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Volumetric Water Content (volumetric + water_content)
Selected AbstractsSphagnum under pressure: towards an ecohydrological approach to examining Sphagnum productivityECOHYDROLOGY, Issue 4 2008D. K. Thompson Abstract The genus Sphagnum is the key peat-forming bryophyte in boreal ecosystems. Relying entirely on passive capillary action for water transport, soil moisture is often the limiting factor in Sphagnum production, and hence peat accumulation. While several hydrological models of peat physics and peatland water movement exist, these models do not readily interface with observations and models of peatland carbon accumulation. A conflict of approaches exists, where hydrological studies primarily utilize variables such as hydraulic head, while ecological models of Sphagnum growth adopt the coarse hydrological variables of water table (WT), volumetric water content (VWC) or gravimetric water content (WC). This review examines the potential of soil pressure head as a measurement to link the hydrological and ecological functioning of Sphagnum in peatlands. The non-vascular structure of Sphagnum mosses and the reliance on external capillary transport of water in the mosses make them an ideal candidate for this approach. The main advantage of pressure head is the ability to mechanistically link plot-scale hydrology to cellular-scale water requirements and carbon exchange. Measurement of pressure head may improve photosynthetic process representation in the next generation of peatland models. Copyright © 2008 John Wiley & Sons, Ltd. [source] Comparison of soil moisture and meteorological controls on pine and spruce transpirationECOHYDROLOGY, Issue 3 2008Eric E. Small Abstract Transpiration is an important component of the water balance in the high elevation headwaters of semi-arid drainage basins. We compare the importance of soil moisture and meteorological controls on transpiration and quantify how these controls are different at a ponderosa pine site and a spruce site in the Jemez river drainage basin of northern New Mexico, a sub-basin of the Rio Grande. If only soil moisture controls fluctuations in transpiration, then simple hydrologic models focussed only on soil moisture limitations are reasonable for water balance studies. If meteorological controls are also critical, then more complex models are required. We measured volumetric water content in the soil and sap velocity, and assumed that transpiration is proportional to sap velocity. Ponderosa sap velocity varies with root zone soil moisture. Nearly all of the scatter in the ponderosa sap velocity,soil moisture relationship can be predicted using a simple model of potential evapotranspiration (ET), which depends only on measured incident radiation and air temperature. Therefore, simple hydrologic models of ponderosa pine transpiration are warranted. In contrast, spruce sap velocity does not clearly covary with soil moisture. Including variations in potential evapotranspiration does not clarify the relationship between sap velocity and soil moisture. Likewise, variations in radiation, air temperature, and vapour pressure do not explain the observed fluctuations in sap velocity, at least according to the standard models and parameters for meteorological restrictions on transpiration. Both the simple and more complex models commonly used to predict transpiration are not adequate to model the water balance in the spruce forest studied here. Copyright © 2008 John Wiley & Sons, Ltd. [source] Spatial variation of ammonia volatilization from soil and its scale-dependent correlation with soil propertiesEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 6 2008R. Corstanje Summary Quantitative predictions of ammonia volatilization from soil are useful to environmental managers and policy makers and empirical models have been used with some success. Spatial analysis of the soil properties and their relationship to the ammonia volatilization process is important as predictions will be required at disparate scales from the field to the catchment and beyond. These relationships are known to change across scales and this may affect the performance of an empirical model. This study is concerned with the variation of ammonia volatilization and some controlling soil properties: bulk density, volumetric water content, pH, CEC, soil pH buffer power, and urease activity, over distances of 2, 50, 500, and >2000 m. We sampled a 16 km × 16 km region in eastern England and analyzed the results by a nested analysis of (co)variance, from which variance components and correlations for each scale were obtained. The overall correlations between ammonia volatilization and the soil properties were generally weak: ,0.09 for bulk density, 0.04 for volumetric water content, ,0.22 for CEC, ,0.08 for urease activity, ,0.22 for pH and 0.18 for the soil pH buffer power. Variation in ammonia volatilization was scale-dependent, with substantial variance components at the 2- and 500-m scales. The results from the analysis of covariance show that the relationships between ammonia volatilization and soil properties are complex. At the >2000 m scale, ammonia volatilization was strongly