Matric Potential Measurements Research Articles

Principles and Applications of a New Class of Soil Water Matric Potential Sensors: The Dihedral Tensiometer

Soil tensiometers are instruments to measure directly, without calibration, the soil matric potential between zero and the barometric pressure, but in practice it is functional up to about 85kPa, allowing monitoring continuously the soil water status for irrigation scheduling and other hydrological applications. The major drawback of these instruments is water cavitation, which causes interruption of measurements (tension breakdown) and requires instrument re-saturation. In order to avoid the major problems related with common tensiometers, a new class of instruments named dihedral tensiometers was developed at the Brazilian Agricultural Research Corporation. In this new class of instruments two rectangular hydrophilic flat glass plates or one glass plate with a fine porous flat plate are fixed in angle, defining the dihedral angle. The distance from the vertex to the water meniscus (L) formed after equilibration is linearly proportional to the soil matric potential. Using different spacer thickness between the two flat plates allows constructing sensors for different matric potential ranges and applications. Several dihedral tensiometer prototypes have been designed, constructed and tested for matric potential measurements in pot substrates and soils. Results shows linear responses between the applied water potential, using a Richards chamber as reference, and the distance L. Time responses was of few minutes to about 40min, depending on the range of measurement, on the spacer thickness, and on the porous element properties and length. Results of experiments performed in laboratory are presented and discussed as well as the basic principles and construction details of the dihedral tensiometer are described.

Inverse Estimation of Soil Hydraulic and Root Distribution Parameters from Lysimeter Data

We investigated the feasibility of simultaneous identification of soil hydraulic and root‐distribution parameters by inverse simulation of soil water flow in monolithic lysimeters under atmospheric boundary conditions using the Richards equation and a macroscopic root water uptake model. Weighable lysimeters are powerful test systems for this purpose because the boundary fluxes (precipitation, actual evapotranspiration, and seepage across the bottom) can be determined very precisely. We analyzed the amount of information needed for the unique identification of parameters and investigated the magnitude of their uncertainties. First, we examined synthetic data sets for different scenarios and instrumentation campaigns that differed in their information content and complexity of soil properties. Atmospheric boundary conditions as measured at the lysimeter station in Wagna, Austria, were used as forcing data. The results show that for homogeneous profiles, cumulative outflow and profile‐averaged water content data contain enough system information to allow the simultaneous estimation of soil hydraulic properties and root‐distribution parameters. In contrast, for soil profiles consisting of two layers, unique soil hydraulic parameters and the correct rooting depth could only be estimated if matric potential measurements from both layers were included in the objective function. Finally, soil hydraulic properties of the grass‐reference lysimeter in Wagna were estimated using real measurements. Water dynamics in the lysimeter could be described well by an effective parameterization assuming a homogeneous soil profile. Furthermore, the system behavior under different boundary conditions could be predicted adequately with the estimated parameters. This demonstrates the usefulness of the identified system properties for predictive modeling.

Design and performance of a wireless sensor network for catchment‐scale snow and soil moisture measurements

A wireless sensor network (WSN) was deployed as part of a water balance instrument cluster across a forested 1 km2headwater catchment in the southern Sierra Nevada of California. The network, which integrates readings from over 300 sensors, provides spatially representative measurements of snow depth, solar radiation, relative humidity, soil moisture, and matric potential. The ability of this densely instrumented watershed to capture catchment‐scale snow depth and soil moisture distributions is investigated through comparison with three comprehensive gridded surveys and 1 day of detailed lidar snow data. Statistical analysis shows that the network effectively characterized catchment‐wide distributions of snow depth, while offering a cost‐effective, reliable, and energy‐efficient means for collecting distributed data in real time. A temporal analysis of snow depth variability reveals that canopy cover is the major explanatory variable of snow depth and that under‐canopy measurements persistently show higher variability compared to those in open terrain. An analysis of soil moisture shows lower variability at deeper soil depth and a correlation between mean soil moisture and variability for shallow soils. A three‐phase design procedure was used to optimize the WSN deployment. First, as off‐the‐shelf performance of current WSN platforms for large‐scale, long‐term deployments cannot be guaranteed, statistics from a prototype deployment were analyzed. Two indicators of network performance, the packet delivery ratio and received signal strength indicator, showed that for our site conditions, a conservative 50 m node‐to‐node spacing would ensure low‐power, reliable, and robust network communications. Second, results from the prototype were used to refine hardware specifications and to guide the layout of the full 57‐node wireless network. Of these nodes, 23 were used actively for sensing, while the remaining 34 nodes were used as signal repeaters to ensure proper spatial radio coverage and robust network operations. Further analysis of network statistics is conducted during the third, operational, phase to validate system performance.

