Pathways Of Water Movement Research Articles

Water seepage and retention in purple soil bunds on sloping farmland in the Three Gorges Reservoir area, China

AbstractSoil bunds are a common land reconstruction practice in the Three Gorges Reservoir area of China, and play a key role in runoff regulation and soil conservation on cultivated hillslopes. This study investigated water movement pathways and retaining effects of soil bunds using water transport trajectory observations inferred from field dye tracing experiments. It was found that water infiltration in the bund was characterised by spatial heterogeneity in three dimensions. The stained area ratio (SAR) of the bund was much lower than that of the adjacent field at all depths. Dye patterns resembled inclined strips in the bund, and lateral seepage occurred mainly on the top of hard pan. SAR, stained path width (SPW) <20 mm, and stained path number (SPN) all decreased with increasing horizontal distance. The SPW <20 mm proportion and SAR both decreased according to a logarithmic function, while the changing trend of SPN followed a first‐order exponential decay. Three pathways of water movement in the bund could be distinguished, namely, vertical infiltration, lateral seepage and flow along the bund–bedrock interface. The primary water seepage channel in the bund was likely to be the bund–bedrock interface (accounting for around 80% of the total flow). These results provide preliminary realisation of water movement mechanisms in the purple soil bunds on sloping farmland and inform management options for bund stability maintenance.

Effects of drought stress on germination and seedling growth of peanut ( Arachis hypogaea L.)

Peanut is am essential legume and has many uses, such as producing oil, food, and fodder. However, with the negative effects of climate change, drought is one especially of the important issues that reduce the yield of peanut. Thus, in this study, the impact of drought stress on the peanut growth was investigated by using PEG-6000 to block pathways of water movement. The changes in morphological, physiological, and biochemical during the peanut growth under drought were analyzed. In the drought condition (-2 bar), the germination time of seed increased but the percentage of germination seeds decreased by approximately 50% compared to control. Besides, the shoot height, the number of leaves, the total leaf area, root length, and fresh weight were lower than that of control. Drought stress made the formation quickly of secondary xylem and phloem. Also, the process of lignification in the phloem parenchyma cell increased. These cell walls were much thicker than those in the control root. In the drought stress, the physiological and biochemical analysis showed that the content of chlorophyll a, leaf relative water content, and starch content reduced significantly in comparison to control. Similarly, the photosynthetic intensity, the activity of cytokinin, and gibberellin decreased. The reverse pattern can be seen in the content of carotenoid, epicuticular wax, proline, and total soluble sugar, respiratory intensity, the activity of catalase, auxin, and ABA activity.

Observed relationships between leaf H218O Péclet effective length and leaf hydraulic conductance reflect assumptions in Craig-Gordon model calculations.

Stable oxygen isotope techniques may be a useful tool to investigate the pathways of water movement within leaves. However, implementation of such methods is limited due to uncertainty in the effective path length (L) for the Péclet effect in leaf water enrichment models. Previous studies have found relationships between L and physiological parameters such as transpiration rate (E) and leaf hydraulic conductance (k(leaf)) both within and between species. However, these studies relied on assumptions in their calculation of L, which were not directly tested. Eucalyptus paniculata Smith was used to evaluate the relationships between L, k(leaf) and E under differing water availability and a range of leaf temperatures. Coupled gas exchange and transpiration isotope measurements allowed previous assumptions to be directly tested. L was significantly and negatively related to both k(leaf) and E when the isotopic signature of water vapour was assumed to be in equilibrium with source water, was equivalent to the room vapour or equal to source water. However, the relationship between L and k(leaf) was non-significant when measured δ( 18)O of transpired vapour was used and disappeared entirely when non-steady-state leaves were excluded, and when evaporation site water was calculated from coupled gas exchange and transpiration isotope values. These results suggest that great care must be taken when calculating L, particularly regarding assumptions of isotopic steady state and δ( 18)O of vapour. Previous suggestions of changes in pathways for water movement as transpiration rate varied need to be reassessed in light of these observations.