correlated with pH (,0.82) and CEC (,0.55), which is probably the result of differences in parent material. We also observed weaker correlations at the 500-m scale with bulk density (,0.61), volumetric water content (0.48), urease activity (,0.42), pH (,0.55) and soil pH buffer power (0.38). Nested analysis showed that overall correlations may mask relationships at scales of interest and the effect of soil variables on these soil processes is scale-dependent. [source] Relationship between thermal conductivity and water content of soils using numerical modellingEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2003P. Cosenza Summary There is no simple and general relationship between the thermal conductivity of a soil, ,, and its volumetric water content, ,, because the porosity, n, and the thermal conductivity of the solid fraction, ,s, play a major part. Experimental data including measurements of all the variables are scarce. Using a numerical modelling approach, we have shown that the microscopic arrangement of water influences the relation between , and ,. Simulated values for n ranging from 0.4 to 0.6, ,s ranging from 2 to 5 W m,1 K,1 and , from 0.1 to 0.4 can be fitted by a simple linear formula that takes into account n, ,s and ,. The results given by this formula and by the quadratic parallel (QP) model widely used in physical property studies are in satisfactory agreement with published data both for saturated rocks and for unsaturated soils. Consequently, the linear formula and the QP model can be used as practical and efficient tools to investigate the effects of water content and porosity on the thermal conductivity of the soil and hence to optimize the design of thermal in situ techniques for monitoring water content. [source] Vertical partitioning of CO2 production within a temperate forest soilGLOBAL CHANGE BIOLOGY, Issue 6 2006ERIC A. DAVIDSON Abstract The major driving factors of soil CO2 production , substrate supply, temperature, and water content , vary vertically within the soil profile, with the greatest temporal variations of these factors usually near the soil surface. Several studies have demonstrated that wetting and drying of the organic horizon contributes to temporal variation in summertime soil CO2 efflux in forests, but this contribution is difficult to quantify. The objectives of this study were to partition CO2 production vertically in a mixed hardwood stand of the Harvard Forest, Massachusetts, USA, and then to use that partitioning to evaluate how the relative contributions of CO2 production by genetic soil horizon vary seasonally and interannually. We measured surface CO2 efflux and vertical soil profiles of CO2 concentration, temperature, water content, and soil physical characteristics. These data were applied to a model of effective diffusivity to estimate CO2 flux at the top of each genetic soil horizon and the production within each horizon. A sensitivity analysis revealed sources of uncertainty when applying a diffusivity model to a rocky soil with large spatial heterogeneity, especially estimates of bulk density and volumetric water content and matching measurements of profiles and surface fluxes. We conservatively estimate that the O horizon contributed 40,48% of the total annual soil CO2 efflux. Although the temperature sensitivity of CO2 production varied across soil horizons, the partitioning of CO2 production by horizon did not improve the overall prediction of surface CO2 effluxes based on temperature functions. However, vertical partitioning revealed that water content covaried with CO2 production only in the O horizon. Large interannual variations in estimates of O horizon CO2 production indicate that this layer could be an important transient interannual source or sink of ecosystem C. [source] Application of a coil-type TDR probe for measuring the volumetric water content in weathered granitic bedrockHYDROLOGICAL PROCESSES, Issue 6 2008Shin'ya Katsura Abstract As a first step toward describing water flow processes in bedrock, a coil-type time domain reflectometry (TDR) probe capable of measuring volumetric water content, ,, in weathered bedrock at three depths was prepared. Because the coil-type TDR probe is large in diameter (19 mm), it can be installed even in highly weathered bedrock more easily and appropriately than conventional TDR probes that consists of two or three rods of small diameter (5-8 mm). The probe calibrations suggest that the values measured by the probe are very sensitive to changes in ,. Using the calibrated probe together with commercially available profile soil moisture sensors, the , profile was monitored for 1 year. Even rainfall events with relatively small cumulative rainfall of 15 mm increased the bedrock ,, and the increments were comparable to those in the soil. After the end of the rainfall events, the bedrock , displayed a more rapid drop than the soil, and varied little during the period of no rainfall. The water storage showed similar tendencies. These observations suggest that the bedrock , is controlled by clearly distinguishable macropores and micropores within the bedrock. It is concluded that the coil-type TDR probe is very effective in determining , in weathered