Matric Potential Measurements Research Articles

Dynamics of water movement on Chalkland

The dynamics of water movement through the unsaturated zone of Upper Chalk in Southeast England were investigated during the winters of 1995–1996 and 1996–1997. Regular measurements of soil water content and soil matric potential were made weekly, by neutron probe (to 3.6m) and ordinary tensiometers (to 2.4m) and half-hourly by theta probe and pressure transducer tensiometers (both to 1.2m). Drainage was determined using the water balance method.Weekly measurements of water content showed changes of 5–8% in the upper 2.0m and about 2% at depths >2.0m during the season with only slight changes from week to week at 0.4m and 0.2m in response to rain showers. Matric potentials decreased with depth throughout the winter seasons with potentials in the soil ranging from −25 to −550cm water and those below 1m typically −100 to −950cm water. The mean daily rate of drainage for the two seasons was 1.6mmd−1 and 1.5mmd−1 at 1.2m depth and 1.45mmd−1 and 1.1mmd−1 at 3.6m. The mean maximum drainage rates calculated from weekly data were 4.6mmd−1 at 1.2m and 3.1mmd−1 at 3.6m. These weekly measurements suggested that flow was through the matrix because potentials were continuously low and the weekly rates of drainage were smaller than the hydraulic conductivity of the chalk matrix.However, half-hourly measurements showed rapid increases of water content and water potential at all depths to 1.0m under a range of conditions after rain giving short-term maximum rates of drainage at 1.2m of up to 26mmd−1. Typically, water took about 7–9h to travel from the surface to 1.2m depth depending on the rainfall and antecedent conditions. These results indicate that fissures conducted water in the upper 1.2m immediately after rainfall although the matric potential at 1.0m was >−50cm water on only one occasion in the two seasons and frequently <−300cm even after rain.

Hillslope and wetland hydrodynamics in a tussock grassland, South Island, New Zealand

AbstractPrevious research on two experimental catchments at the Glendhu Forest (Otago) showed that runoff dynamics from this landscape differ substantially from other areas in New Zealand. In particular, the recession curves from these catchments have two distinct phases: a rapid decline for a period of several hours, followed by a slow decline that may last many days to weeks. In the current study, soil water matric potential measurements indicated that perched saturated zones develop very rapidly in the surface (0–30 cm) soil horizons. Deep (50–250 cm) loess horizons remain moist throughout the year and may be near or at saturation at the bedrock contact, and respond only slowly to rain events. Soils in the middle horizons are often drier than the surface soil above or the deep soils below. Throughflow measurements showed that storm runoff occurs primarily as interflow through a retentive but pervious mat of mosses, litter, and roots at the soil surface. Despite the presence of abundant earthworm burrows, less water is transmitted through the soil horizon just below this surface mat. Only a small fraction of stormflow was transmitted through the deep, mineral horizons. However, baseflow could be sustained for long periods (up to 4 weeks) from unsaturated flow of soil moisture stored in the deep loess horizons. Water storage in bog peats and associated riparian soils (which are numerous in this area) is insufficient to sustain baseflow for more than a few days. The results from this study are consistent with previous hydrometric, soils, and isotopic observations and suggest that any land‐use practices that disrupt the surface soil horizons are likely to change the runoff characteristics of this area. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Improved Installation Procedures for Deep-Layer Soil Moisture Measurements

Abstract The Oklahoma Mesonet, an automated network of 115 meteorological observing stations, includes soil moisture monitoring devices at 60 locations. The Campbell Scientific model 229-L matric potential (water potential) sensor was chosen for operational use based on its capability to perform as a fully automated soil water measuring device. Extensive laboratory calibrations were performed on each sensor to ensure the quality of the matric potential measurements. Examination of the data from the Norman site during July 1997 revealed significant inconsistencies between near-surface (5 and 25 cm) measurements of soil moisture and deep-layer (60 and 75 cm) measurements of soil moisture. In particular, a heavy precipitation event was followed by only a small increase in near-surface soil water potential values, while a much larger increase occurred in the deep-layer values. It is theorized that an installation flaw is the cause for these inconsistencies. A solution is proposed in the hope that future effor...