Varietal differences in growth and Cs allocation of blackgram (Vigna mungo) under water stress

Plant uptake and distribution of radioactive caesium (137Cs) was examined in an effort to determine how plant-water interactions differ between two blackgram plant varieties with different salt tolerance. Salt-tolerant U Taung-2 (BT) and the salt-sensitive variety Mutpe Khaing To (BS) were grown in pots filled with 137Cs-contaminated soil obtained from Nihonmatsu, Fukushima, Japan. The average soil 137Cs concentration was 1084Bqkg−1, and the soil type comprised sandy clay loam, where the sand: silt: clay proportion was 62:18:20. After growing for 15 days in a temperature-controlled growth chamber, plants were subjected to drought stress conditions by addition of either 10 or 20% polyethylene glycol (PEG 6000) to the soil to reduce the osmotic potential of the soil, or to field capacity conditions (0% PEG).Analysis of the growth traits and water relations revealed the different strategies employed by the two blackgram varieties under water stress conditions. No differences were observed among the two varieties under field capacity conditions. However, under both drought stress conditions (10 or 20% PEG), the reduction in total leaf area, total biomass and plant growth analysis parameters, in addition to the relative water content, were higher in BS than in BT plants. Both plant varieties exhibited lower 137Cs concentrations in the leaves, shoots and roots under drought stress conditions compared to plants subjected to field capacity conditions. Under drought stress conditions, BT plants showed uptake and translocation of 137Cs from the roots to shoots and leaves, while this activity was seriously limited in BS plants, and especially in the leaf portion of BS plants treated with 20% PEG. In BT plants, potassium moved more freely than Cs from roots to the aerial parts of the plant under water stress conditions, while this was not the case in BS plants. Examination of the hydraulic conductivity showed a positive correlation with the 137Cs concentration in leaf and stem. Our results showed that the accumulation and distribution patterns of 137Cs among part of the blackgram tissues were dependent on plant growth performance, water movement pathways, and selective absorption.

Transmembrane water-flux through SLC4A11: a route defective in genetic corneal diseases

Three genetic corneal dystrophies [congenital hereditary endothelial dystrophy type 2 (CHED2), Harboyan syndrome and Fuchs endothelial corneal dystrophy] arise from mutations of the SLC4a11 gene, which cause blindness from fluid accumulation in the corneal stroma. Selective transmembrane water conductance controls cell size, renal fluid reabsorption and cell division. All known water-channelling proteins belong to the major intrinsic protein family, exemplified by aquaporins (AQPs). Here we identified SLC4A11, a member of the solute carrier family 4 of bicarbonate transporters, as an unexpected addition to known transmembrane water movement facilitators. The rate of osmotic-gradient driven cell-swelling was monitored in Xenopus laevis oocytes and HEK293 cells, expressing human AQP1, NIP5;1 (a water channel protein from plant), hCNT3 (a human nucleoside transporter) and human SLC4A11. hCNT3-expressing cells swelled no faster than control cells, whereas SLC4A11-mediated water permeation at a rate about half that of some AQP proteins. SLC4A11-mediated water movement was: (i) similar to some AQPs in rate; (ii) uncoupled from solute-flux; (iii) inhibited by stilbene disulfonates (classical SLC4 inhibitors); (iv) inactivated in one CHED2 mutant (R125H). Localization of AQP1 and SLC4A11 in human and murine corneal (apical and basolateral, respectively) suggests a cooperative role in mediating trans-endothelial water reabsorption. Slc4a11−/− mice manifest corneal oedema and distorted endothelial cells, consistent with loss of a water-flux. Observed water-flux through SLC4A11 extends the repertoire of known water movement pathways and call for a re-examination of explanations for water movement in human tissues.

Exploring the energetics of water permeation in photosystem II by multiple steered molecular dynamics simulations

The Mn4Ca cluster of the oxygen-evolving complex (OEC) of photosynthesis catalyzes the light-driven splitting of water into molecular oxygen, protons, and electrons. The OEC is buried within photosystem II (PSII), a multisubunit integral membrane protein complex, and water must find its way to the Mn4Ca cluster by moving through protein. Molecular dynamics simulations were used to determine the energetic barriers for water permeation though PSII extrinsic proteins. Potentials of mean force (PMFs) for water were derived by using the technique of multiple steered molecular dynamics (MSMD). Calculation of free energy profiles for water permeation allowed us to characterize previously identified water channels, and discover new pathways for water movement toward the Mn4Ca cluster. Our results identify the main constriction sites in these pathways which may serve as selectivity filters that restrict both the access of solutes detrimental to the water oxidation reaction and loss of Ca2+ and Cl− from the active site.