bedrock, and that bedrock, conventionally defined by conducting cone penetration tests and treated as impermeable, does conduct and hold substantial amounts of water, and therefore contribute greatly to hydrological processes in headwater catchments. Copyright © 2007 John Wiley & Sons, Ltd. [source] Development of a technique for modelling clay liner desiccationINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 6 2003Y. Zhou Abstract This paper presents a model for the analysis of clay liner desiccation in a landfill barrier system due to temperature effects. The model incorporates consideration of fully coupled heat-moisture-air flow, a non-linear constitutive relationship, the dependence of void ratio and volumetric water content on stress, capillary pressure and temperature, and the effect of mechanical deformation on all governing equations. Mass conservative numerical schemes are proposed to improve the accuracy of the finite element solution to the governing equations. The application of the model is then demonstrated by examining three test problems, including isothermal infiltration, heat conduction and non-isothermal water and heat transport. Comparisons are made with results from literature, and good agreement is observed. Copyright © 2003 John Wiley & Sons, Ltd. [source] Experimentelle Bestimmung der hygrischen Sorptionsisotherme und des Feuchtetransportes unter instationären BedingungenBAUPHYSIK, Issue 2 2006Assistent und Laborleiter Rudolf Plagge Dr.-Ing. Mit der vorgestellten Augenblicksprofil-Methode (APM) werden sowohl die relative Luftfeuchte und die Temperatur, als auch der volumetrische Wassergehalt in bestimmten Positionen in einem porösen Material bestimmt. Die Messungen werden kontinuierlich unter instationären Bedingungen durchgeführt. Damit erlaubt die APM eine dynamische und gleichzeitige Messung der hygroskopischen Sorptionsisotherme und der hygrischen Feuchteleitfähigkeit für einzelne Kompartimente innerhalb der Materialprobe. Die Feuchteleitfähigkeit wird aus den sich zeitlich ändernden Potentialgradienten und den dazugehörigen Feuchteverteilungen für die jeweiligen Kompartimente berechnet. Die Anwendung nicht konstanter Randbedingungen in der APM erlaubt die Untersuchung des hygrodynamischen Verhaltes von porösen Materialien. In der vorliegenden Studie werden die zeit- und prozeßabhängige Feuchtespeicherung und der Feuchtetransport bestimmt. Die vorgestellten Adsorptions- und Desorptionsexperimente wurden an dem kapillar- aktiven Wärmedämmstoff Calciumsilikat durchgeführt. Die Ergebnisse geben das Hystereseverhalten und den Einfluß der Dynamik der Prozesse wider. In Positionen mit schnellen Feuchteänderungen wird die Feuchtespeicherfunktion im Vergleich mit Regionen langsamer Feuchteänderung nach oben verschoben. Die Feuchteleitfähigkeit als Funktion der relativen Luftfeuchte zeigt eine bedeutende Hysterese. Hingegen ist die Feuchteleitfähigkeit in Relation zum Wassergehalt nicht hysteretisch. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Experimental determination of the hygroscopie sorption isotherm and the moisture conductvity under transient conditions. By means of the proposed Instantaneous Profile Method (IPM) the relative humidity or the capillary pressure as well as the volumetric water content at specific locations inside a porous medium can be determined. The measurements are carried out under transient conditions and continuously in time. Thus, the IPM allows dynamic measurements of the hygroscopic sorption isotherm and the hygroscopic moisture conductivity. In addition, the moisture conductivity can be obtained via calculation of the moisture flow distribution from the temporal change of moisture contents in the compartments of the sample. The application of non-constant boundary conditions in the IPM allows investigation of the hygrodynamic behaviour of porous materials. In the presented study, the time and process dependent moisture retention characteristic and moisture conductivity are determined. The adsorption and successive desorption experiments presented here have been performed on the capillary active insulation material Calcium Silicate. The results show a hysteretic behaviour with a pregnant influence of the process dynamics. At locations with a rapid moisture increase, the moisture retention characteristic is shifted up in comparison to regions with slow moisture change. The moisture conductivity as function of relative humidity shows a remarkable hysteresis. However, the moisture conductivity in relation to the water content turned out to be non-hysteretic. [source] Moisture controls on carbon dioxide dynamics of peat- Sphagnum monolithsECOHYDROLOGY, Issue 1 2009M. Strack Abstract Sphagnum moss is the major peat-forming vegetation component in boreal peatlands. The relationship between Sphagnum productivity and moss moisture content has been documented; however, the link between moss moisture content and conditions in the underlying peat column is less clear. We conducted a pilot study in which we monitored volumetric moisture content with depth and gravimetric water content of Sphagnum capitula and CO2 exchange for two peat monoliths with intact moss layer dominated by Sphagnum fuscum and