Effects of Drip Irrigation Configuration and Rate on Yield and Fruit Quality of Young Highbush Blueberry Plants

A 4-year study was conducted to establish the effects of drip irrigation configuration and rate on fruit yield and quality of young highbush blueberry plants (Vaccinium corymbosum L. ‘Duke’). Plants were grown in a silt loam soil on raised beds and were non-irrigated or irrigated using either one or two lines of suspended drip tape. Each line configuration had in-line emitters spaced every 0.3 or 0.45 m for a total of four drip configurations. Water was applied by each drip configuration at two rates, a moderate rate of 5 L/plant per irrigation event, and a heavy rate of 10 L/plant. The frequency of irrigation was guided by measurements of soil matric potential. Irrigation was applied each year, and plants were cropped beginning the second year after planting. Rainfall was above normal in the first 2 years of the study, and differences in soil moisture were most evident in the last 2 years, in which soil matric potential increased with irrigation volume. Neither the number of irrigation lines nor emitter spacing had an effect on yield or fruit quality. Yield was unaffected by irrigation rate until the fourth year after planting and was only higher when 5 L/plant was applied. The yield increase was the result of differences in fruit weight during the second of two harvests and was associated with delays in fruit maturation. Irrigation affected plant mineral concentrations but leaves and berries responded differently; affected minerals tended to decrease in leaves but increase in the fruit. Many irrigation-induced changes in fruit quality were evident 1 or 2 years before changes in yield. Higher irrigation volume increased fruit size and water content but reduced fruit firmness and soluble solids. Irrigation reduced fruit water loss during storage and thereby promoted longer shelf life. Irrigation also resulted in a change in anthocyanin composition in the fruit but did not affect antioxidants or total anthocyanin content.

Irrigation scheduling research: South African experiences and future prospects

Many scheduling approaches have been developed with Water Research Commission funding over the past 4 decades and deployed with varying levels of success; 2 approaches have won prestigious international awards. Soil-based approaches which include measurement of matric potential (tensiometry), water content (neutron probes, capacitance sensors) and depth of wetting (wetting front detectors) have been relatively well accepted by farmers. Atmospheric-based approaches apply, through biophysical modelling of the soil-crop-atmosphere system, thermodynamic limits to the amount of water that can evaporate from a cropped surface under particular environmental conditions. Modelling approaches have been quite empirical or somewhat more mechanistic, generic or crop specific, with pre-programmed (e.g. irrigation calendars) or real-time output. Novel mechanisms have been developed to deliver recommendations to farmers, including resource-poor irrigators. Although general adoption of objective irrigation scheduling in South Africa is still low, the high cost of electricity and nitrogen, and scarcity of water is reviving the interest of consultants and irrigators in the application of these tools to use water more efficiently. Where adoption has been relatively high, intensive support and farmer-researcher-consultant interac tions have been key contributing factors. We propose 4 avenues in the RD and secondly, to further develop new and emerging technologies, like the use of remote sensing. Thirdly, the user-friendliness should be improved as should systems that support existing scheduling tools; and finally, we need to appreciate that farmers are intuitively adaptive man agers, and we need to develop simple monitoring tools and conceptual frameworks that enable structured learning.