Arbuscular mycorrhizal symbiosis increases relative apoplastic water flow in roots of the host plant under both well-watered and drought stress conditions

The movement of water through mycorrhizal fungal tissues and between the fungus and roots is little understood. It has been demonstrated that arbuscular mycorrhizal (AM) symbiosis regulates root hydraulic properties, including root hydraulic conductivity. However, it is not clear whether this effect is due to a regulation of root aquaporins (cell-to-cell pathway) or to enhanced apoplastic water flow. Here we measured the relative contributions of the apoplastic versus the cell-to-cell pathway for water movement in roots of AM and non-AM plants. We used a combination of two experiments using the apoplastic tracer dye light green SF yellowish and sodium azide as an inhibitor of aquaporin activity. Plant water and physiological status, root hydraulic conductivity and apoplastic water flow were measured. Roots of AM plants enhanced significantly relative apoplastic water flow as compared with non-AM plants and this increase was evident under both well-watered and drought stress conditions. The presence of the AM fungus in the roots of the host plants was able to modulate the switching between apoplastic and cell-to-cell water transport pathways. The ability of AM plants to switch between water transport pathways could allow a higher flexibility in the response of these plants to water shortage according to the demand from the shoot.

Response of stomatal numbers to CO2 and humidity: control by transpiration rate and abscisic acid.

The observation that stomatal density (number mm(-2)) on herbarium leaves had decreased over the last century represents clear evidence that plants have responded to anthropogenic increases in CO2 concentration. The mechanism of the response has proved elusive but here it is shown that density responses to both CO2 concentration and humidity are correlated with changes in whole-plant transpiration and leaf abscisic acid (ABA) concentration. The transpiration rate of a range of accessions of Arabidopsis thaliana was manipulated by changing CO2 concentration, humidity and by exogenous application of ABA. Stomatal density increased with transpiration and leaf ABA concentration. A common property of signal transduction systems is that they rapidly lose their ability to respond to the co-associated stimulus. Pathways of water movement within the plant are connected and so variations in supply and demand can be signalled throughout the plant directly, modifying stomatal aperture of mature leaves and stomatal density of developing leaves. Furthermore, the system identified here does not conform to the loss of ability to respond. A putative mechanism is proposed for the control of stomatal density by transpiration rate and leaf ABA concentration.

Peatland hydrology and carbon release: why small-scale process matters

Peatlands cover over 400 million hectares of the Earth's surface and store between one-third and one-half of the world's soil carbon pool. The long-term ability of peatlands to absorb carbon dioxide from the atmosphere means that they play a major role in moderating global climate. Peatlands can also either attenuate or accentuate flooding. Changing climate or management can alter peatland hydrological processes and pathways for water movement across and below the peat surface. It is the movement of water in peats that drives carbon storage and flux. These small-scale processes can have global impacts through exacerbated terrestrial carbon release. This paper will describe advances in understanding environmental processes operating in peatlands. Recent (and future) advances in high-resolution topographic data collection and hydrological modelling provide an insight into the spatial impacts of land management and climate change in peatlands. Nevertheless, there are still some major challenges for future research. These include the problem that impacts of disturbance in peat can be irreversible, at least on human time-scales. This has implications for the perceived success and understanding of peatland restoration strategies. In some circumstances, peatland restoration may lead to exacerbated carbon loss. This will also be important if we decide to start to create peatlands in order to counter the threat from enhanced atmospheric carbon.

Significance and dynamics of drip water responding to rainfall in four caves of Guizhou, China

In rainy season, NaCl is adopted to trace sources of cave drip water, time scales of drip water responding to precipitation, and processes of water dynamics in four caves of Pearl watershed in Guizhou, China (Liangfeng cave in Libo, Qixing cave in Duyun, Jiangjun cave in Anshun and Xiniu cave in Zhenning). Because of the variety of karst cave surroundings, interconnections of water transporting ways, water dynamics processes etc., time scales of drip-water in four caves responding to rainfall is 0–40 d. According to the characteristics of water transport in cave roof, pathways of water movement, types of water head etc., drip water of four caves can be divided into five hydrodynamics types. The differences of time scales, and ways of water-soil and water-rock interaction during water transporting in cave roof make it difficult to correctly measure speleothem record and trace material sources. In addition, there exist great differences in water dynamic conditions among the four caves. So the interpretation of the paleoenvironment records of speleothem must be supported by the understanding of hydrodynamics conditions of different drip sites. Based on the data got from drip sites in four caves, drip conductivity accords with precipitation, which indicates that element contents in speleothem formed by drip water record the change of karst paleoenvironment. But results of multi-points study are needed to guarantee the correctness of interpretation.