Variabilidade dos parâmetros da equação que relaciona a condutividade hidráulica com a umidade do solo no método do perfil instantâneo

A condutividade hidráulica é uma das mais importantes propriedades do solo para estudos que envolvem a infiltração da água, o movimento da água dentro do perfil, bem como para as raízes das plantas e para a drenagem interna. O conhecimento da função condutividade hidráulica versus umidade (K(θ)) é essencial para esses estudos; no entanto, a informação disponível sobre a variabilidade dos parâmetros empíricos dessa função é escassa. Com o objetivo de conhecer a variabilidade desses parâmetros, realizaram-se, neste trabalho, em doze locais do mesmo solo, observações da condutividade hidráulica em função da umidade do solo pelo método do perfil instantâneo, dentro de uma área de 1.000 m². A equação utilizada para K(θ) foi K = K0.exp[γ(θ-θ0)], em que K0 e θ0 são os valores de K e θ, respectivamente, para o tempo zero de redistribuição da água. As medições de umidade e potencial mátrico foram feitas ao longo de 50 dias, aproximadamente, em nove profundidades entre 0,2 e 1,0 m. Os resultados mostraram que coeficientes de variação em torno de dois a três foram obtidos para os valores de K0 na maioria das profundidades e que os valores de γ apresentaram coeficientes de variação menores, em torno de 0,2 a 0,5, maiores na superfície e menores em profundidade. Concluiu-se que os valores de umidade versus tempo ajustaram-se muito bem a uma equação potencial e os de potencial total versus profundidade muito bem a uma equação polinomial de segundo grau. Os resultados indicaram que a relação de K versus θ, determinada pelo método do perfil instantâneo em uma área de alguns metros quadrados, não foi representativa de área muito maior.

Time Domain Reflectometry Probe for Simultaneous Measurement of Soil Matric Potential and Water Content

Simultaneous measurement of matric potential, ψ, and water content, θ, is demanded in many disciplines. Time domain reflectometry (TDR) has been routinely used to measure water content and electrical conductivity of soil. Previous efforts to combine TDR probes with porous materials functioning as tensiometers were successful, but these probes were still constrained by characteristics of tensiometers, such as the need to supply water and measuring ranges ψ &gt; −85kPa. A new ψ–θ TDR probe was developed to overcome shortcomings of the previous work. A portion of the TDR rod was embedded in a dental plaster (gypsum), whose matric potential equilibrated with surrounding soil. The rest of the TDR rod was inserted into the soil. The TDR technique was used to determine dielectric constants, κ, of both the gypsum and the soil. The new ψ–θ TDR probes were tested in clay loam soil using a pressure‐plate apparatus to produce κ–ψ relationships of the gypsum and the soil for −1000 ≤ ψ ≤ −10 kPa. Changes in κ of the gypsum corresponded well to applied pressures for −1000 ≤ ψ ≤ −30 kPa, but κ values did not noticeably change for ψ &gt; −30 kPa. Values of κ of the soil corresponded well to the whole ψ range tested. The new probes accurately measure θ and ψ of soil when soil water content gradually decreases or increases. The newly developed ψ–θ TDR probe requires no more maintenance than ordinary TDR probes and requires no additional instrumentation.

Water use and salt accumulation by Eucalyptus camaldulensis and Casuarina cunninghamiana on a site with shallow saline groundwater