Analysis of observed soil moisture patterns under different land covers in Western Ghats, India

Summary An understanding of the soil moisture variability is necessary to characterize the linkages between a region’s hydrology, ecology and physiography. In the changing land use scenario of Western Ghats, India, where deforestation along with extensive afforestation with exotic species is being undertaken, there is an urgent need to evaluate the impacts of these changes on regional hydrology. The objectives of the present study were: (a) to understand spatio-temporal variability of soil water potential and soil moisture content under different land covers in the humid tropical Western Ghats region and (b) to evaluate differences if any in spatial and temporal patterns of soil moisture content as influenced by nature of land cover. To this end, experimental watersheds located in the Western Ghats of Uttara Kannada District, Karnataka State, India, were established for monitoring of soil moisture. These watersheds possessed homogenous land covers of acacia plantation, natural forest and degraded forest. In addition to the measurements of hydro-meteorological parameters, soil matric potential measurements were made at four locations in each watershed at 50 cm, 100 cm and 150 cm depths at weekly time intervals during the period October 2004–December 2008. Soil moisture contents derived from potential measurements collected were analyzed to characterize the spatial and temporal variations across the three land covers. The results of ANOVA (p

A Simple Approach for Demonstrating Soil Water Retention and Field Capacity

It is difficult to demonstrate the soil water retention relationship and related concepts because the specialized equipment required for performing these measurements is unavailable in most classrooms. This article outlines a low‐cost, easily visualized method by which these concepts can be demonstrated in most any classroom. Columns (62.5 cm tall) were constructed using 25, 2.5 cm tall sections of 7.62‐cm (3‐inch) i.d. polyvinyl chloride pipe, which were connected using transparent tape. Three different soil materials were packed to specified bulk densities in the columns, and saturated with water. These vertical columns were then allowed to drain into a simulated water table 2.5 cm above the bottom of the soil volume until drainage ceased. After drainage, columns were sectioned to determine water content distribution with depth along the column. It was assumed that matric potential was inversely related to height above the water table. Therefore, water content measurements and assumed potentials for each section provided data for a water retention curve with minimum potential of approximately –60 cm. During drainage, measurements of soil matric potential were taken at regular intervals using tensiometers installed within the column, validating assumptions about matric potential. Among soil materials tested, those with narrow particle‐size distributions, ∼100% sand, gave the widest distribution of water contents in the observed matric potential ranges. This method, with proper explanation and execution, may be a valuable learning tool by which visual, auditory, and kinesthetic learners may be better able to understand the concepts pertaining to soil–water retention relationships.

A model for flow in the chalk unsaturated zone incorporating progressive weathering

Summary Groundwater from unconfined chalk aquifers constitutes a major water resource in the UK. The unsaturated zone in such systems plays a crucial role in the hydrological cycle, determining the timing and magnitude of recharge, and the transport and fate of nutrients. However, despite more than three decades of study, our physical understanding of this system is incomplete. In this research, state of the art instrumentation provided high temporal resolution readings of soil moisture status, rainfall and actual evaporation from two sites in the Pang and Lambourn catchments (Berkshire, UK), for a continuous two year period (2004/5). A parsimonious, physically based model for the flow of water through the chalk unsaturated zone, including a novel representation of the soil and weathered chalk layers, was developed. The parameters were identified by inverse modelling using field measurements of water content and matric potential. The model was driven by rainfall and evaporation data, and simulated fluxes throughout the profile (including the discrete matrix and fracture components), down to the water table (but not the water table response). Results showed that the model was able to reproduce closely the observed changes in soil moisture status. Recharge was predominantly through the matrix, and the recharge response was strongly attenuated with depth. However, the activation of fast recharge pathways through fractures in the chalk unsaturated zone was highly sensitive to rainfall intensity. Relatively modest increases in rainfall led to dramatically different recharge patterns, with potentially important implications for groundwater flooding. The development and migration of zero flux planes with time and depth were simulated. The simulations also provided strong evidence that, for water table depths greater than 5 m, recharge fluxes persist throughout the entire year, even during drought conditions, with important implications for the calculation of specific yield from baseflow estimates and the representation of recharge in groundwater models.

Matric Potential Measurements by Polymer Tensiometers in Cropped Lysimeters under Water‐Stressed Conditions

In many regions of the world, plant growth and productivity are limited by water deficits. As a result of more frequent and intense droughts, the area of land characterized as very dry has more than doubled since the 1970s. Consequently, understanding root water uptake under water‐stressed conditions is gaining importance. The performance of a recently developed polymer tensiometer (POT) designed to measure matric potentials down to −1.6 MPa was evaluated and compared with volumetric moisture content measurements in dry soil. Three irrigation intensities created severe, intermediate, and no water stress conditions in lysimeters with growing maize (Zea mays L.) plants. By monitoring matric potentials using POTs, levels of local water stress in our experiments were better defined. When the defined stress levels were reached, volumetric moisture measurements for this particular loam soil were below 0.1, thus less informative compared with matric potential measurements. The observed matric potential profiles indicate significant root water uptake between 0.3‐ and 0.5‐m depth in the later growth stages under water‐stressed conditions. The temporal pattern of matric potential profiles reflected changing root water uptake behavior under dry conditions. As the total soil water potential is a direct indication of the amount of energy required by plants to take up water, POTs may contribute to quantifying root water uptake in dry soils.