Do pathways of water movement and leaf anatomical dimensions allow development of gradients in H218O between veins and the sites of evaporation within leaves?

ABSTRACTThe oxygen isotope enrichment of bulk leaf water (ΔL) is often observed to be poorly predicted by the Craig–Gordon‐type models developed for evaporative enrichment from a body of water (Δe). The discrepancy between ΔL and Δe may be explained by gradients in enrichment within the leaf as a result of convection of unenriched water to the sites of evaporation opposing the diffusion of enrichment away from the sites; a Péclet effect. However, this effect is difficult to quantify because the velocities of water movement within the leaf are unknown. This paper attempts to model the complex anatomy of a leaf, and hence such velocities, to assess if the gradients in H218O required for a significant Péclet effect between the vein and the evaporation sites are possible within a leaf. Published dimensions of cells in wheat leaves are used to calculate the cross‐sectional areas perpendicular to the flow velocities of water through assumed pathways. By combining the ratio of actual to ‘slab’ velocities with anatomical lengths, equivalent lengths (L) emerge. In this way, it is concluded that if water moves only through the cell walls, or from cell to cell via either aquaporins or plasmodesmata, and evaporates from mesophyll cells, or the substomatal cells, or from the peristomatal region (a total of 15 combinations of assumptions), then the 15 central estimates of the values of L are between 9 and 200 mm. Each of these central estimates is subject to uncertainty, but overall their magnitude is important and estimates of L are comparable with those made from fitting to isotopic data (8 mm for wheat). It is concluded that significant gradients in enrichment between the vein and the evaporation sites are likely.

Effectiveness of groundwater nitrate removal in a river riparian area: the importance of hydrogeological conditions

Riparian areas are often considered important ecological ecotones that decrease the nitrate load of groundwater discharging into rivers. In this paper, the effectiveness of nitrogen removal from groundwater in a riparian area has been evaluated. We used a quantitative hydrogeological approach which delimited homogeneous stream tubes based on the water table contour. We constructed water and mass balances and used the results to estimate the mass of nitrogen removed by biological processes. Nitrogen removal effectiveness was calculated as the ratio of nitrogen removed to the total nitrogen mass flowing through the riparian area. This approach requires knowledge of the hydrogeological settings and coverage of an area larger than the study site. The removal we observed was seasonally variable, ranging between 2.2 and 7.6 mg N m −2 day −1, and represented up to 95% of the total nitrogen mass entering the riparian area in late summer. Removal effectiveness was only 27–38% in winter, due to the combination of a high nitrogen input and a low plant uptake. Nitrogen removal was highest in spring, but effectiveness was about 60% because the input was as high as in winter. Rainwater infiltrating the riparian area could represent almost the same quantity as groundwater input. Under such conditions, the dilution effect is very important in riparian areas that are not nitrate sources and it is essential to maintain this non-nitrate-emissive use. This study showed that the effectiveness of nitrogen removal in a riparian area is highly dependent on the pathway of water movement through its biologically active layers.

SGS Water Theme: influence of soil, pasture type and management on water use in grazing systems across the high rainfall zone of southern Australia