S. magellanicum. Measurements were made under drying conditions and rewetting from below and following simulated precipitation events. Capitulum moisture content was related to water table position but varied between species. Both capitulum moisture content and water table position could be used to explain net CO2 exchange and respiration during drying and rewetting from below, although hysteresis was apparent where respiration was lower on rewetting than drying for the same water table position. Precipitation complicated these relationships because small events (<5 mm) rewetted the upper few centimeters of moss resulting in a change in capitulum moisture content equivalent to a rise in water table position of ,20 cm. This change in capitulum moisture content resulted in substantial shifts in both photosynthesis and respiration rates without affecting water table position or subsurface volumetric water contents as shallow as 5 cm below the surface. While these small events will be difficult to measure in the field, this study suggests they are essential to effectively track or model Sphagnum productivity because they may contribute significantly to seasonal carbon balance. Copyright © 2009 John Wiley & Sons, Ltd. [source] Etude comparative de la disponibilité de l'eau en irrigation goutte à goutteIRRIGATION AND DRAINAGE, Issue 3 2001A.V. Ould Mohamed El-Hafedh goutteur; écartement; durée d'irrigation; fréquence d'arrosage Abstract Dans le but de déterminer la meilleure combinaison entre période et durée d'irrigation pour différents écartements inter-goutteurs, nous avons essayé d'analyser la disponibilité de l'eau au sein de la zone racinaire d'une culture de tomate irriguée à l'aide d'une rampe de goutteurs débitant chacun 4 l h,1. Trois écartements ont été étudiés à savoir 30, 50 et 70 cm. Lorsqu'on a pris une durée d'arrosage systématique de 4h30mn, nous avons observé à la fin des irrigations des teneurs en eau volumiques moyennes dépassant la capacité au champ pour les trois écartements. En effet, à l'examen de l'évolution des teneurs en eau au sein du bulbe, il est remarqué que la teneur en eau à la capacité au champ a été atteinte après 25 minutes, 50 minutes et deux heures respectivement pour 30, 50 et 70 cm. Mais l'humidification de toute la ligne de culture n'a été observée qu'après des temps respectifs d'une heure, deux heures et quatre heures (Ould Mohamed El-Hafedh et al., 2000). Les irrigations ont été reprises avec ces durées réduites (une heure, deux heures et quatre heures respectivement pour les écartements 30, 50 et 70 cm) en vue d'évaluer et comparer les consommations en eau de la culture sous les différents traitements. Dans le cas de l'irrigation d'une durée de 4h30mn, la période séparant deux arrosages successifs a été de cinq, quatre et trois jours respectivement pour les écartements 30, 50 et 70 cm. Pour les irrigations des durées réduites, on a constaté qu'il est impératif d'irriguer après trois jours pour les deux écartements 50 et 70 cm et après deux jours pour l'écartement 30 cm. En comparant les consommations en eau pour les durées réduites et la durée systématique de 4h30mn, on a observé une économie d'eau de l'ordre de 20, 15 et 5% respectivement pour 30, 50 et 70 cm d'écartement. D'autre part, la comparaison entre les durées réduites montre que la plus importante économie en eau a été réalisée avec 50 cm d'écartement. Copyright © 2001 John Wiley & Sons, Ltd. In order to determine the best combination between duration and frequency of drip irrigation for various inter-dripper spacing, we analysed the availability of water within the root zone of a tomato culture irrigated using lateral drippers each outputting 4 l h,1 discharge rate. Three spacings were studied, namely 30, 50 and 70 cm. Studying systematic irrigation duration of 4½ hours, we observed at the end of each irrigation average volumetric water contents exceeding the field capacity. Indeed, with the examination of the evolution of the water contents within the bulb, it is noticed that the water content at the field capacity was reached after 25 minutes, 50 minutes and 2 hours respectively for 30, 50 and 70 cm spacing. But the humidification of the whole culture line was observed only after the respective times of 1, 2 and 4 hours (Ould Mohamed El-Hafedh et al., 2000). The irrigations were taken again with these reduced durations (1, 2 and 4 hours for 30, 50 and 70 cm spacing respectively) in order to evaluate and compare water consumption of the culture under the various treatments. In case of systematic irrigation duration, the period separating two successive waterings was five, four and three days for 30, 50 and 70 cm spacing respectively. For the reduced irrigation durations, it is imperative to irrigate after three days for the 50 and 70 cm spacings and after two days for the 30 cm spacing. Comparing water consumption for the reduced durations and those of the systematic duration, we observed water savings of about 20, 15 and 5% respectively for 30, 50 and 70-cm spacing. The comparison between the reduced durations shows that the most significant water saving was obtained with the 50 cm spacing. Copyright © 2001 John Wiley & Sons, Ltd. [source] | |||||||||||||