Modeling Soil Moisture Processes and Recharge under a Melting Snowpack

Recharge into granitic bedrock under a melting snowpack is being investigated as part of a study designed to understand hydrologic processes involving snow at Yosemite National Park in the Sierra Nevada Mountains of California. Snowpack measurements, accompanied by water content and matric potential measurements of the soil under the snowpack, allowed for estimates of infiltration into the soil during snowmelt and percolation into the bedrock. During portions of the snowmelt period, infiltration rates into the soil exceeded the permeability of the bedrock and caused ponding to be sustained at the soil–bedrock interface. During a 5‐d period with little measured snowmelt, drainage of the ponded water into the underlying fractured granitic bedrock was estimated to be 1.6 cm d−1, which is used as an estimate of bedrock permeability. The numerical simulator TOUGH2 was used to reproduce the field data and evaluate the potential for vertical flow into the fractured bedrock or lateral flow at the bedrock–soil interface. During most of the snowmelt season, the snowmelt rates were near or below the bedrock permeability. The field data and model results support the notion that snowmelt on the shallow soil overlying low permeability bedrock becomes direct infiltration unless the snowmelt rate greatly exceeds the bedrock permeability. Late in the season, melt rates are double that of the bedrock permeability (although only for a few days) and may tend to move laterally at the soil–bedrock interface downgradient and contribute directly to streamflow.

Sampling Silica and Ferrihydrite Colloids with Fiberglass Wicks under Unsaturated Conditions

The suitability of passive capillary samplers (PCAPS) for collection of representative colloid samples under partially saturated conditions was evaluated by investigating the transport of negatively and positively charged colloids in fiberglass wicks. A synthetic pore water solution was used to suspend silica microspheres (330 nm in diameter) and ferrihydrite (172 nm in diameter) for transport experiments on fiberglass wicks. Breakthrough curves were collected for three unsaturated flow rates with silica microspheres and one unsaturated flow rate with ferrihydrite colloids. A moisture characteristic curve, relating tensiometer measurements of matric potential to moisture content, was developed for the fiberglass wick. Results indicate that retention of the silica and the ferrihydrite on the wick occurred; that is, the wicks did not facilitate quantitative sampling of the colloids. For silica microspheres, 90% of the colloids were transmitted through the wicks. For ferrihydrite, 80 to 90% of the colloids were transmitted. The mechanisms responsible for the retention of the colloids on the fiberglass wicks appeared to be physicochemical attachment and not thin-film, triple-phase entrapment, or mechanical straining. Visualization of pathways by iron staining indicates that flow is preferential at the center of twisted bundles of filaments. Although axial preferential flow in PCAPS may enhance their hydraulic suitability for sampling mobile colloids, we conclude that without specific preparation to reduce attachment or retention, fiberglass wicks should only be used for qualitative sampling of pore water colloids.

A Small‐Scale Matric Potential Sensor Based on Time Domain Reflectometry

Measurements of soil matric potential (ψ) are needed in many soil science applications. In the present study, a small matric potential sensor having a length of 30 mm and a diameter of 9.6 mm was developed. The sensor consists of two coils made of lacquer‐coated copper wires embedded in gypsum. The dielectric constant of the gypsum (Kgypsum) was measured with time domain reflectometry (TDR). The gypsum probes were tested in a sandy loam soil using a pressure extractor to obtain the relationship between Kgypsum and ψ, allowing inference of the soil ψ in equilibrium with the sensor. It was shown that the gypsum probe could give ψ estimates with a root mean square error (RMSE) of around 50 kPa in the range of −20 to −1500 kPa. Most of the sensitivity of the sensor was in the −50‐ to −1000‐kPa range.