Eleven experimental sites in the Sustainable Grazing Systems (SGS) national experiment were established in the high rainfall zone (HRZ, >600 mm/year) of Western Australia, Victoria and New South Wales to measure components of the water balance, and pathways of water movement, for a range of pastures from 1997 to 2001. The effect of widely spaced river red gums (Eucalyptus camaldulensis) in pasture, and of belts of plantation blue gums (E. globulus), was studied at 2 of the sites. The soil types tested ranged from Kurosols, Chromosols and Sodosols, with different subsoil permeabilities, to Hydrosols and Tenosols. The pasture types tested were kikuyu (Pennisetum clandestinum), phalaris (Phalaris aquatica), redgrass (Bothriochloa macra) and annual ryegrass (Lolium rigidum), with subterranean clover (Trifolium subterraneum) included. Management variables were set stocking v. rotational grazing, adjustable stocking rates, and level of fertiliser input. Soil, pasture and animal measurements were used to set parameters for the biophysical SGS pasture model, which simulated the long-term effects of soil, pasture type, grazing method and management on water use and movement, using as inputs daily weather data for 31 years from selected sites representing a range of climates. Measurements of mean maximum soil water deficit Sm were used to estimate the probability of surplus water occurring in winter, and the average amount of this surplus, which was highest (97–201 mm/year) for pastures in the cooler, winter-rainfall dominant regions of north-east and western Victoria and lowest (3–11 mm/year) in the warmer, lower rainfall regions of the eastern Riverina and Esperance, Western Australia. Kikuyu in Western Australia achieved the largest increase in Sm compared with annual pasture (55–71 mm), while increases due to phalaris were 18–45 mm, and those of native perennials were small and variable. Long-term model simulations suggested rooting depth was crucial in decreasing deep drainage, to about 50 mm/year for kikuyu rooting to 2.5 m, compared with 70–200 mm/year for annuals rooting to only 0.8 m. Plantation blue gums dried the soil profile to 5.25 m by an average of 400 mm more than kikuyu pasture, reducing the probability of winter surplus water to zero, and eliminating drainage below the root zone. Widely spaced river red gums had a much smaller effect on water use, and would need to number at least 14 trees per hectare to achieve extra soil drying of about 50 mm over a catchment. Soil type affected water use primarily through controlling the rooting depth of the vegetation, but it also changed the partitioning of surplus water between runoff and deep drainage. Strongly duplex soils such as Sodosols shed 50% or more surplus water as runoff, which is important for flushing streams, provided the water is of good quality. Grazing method and pasture management had only a marginal effect in increasing water use, but could have a positive effect on farm profitability through increased livestock production per hectare and improved persistence of perennial species.

Co-ordination of vapour and liquid phase water transport properties in plants.

The pathway for water movement from the soil through plants to the atmosphere can be represented by a series of liquid and vapour phase resistances. Stomatal regulation of vapour phase resistance balances transpiration with the efficiency of water supply to the leaves, avoiding leaf desiccation at one extreme, and unnecessary restriction of carbon dioxide uptake at the other. In addition to maintaining a long-term balance between vapour and liquid phase water transport resistances in plants, stomata are exquisitely sensitive to short-term, dynamic perturbations of liquid water transport. In balancing vapour and liquid phase water transport, stomata do not seem to distinguish among potential sources of variation in the apparent efficiency of delivery of water per guard cell complex. Therefore, an apparent soil-to-leaf hydraulic conductance based on relationships between liquid water fluxes and driving forces in situ seems to be the most versatile for interpretation of stomatal regulatory behaviour that achieves relative homeostasis of leaf water status in intact plants. Components of dynamic variation in apparent hydraulic conductance in intact plants include, exchange of water between the transpiration stream and internal storage compartments via capacitive discharge and recharge, cavitation and its reversal, temperature-induced changes in the viscosity of water, direct effects of xylem sap composition on xylem hydraulic properties, and endogenous and environmentally induced variation in the activity of membrane water channels in the hydraulic pathway. Stomatal responses to humidity must also be considered in interpreting co-ordination of vapour and liquid phase water transport because homeostasis of bulk leaf water status can only be achieved through regulation of the actual transpirational flux. Results of studies conducted with multiple species point to considerable convergence with regard to co-ordination of stomatal and hydraulic properties. Because stomata apparently sense and respond to integrated and dynamic soil-to-leaf water transport properties, studies involving intact plants under both natural and controlled conditions are likely to yield the most useful new insights concerning stomatal co-ordination of transpiration with soil and plant hydraulic properties.

