate and water flow are considerable, causing varying effects of human activities on land and water quality and quantity, depending on location within a watershed, geology, biology, physiographic characteristics, and climate. These natural characteristics also greatly control human activities, which will, in turn, modify (or affect) the natural composition of water. One of the most important issues for effective resource management is recognition of cyclical and cascading effects of human activiti...