Evaluation of the Watermark sensor for use with drip irrigated vegetable crops

The Watermark 200SS sensor was evaluated for the measurement of soil matric potential (SMP) with drip-irrigated vegetable crops. Pepper and melon crops were grown sequentially during autumn-winter and spring-summer, in a sandy loam soil in a greenhouse. Ranges of SMP were generated by applying three different irrigation treatments — 100, 50 and 0% of crop water requirements, during two treatment periods (16 December 2002–7 January 2003; 20 January–10 February 2003) in pepper and one treatment period (26 May–6 June 2003) in melon. Watermark sensors and tensiometers were positioned, at identical distances from irrigation emitters, at 10 cm soil depth, with four replicate sensors for each measurement. Electrical resistance from Watermark sensors and SMP from tensiometers were recorded at 30-min intervals. An in-situ calibration equation was derived using data from the first pepper treatment period. For data in the three treatment periods, SMP was calculated from Watermark electrical resistance using the in-situ, Thomson and Armstrong (in Appl Eng Agric 3:186–189 1987), Shock et al. (1998) and Allen (2000) calibration equations. Additionally, the Thomson and Armstrong (in Appl Eng Agric 3:186–189 1987) and Shock et al. (1998) equations were re-parameterised with the SOLVER® function of Microsoft Excel 2000® using data from the first pepper treatment period. Watermark-derived SMP, for each equation, were compared with tensiometer-measured SMP, for −30 kPa, measuring to −2.5 kPa. Where the soil was not drying rapidly, the Shock et al. (1998) equation generally provided an accurate indication of SMP at −30 to −80 kPa. The use of dynamic data (collected every 30 min) compared to static data (collected only at 6 a.m.) did not influence the evaluation of calibration equations. This study suggested that the Watermark sensor can provide an accurate indication of SMP provided that a suitable calibration equation is derived/verified for the specific cropping conditions, and that the performance characteristics of the sensor are considered.

POROUS MEDIA MATRIC POTENTIAL AND WATER CONTENT MEASUREMENTS DURING PARABOLIC FLIGHT

Control of water and air in the root zone of plants remains a challenge in the microgravity environment of space. Due to limited flight opportunities, research aimed at resolving microgravity porous media fluid dynamics must often be conducted on Earth. The NASA KC-135 reduced gravity flight program offers an opportunity for Earth-based researchers to study physical processes in a variable gravity environment. The objectives of this study were to obtain measurements of water content and matric potential during the parabolic profile flown by the KC-135 aircraft. The flight profile provided 20-25 s of microgravity at the top of the parabola, while pulling 1.8 g at the bottom. The soil moisture sensors (Temperature and Moisture Acquisition System: Orbital Technologies, Madison, WI) used a heat-pulse method to indirectly estimate water content from heat dissipation. Tensiometers were constructed using a stainless steel porous cup with a pressure transducer and were used to measure the matric potential of the medium. The two types of sensors were placed at different depths in a substrate compartment filled with 1-2 mm Turface (calcined clay). The ability of the heat-pulse sensors to monitor overall changes in water content in the substrate compartment decreased with water content. Differences in measured water content data recorded at 0, 1, and 1.8 g were not significant. Tensiometer readings tracked pressure differences due to the hydrostatic force changes with variable gravity. The readings may have been affected by changes in cabin air pressure that occurred during each parabola. Tensiometer porous membrane conductivity (function of pore size) and fluid volume both influence response time. Porous media sample height and water content influence time-to-equilibrium, where shorter samples and higher water content achieve faster equilibrium. Further testing is needed to develop these sensors for space flight applications.

Influence of transitional volcanic strata on lateral diversion at Yucca Mountain, Nevada

Natural hydraulic barriers exist at Yucca Mountain, Nevada, a potential high‐level nuclear waste repository, that have been identified as possible lateral diversions for reducing deep percolation through the waste storage area. Historical development of the conceptual model of lateral diversion has been limited by available field data, but numerical investigations presented the possibility of significant lateral diversion due to the presence of a thin, porous rock layer, the Paintbrush nonwelded tuffs. Analytical analyses of the influence of transitional changes in properties suggest that minimal lateral diversion is likely at Yucca Mountain. Numerical models, to this point, have not accounted for the gradual transition of properties or the existence of multiple layers that could inadvertently influence the simulation of lateral diversion as an artifact of numerical model discretization. Analyses were made of subsurface matric potential measurements, and comparisons were made of surface infiltration estimates with deeper percolation flux calculations using chloride‐mass‐balance calculations and simulations of measured temperature profiles. These analyses suggest that insignificant lateral diversion has occurred above the repository horizon and that water generally moves vertically through the Paintbrush nonwelded tuffs.