The pathways of water movement in leaves modified into tents by bats

A number of species of bats modify leaves into tents, which they use as roost-sites. Through this process, some areas of the leaf lamina are damaged or become detached from the midrib. Such injuries do not cause death of the leaf or the detached areas, indicating that water supply to these areas must be maintained. We examined the anatomy of the vascular systems and water transport in the leaves of three species of plants Heliconia pogonantha L Manicaria plukenetii Griseb. & H. Wendland Cryosophila warcsewiczii (H. Wend.) Bartlett. In altered leaves of all three species, detached areas of the laminae were supplied with water by minor transverse veins branching from the first major parallel vein that remained intact next to the cut. These transverse veins conducted water through single xylem elements of narrow diameter (approximately 10 μm) previously thought to supply water only to mesophyll cells in their immediate vicinity. The short lengths of these veins compensates their high resistance to water flow (a consequence of their small diameter xylem elements), indicating that small transverse veins have a large capacity for water transport. Water typically flowed through transverse veins into detached major and minor parallel veins, filled these parallel veins in both directions (i.e. toward the midrib and the leaf edge), and continued on into subsequent transverse and parallel veins, thereby supplying water to the entire leaf. Water conduction through these small transverse veins could support large areas of leaf lamina, keeping the leaf-tent alive for at least several months. The maintenance of the flow of water and nutrients to areas of leaves detached by bats during the tent-making process increases the longevity and decreases the conspicuousness of leaf-tents, and is likely a key factor in the success of this roosting strategy.

The pathways of water movement in leaves modified into tents by bats

Abstract A number of species of bats modify leaves into tents, which they use as roost-sites. Through this process, some areas of the leaf lamina are damaged or become detached from the midrib. Such injuries do not cause death of the leaf or the detached areas, indicating that water supply to these areas must be maintained. We examined the anatomy of the vascular systems and water transport in the leaves of three species of plants Heliconia pogonantha L Manicaria plukenetii Griseb. & H. Wendland Cryosophila warcsewiczii (H. Wend.) Bartlett. In altered leaves of all three species, detached areas of the laminae were supplied with water by minor transverse veins branching from the first major parallel vein that remained intact next to the cut. These transverse veins conducted water through single xylem elements of narrow diameter (approximately 10 μm) previously thought to supply water only to mesophyll cells in their immediate vicinity. The short lengths of these veins compensates their high resistance to water flow (a consequence of their small diameter xylem elements), indicating that small transverse veins have a large capacity for water transport. Water typically flowed through transverse veins into detached major and minor parallel veins, filled these parallel veins in both directions (i.e. toward the midrib and the leaf edge), and continued on into subsequent transverse and parallel veins, thereby supplying water to the entire leaf. Water conduction through these small transverse veins could support large areas of leaf lamina, keeping the leaf-tent alive for at least several months. The maintenance of the flow of water and nutrients to areas of leaves detached by bats during the tent-making process increases the longevity and decreases the conspicuousness of leaf-tents, and is likely a key factor in the success of this roosting strategy.

Aquaporin-1 expression in visceral smooth muscle cells of female rat reproductive tract.

The aquaporins (AQ-s) are a group of intrinsic membrane proteins which facilitate movement of water across cell membranes; their recent identification in the kidney has led to the reappraisal of the mechanisms and pathways of water movement across epithelia. Aquaporin-1, (CHIP-28) is reported distributed in cardiac myocytes and vascular smooth muscle cells of large arteries. A related protein, AQ-4, has been identified in the sarcolemma of skeletal muscle fibres. We report aquaporin expression in the cell membrane of smooth muscle cells of the rat genital tract; fluorescence immunohistochemistry of rat uterine (fallopian) tube and vagina demonstrated AQ-1 in visceral smooth muscle of these tissues. In the uterine tube, AQ-1 labelling is most pronounced in the innermost longitudinal and the inner cells of the circular muscle layer and is absent from the outer longitudinal muscle layer of the myosalpinx. The possibility of a specific role for AQ-1 in tubal transport by altering the tubal luminal diameter during the estrus cycle is suggested.

SWBCM: a soil water balance capacity model for environmental applications in the UK