Tree growth, water use, climate, soil salinity and groundwater conditions were monitored over a 3-year period in an unirrigated 5–8-year-old plantation in northern Victoria, within an area where groundwater pumping is in use for control of shallow watertables and salinisation of irrigated farmland. Diameter and sapwood area of Casuarina cunninghamiana increased twice as rapidly as Eucalyptus camaldulensis, in plots with initially equal mean diameter. C. cunninghamiana had greater sapwood area but lower sap flux density, so that the daily and annual water use did not differ significantly between species. Average single tree water use determined by the heat pulse method varied from less than 10 l day −1 in winter to over 30 l day −1 in summer. Stand water use averaged 0.9–1.0 mm day −1 over 2 years, and was evidently limited by soil water availability. The leaf area index (LAI) of the E. camaldulensis stand was estimated as 2.07 and the mean ratio of leaf area to sapwood area as 3815 m 2 m −2. This ratio increased with sapwood area in small trees. Hydraulic properties of the soil profile were inferred from measurements of moisture content and matric potential at 30 cm depth intervals to 390 cm, indicating the presence of a medium clay layer with heavy clay above and below. Salinity and chloride distribution were also determined, indicating slight-to-moderate salinity below 30 cm depth reaching a maximum (saturation extract electrical conductivity 8 dS m −1) at around 200 cm. Saturated hydraulic conductivity of the heavy clay was estimated as 3 mm day −1. Daily watertable depth observations were used to parameterise a water balance model to predict the uptake of water from unsaturated soil and shallow groundwater based on measured daily water use and rainfall, and accounting for the influence of groundwater pumping. Modelled results indicated that more than half the tree water uptake (170–220 mm year −1) was drawn from the groundwater. Both recharge and discharge of the groundwater occurred annually, but discharge was predominant. Intensive monitoring of soil salinity at 100–400 cm depth using buried sensors demonstrated substantial seasonal variation, apparently related to watertable movements. The average soil solution salinity rose by over 5 dS m −1 in 32 months of monitoring, estimated to correspond to chloride accumulation of more than 4 kg m −2. These values cannot be reconciled with the observed water use and salinity below the maximum watertable depth, and may indicate lateral inflow of more saline perched groundwater within the medium clay layer. The results are expected to contribute toward the development and validation of models for predicting the sustainability and environmental benefit of tree plantations in areas with shallow saline groundwater.

PSEUDO-SEÇÕES ELÉTRICAS NA AVALIAÇÃO DA CONTAMINAÇÃO DO SUBSOLO

The geophysical methods are used for indirect determination of the nature of subsurface structures. One geophysical rnethod frequently used in groundwater studies is the electrical resistivity method. The purpose of this work is to show how the electrical resistivity method can be associated to other monitoring techniques, for evaluating contaminant migration in the soil. In this work, the data obtained using geophysics were complemented with matric potential measurements in the vadose zone. The study area is located at the campus of the Brasilia University, in Distrito Federal, and corresponds to an infiltration area, which receives the effluent from a septic tank. The results obtained show, qualitatively, the influence of the moisture increase over the electrical resistivity reduction.

Variabilidade do potencial mátrico e do conteúdo de água no solo em experimento de manejo de irrigação

Desenvolveu-se um experimento de manejo de irrigação de milho (Zea mays L.), cultivado em Latossolo Roxo, de julho a dezembro de 1994, em Guaíra (SP). Os tratamentos, com 4 repetições, foram: tr000 - aplicação de metade da lâmina de água recomendada durante todo o ciclo da cultura; tr0I0 - aplicação da lâmina recomendada somente na fase intermediária da cultura, e metade dessa lâmina na fase inicial e final; e trIII - aplicação da lâmina recomendada durante todo o ciclo da cultura. Utilizando tensiômetros, mediu-se o potencial mátrico de 0,10 a 1,00 m de profundidade, de 0,10 em 0,10 m e, a partir dos valores obtidos, analisaram-se a variabilidade e a propagação desta para o conteúdo de água no solo. Os resultados mostraram que: (1) eventualmente, nas primeiras leituras realizadas após irrigação ou precipitação pluvial, o potencial mátrico e o conteúdo de água no solo mostraram acréscimos em suas variabilidades, pelo fato de, em alguma repetição, a frente de molhamento não ter atingido a cápsula porosa do tensiômetro; (2) o valor do potencial mátrico aumentou em profundidade, tendência que foi acompanhada de um aumento da estabilidade temporal e de uma redução da variabilidade espacial; (3) o potencial mátrico apresentou variabilidade elevada e heterogeneidade de variância; (4) a variabilidade do conteúdo de água no solo foi menos acentuada que a do potencial mátrico, para toda a faixa estudada. Mediante tais resultados, foi possível concluir que a irrigação pode ser manejada com alto nível de confiança estatística quando os resultados de potencial mátrico são transformados para conteúdo de água no solo, transformação esta que minimiza os problemas de variabilidade.