The paper presents a daily-time step, multi-horizon capacity model of soil-water balance (SWBCM—Soil Water Balance Capacity Model) suitable for ecological and environmental applications investigating the spatial and temporal variability of soil water content determined by changes in soil hydraulic conductivity, soil water storage capacity and the pathways of water movement through the soil and across soil types. SWBCM simulates soil water content at horizon level and encompasses limits on the amount of drainage from one horizon to the next to allow the formation of temporary perched water tables, lateral drainage, matric potential and surface runoff. The model incorporates a dynamic sub-model of grass growth (SWARD—Dowle, K., Armstrong, A.C., 1990. A model for investment appraisal of grassland drainage schemes on farms in the UK. Agric. Water Manage. 18, 101–120; Armstrong, A.C., Castel, D.A., Tyson, K.C., 1995. SWARD: a model of grass growth and the economic utilisation of grassland. In: Pereira L.S., van den Broek B.J., Kabat P., Allen R.G. (Eds.), Crop-Water Simulation Models in Practice. Wageningen Press, Wageningen, The Netherlands, pp. 189–197). SWBCM’s predictive ability is tested across a range of soil types under permanent grass in the UK and outputs are compared with predictions made by MACRO (Jarvis, N.J., 1994. The MACRO Model Version 3.1. Technical Description and Sample Simulations. Swedish University of Agricultural Sciences, Department of Soil Sciences, Reports and Dissertations 19, Uppsala, Sweden, 51 pp.), a mechanistic solute transport model which incorporates a physically-based preferential flow model in which total soil porosity is divided into two flow domains (macro-pores and micro-pores), each characterised by a flow rate; soil water flow in the micro-pore domain is modelled using Richards’ equation. In the modelling experiment, SWBCM simulations have been shown to provide good approximations of point-scale experimental data under a range of soil, climate and drainage management conditions in the UK. SWBCM simulations are close to those developed by the mechanistic MACRO model, suggesting that the capacity model can be applied to describe the water balance of multi-horizon UK soil profiles. The modelling approach used is considered to be applicable to the wide range of soil lower boundary conditions, ranging from free-draining to impermeable, occurring in the field and to simulate transient perched water tables that commonly occur in most temperate, high latitude countries such as the UK.

Evaluation of the use of soil ion exchange properties for predicting streamwater chemistry in upland catchments

The potential of soil ion exchange chemistry for predicting streamwater quality is evaluated using soil and streamwater chemical data from ten upland catchments in NE Scotland. The study catchments vary from those dominated by acid hill peats, alpine soils and podzols to those dominated by more base-rich soils, including cambisols and gleysols. Soil and streamwater chemical data combined with precipitation and parent material chemistry are also used to investigate sources and pathways of water movement. In all soils studied, Ca and Mg are the dominant exchangeable base cations in the surface soil horizons. In most soils, Na becomes increasingly important on the exchange complex with depth down the soil profile. Plots of the relative proportions of Na:Ca:Mg in streamwater show that, during periods of high discharge, streamwater chemistry tends to become relatively more Na-rich compared with Ca and Mg. Using triangular diagrams, streamwater chemistry can be described as a mixture of geochemically distinct packages of water derived from precipitation inputs, specific parent materials and key soil horizons, although spatially important soils within catchments may be relatively unimportant in controlling streamwater chemistry. Changes in streamwater chemistry at high flow can be explained by dilution of water derived from groundwater sources or the B/C horizon, either with laterally flowing water from the upper soil horizons, or with precipitation. In conclusion, changes in the relative proportions of Na:Ca:Mg in streamwater during storms suggest that precipitation chemistry may play a greater role than hitherto suggested in modifying solute chemistry during periods of high flow.

Developmental changes in rabbit juxtamedullary proximal convoluted tubule water permeability.

The mammalian proximal tubule reabsorbs the bulk of the glomerular filtrate in a nearly isosmotic fashion due to the high osmotic water permeability (Pf) of this segment. Although the characteristics of proximal tubule water transport have been studied in the adult proximal tubule, little is known about the neonatal segment. The present study directly measured the Pf and diffusional water permeability (PDW) of neonatal (10 +/- 2 day old) and adult rabbit juxtamedullary proximal convoluted tubules (PCT) using in vitro microperfusion. The Pf of neonatal juxtamedullary PCT was greater than the Pf of adult juxtamedullary PCT. In contrast, the PDW was not different between the two groups. The Pf and PDW values of both neonatal and adult tubules were inhibited to the same degree by p-chloromercuribenzene sulfonate and had identical activation energies. The transepithelial reflection coefficients of NaCl and NaHCO3 were also found to be similar in both the neonatal and adult proximal tubules. Thus neonatal and adult juxtamedullary PCT have many characteristics of water transport that are identical; however, neonatal Pf is three to five times that of the adult value. This difference in Pf with identical PDW values may give an insight into the transepithelial pathway for water movement in the neonatal tubule.