The well-shape factor for the measurement of soil hydraulic properties using the Guelph Permeameter

The Guelph Permeameter method is one of the most frequently used field methods to measure soil hydraulic properties in situ. The well-shape factor which is necessary in the calculation must be obtained visually from a chart. We provide three empirical formulae which can almost perfectly describe the shape factor calculated numerically by Reynolds (see, e.g., Reynolds, 1986. The Guelph Permeameter method for in situ measurement of field-saturated hydraulic conductivity and matric flux potential. Ph.D. Dissertation. University of Guelph, Guelph, Ont.). We conclude that the three empirical formulae can replace the three shape factor curves in the Operating Instructions of Model 2800K1 Guelph Permeameter ( Soil Moisture Corp., 1987).

In situ estimation of soil hydraulic functions using a multistep soil‐water extraction technique

Estimation of the retention and unsaturated hydraulic conductivity functions is essential to effectively provide input for water flow and transport simulation and prediction. A parameter optimization procedure is shown as a promising tool to estimate inversely these hydraulic function parameters from transient soil matric potential and cumulative soil solution extraction measurements. Sensitivity analyses from synthetic data generated from forward numerical model simulations showed that optimum tensiometer locations will depend on soil type. Experiments were carried out in both a laboratory column (Columbia sandy loam) and in the field (Yolo silt loam). In both cases a series of vacuum extraction pressures was applied to a ceramic soil solution sampler, and cumulative soil solution extraction volume and matric potentials at various positions near the extraction device were monitored as the soil solution was extracted. In the laboratory a zero‐flux boundary condition was maintained at the bottom of the column, whereas matric potential measurements were used in the field to define the lower boundary. In both the field and laboratory experiments, flow at the upper boundary was zero. Cumulative extraction volume and matric potential data were included in the objective function to be minimized to estimate the hydraulic function parameters. We determined that the optimized solution was sensitive to the contact between the ceramic ring and the surrounding soil. By also optimizing the hydraulic resistance of the ceramic ring of the extraction device, optimization improved the fit between measured and optimized flow variables. Comparison of the optimized with the independently measured hydraulic functions indicated that the in situ estimation using a multistep extraction procedure can provide accurate soil hydraulic data.

Recharge components in a semi-arid area: the Sana’a Basin, Yemen

Abstract Credible estimates of recharge are essential for assessment of sustainable water resources in semi-arid areas, which rely on groundwater for irrigation and domestic supply. Investigation of the mechanisms and rates of recharge to the aquifers of the Sana’a Basin, Yemen, showed that the main components of recharge are infiltration of surface flows, irrigation return and urban leakage. Limited data and inaccessible, mountainous terrain required the use of robust, low-cost methods of quantifying the recharge components. Infiltration of surface flows from 12 wadis averages 39 Mm 3 a −1 , with high interannual variation. The recharge was calculated using a combination of a rainfall-run-off model and computation of the water balance for the wadi. Irrigation return flows of 30 Mm 3 a −1 , 20 to 40% of the water applied, were derived from measurements of soil moisture and matric potential for the three most extensive crops. Urban recharge from mains leakage and seepage through septic tanks was estimated from a well inventory, water supply records and usage rates, as 59% of the water supplied. The total recharge to the aquifers of the Sana’a Basin in 1993 was estimated to be 102 Mm 3 , significantly less than the total groundwater abstraction, of about 184 Mm 3 .

Soil physical factors limiting establishment of sorghum and cowpea in two contrasting soil types in the semi-arid tropics

Experiments were carried out to study which soil physical factors, if any, would restrict germination and emergence of sorghum ( Sorghum bicolor (L.) Moench.) and cowpea ( Vigna unguiculata (L.) Walp.), under field conditions in Tanzania. The study also aimed to consider the usefulness of mulching as a strategy to overcome such problems, and to contribute to the development of a strategy for modelling seedling emergence in seedbeds in the field. Sorghum and cowpea were planted at two sites in experimental plots which simulated local farming practices, with both bare soil and mulched treatments. The soil types were a sandy clay loam (Paleustaulf) and a sandy loam (Typic Untorthent), both of which were hardsetting. Sequential, daily harvests of the seedlings were related to measurements of soil matric potential and temperature at different depths. Penetration resistances (PRs) were also estimated from matric potentials, and strength characteristics obtained from the same soils. Germination of both crops was high (> 85%) in all of the trials. Emergence of sorghum seedlings was poor (0–30%) in all cases. In the sandy clay loam it was primarily the large mechanical impedance that prevented emergence, whereas a combination of high temperature, water stress and mechanical impedance all contributed to poor emergence in the sandy loam. Cowpea emergence was higher but still only 56% or less in the sandy clay loam due to the increase in mechanical impedance to shoot growth as the soil dried. In the sandy loam, emergence of cowpea was very good (> 85%). The greater percentage emergence of cowpea was related to its greater initial rates of root and shoot growth, which allowed it to avoid the greater stresses caused by temperature, drought and mechanical impedance, experienced by the sorghum. In comparison to sorghum, the larger cowpea shoots were also able to crack a greater thickness of overlying soil when they approached the soil surface and hence overcome mechanical impedance that might otherwise have prevented emergence. The mulched treatment reduced soil surface temperatures by up to 20°C, and delayed the onset of water stress and high mechanical impedance. However, in comparison to the bare soil treatment, the overall effect of these factors varied between a decrease in the percentage emergence of cowpea in the sandy clay loam and an increase in emergence of sorghum in the sandy loam. It is suggested that a modelling approach is required to understand fully these interactions.

Evaluation of a Line Heat Dissipation Sensor for Measuring Soil Matric Potential

AbstractThe accuracy and operational limits of a new heat dissipation sensor for measuring soil matric potential are evaluated. The measurement is based on the rate of temperature rise from a line heat source embedded in a cylindrical porous ceramic that is in equilibrium with soil water. Different heating currents or times can be accounted for by basing calibration on sensor thermal conductivity rather than temperature rise. Six sensors were calibrated against pressure plate and psychrometer measurements of matric potential. Inverse of sensor thermal conductivity was linearly related to the logarithm of matric potential between −10 and −1200 J kg−1. This range of measurable matric potentials is much larger than previous heat dissipation sensor designs. The heat dissipation sensor was accurate to within 20% of the independently measured matric potential. Normalizing sensor readings by sensor thermal conductivity after oven drying results in a calibration that accounts for between‐sensor variability. In a growth chamber experiment with fluctuating soil temperatures, heat dissipation sensor estimates of soil matric potential compared favorably with tensiometer and psychrometer measurements. Heat dissipation sensors based on this design appear useful when a wide range of matric potentials needs to be studied.

Modelling pea (Pisum sativum) root growth in drying soil. A comparison between observations and model predictions

SUMMARYPeas were grown in the glasshouse for 29 d in duplicate 1 m tall containers packed with soil at field capacity (a matric potential of –0·005 MPa) to a bulk density of 1·30 Mg m−3 and with an initial penetration resistance (PR) of 0·35 MPa. Counts of new root contacts on the sides of the containers and matric potential (Ψm) measurements at 0·1 m depth increments were made every two days. Although the plants were not watered, matric potential was never less than –0·041 MPa and was unlikely to have directly limited root growth, but it was used as a predictor of PR and hence of the effect of PR on root growth rate. The effect of PR on root growth was predicted using experimentally determined relationships between root growth rate and PR, and PR and Ψm. These results were used with the ROOTMAP root growth model to give two different model scenarios: (1) variable resistance (VR), allowing mechanical impedance to vary with time and depth in line with the observed values of Ψm and (2) constant resistance (CR), maintaining mechanical impedance at its value at the start of the experiment. CR predictions gave a gross overestimate of the rate at which new root contacts were made on the container walls, whereas VR predictions agreed with experimental observations to within the limits of accuracy of the model and the observations. The comparison between CR and VR predictions gives a clear demonstration of bow the increase in soil strength caused by roots drying the profile can result in a reduction in the size and extent of the whole root system.

Use and accuracy of the filter‐paper technique for measurement of soil matric potential

SummaryA filter–paper technique that can be used to measure the matric potential of field soil is described in detail. When a calibrated batch of filter papers is used, the precision is limited by the variability between individual papers and the limitation of weighing to the nearest mg. The absolute accuracy is also limited by the instruments used for calibration. There was reasonable agreement between batches of the same type of paper, suggesting that our generalized calibration curves can be used for Whatman No. 42 filter papers where accuracy is not paramount.For potentials ≥−2.5 MPa, papers need to be equilibrated with the soil for 6 d. To achieve the same accuracy at lower potentials longer equilibration periods are required. Results suggest that the technique should be effective down to at least −100 MPa if soils are allowed to equilibrate fully with the papers. Coefficients of determination (r2) for the calibration lines were all ≥0.92. A regression of log(‐matric potential) measured by the filter paper technique against measurements made using tensiometers and a psychrometer over a range of potentials between −1 kPa and −10 MPa gave an r2 value of 0.995.

Matric potential evaluations and measurements for gelled substrates

A new method for evaluating the matric potential of gelled media has been developed. The method allows the derivation of the matric potential as a limit of a series of measurements of water potential values from gelled media prepared without added components, from agar powders progressively cleaned of mineral impurities. Three commercial agar brands were tested, and for these the matric potential was found to contribute only between 1 and 2% of their total water potential. Thermodynamic features relating matric and osmotic potentials are described. New hypotheses for understanding the water flux mechanism from gel to tissue cultured explants are discussed. Movement of water along polymeric chains is postulated to be a facilitated step in comparison with bulk movement.

Matric potential evaluations and measurements for gelled substrates

Matric potential evaluations and measurements for gelled substrates

Subsurface Flow Patterns in a Riparian Buffer System

Matric potential was measured in a grass and forest riparian buffer system adjacent to a cropped field in the Georgia Coastal Plain. The soil in the adjacent cropped field is a Tifton loamy sand, containing an argillic subsurface horizon with plinthite at approximately 1 m which has been shown to restrict vertical infiltration and induce lateral flow. Two years of matric potential data and measurements of soil hydraulic characteristics were examined to evaluate and quantify unsaturated water flow in the riparian buffer. The lowest soil matric potential occurred at the grass/forest interface, and the greatest surface infiltration occurred within 10 m downslope of the same interface. The area of low matric potential was likely due to water uptake by trees. Water flowed laterally through the unsaturated soil into the riparian area from the upland field, apparently induced by low vertical conductivity in the subsurface and driven by the high water demand of the forest.

Simulation of field measurement of hydraulic conductivity in unsaturated heterogeneous soil

In this study, the procedures used in common methods for field measurement of unsaturated hydraulic conductivity in heterogeneous soil were simulated through numerical experiments. It was assumed that the heterogeneity of the field soil can be described by a scaling factor δ which is treated as a lognormally distributed, second‐order stationary random variable in space. After the hypothetical random field was generated using property variability and correlation length scales typical of those observed in field measurements, it was used to test various field measurement methods currently in practice. One detailed and three simplified unsteady drainage flux methods were used to estimate the hydraulic conductivity under plausible field measurement conditions. In addition, the soil water retention data were used to predict hydraulic conductivity from a pore‐size distribution model. It was found that even under the assumption that there were no reading or instrument errors, other sources of errors can still be created due to the limitations of the observation techniques. These error sources include limited number of readings in space and time, separation distances between the readings of water content and pressure head, and volume‐averaged readings rather than point estimates with common instrumentation. Under the conditions studied in the simulations, the detailed method of estimation produced reasonable estimates of hydraulic conductivity, but only when very dense measurements of water content and matric potential were used in the estimation. None of the approximate methods yielded accurate measurements of the true hydraulic conductivity of the field region measured.

Microtensiometer technique for in situ measurement of soil matric potential and root water extraction from a sandy soil

The suitability of microtensiometers to measure the spatial variation of soil matric potential and its diurnal change was tested in a pot experiment with pearl millet (Pennisetum americanum [L.] Leeke) in a sandy soil as the soil dried out.