An experimental tracer study of the role of macropores in infiltration in grassland soils

AbstractWater flow in macropores is an important mechanism of infiltration in natural soils and, as such, is crucial for the prediction of runoff generation. The major flow processes controlling macropore flow are the initiation of macropore flow (water supply into macropores) and the water transfer from the macropores into the surrounding soil matrix (interaction). The water movement during infiltration and the resulting flow paths were studied with combined sprinkling and dye tracer experiments under different rainfall intensities and initial soil moisture conditions. The dye tracer was continuously applied with the sprinkling water on 1 m2 plots. After the sprinkling, horizontal and vertical soil sections were prepared for surveying dye patterns, which showed the cumulated flow pathways in the soils. These experiments were carried out on four hillslope sites covered with grassland, where earthworms mainly built the macropore system. The evaluation of the flow processes in the soil was based on classified dye patterns and measurements of water content and matric potential. The results illustrate how flow in earthworm channels influences general hydrological flow processes during extreme rainfall events. Macropore flow was initiated from the soil surface or from a saturated or partially saturated soil layer. Transfer of water from the macropores into the soil matrix was mainly influenced by the soil properties and soil water content. The permeability of the underlying bedrock in combination with this transfer of water controlled the drainage of the macropores. Finally, major effects of macropore flow processes on the hydrological response were extracted. Infiltration excess overland flow was reduced if water bypassed the less permeable layer through macropores, saturation excess overland flow was less affected by macropores, and subsurface flow was activated very rapidly because the infiltrated water bypassed the soil matrix. This study highlights the most important processes that have to be considered in order to understand better and to model infiltration in natural soils in the future. Copyright © 2003 John Wiley & Sons, Ltd.

DETERMINING THE EXTRACTABLE WATER LIMIT FOR WHEAT IN A SUBSTRATE–BASED MEDIA DESIGNED FOR SPACE FLIGHT APPLICATIONS

The objectives of this research were to determine the limit of extractable water for wheat grown in asubstratebased nutrient delivery system designed for spaceflight applications and to compare novel soil moisture sensorsto matric potential measurements using tensiometers. The substrate compartments were constructed of clear polycarbonateplastic with a bottommounted porous tube and filled with sieved 1 to 2mm Turface media. Seeds were germinated andtransplanted into the Turfacefilled substrate compartment. A sheet of plastic was used to cover the substrate to limitevaporation from the medias surface. The compartments were irrigated with halfstrength Hoaglands nutrient solution viathe porous tubes. Plant transpiration rates were monitored using digital scales. Soil moisture levels were also monitored usingpressure sensors mounted to the porous tubes (tensiometer) and novel heatpulse soil moisture sensors that were embeddedat different depths in the substrate. The plants were grown for a minimum of 45 days prior to imposing drought conditions.The drought conditions were maintained until daytoday mass changes in the substrate compartments decreased to aminimum. The Turface media from the compartments was subsequently removed, weighed, dried, and reweighed. The sensorsperformed satisfactorily, but both systems have limitations. The average limit of extractable water on a mass of water basiswas 39.7% based on six replications.

Soil Water Retention Measurements Using a Combined Tensiometer‐Coiled Time Domain Reflectometry Probe

The objective of the presented study was to develop a single probe that can be used to determine soil water retention curves in both laboratory and field conditions, by including a coiled time domain reflectometry (TDR) probe around the porous cup of a standard tensiometer. The combined tensiometer‐coiled TDR probe was constructed by wrapping two copper wires (0.8 mm diam. and 35.5 cm long) along a 5‐cm long porous cup of a standard tensiometer. The dielectric constant of five different soils (Oso Flaco [coarse‐loamy, mixed Typic Cryorthod‐fine‐loamy, mixed, mesic Ustollic Haplargid], Ottawa sand [F‐50‐silica sand], Columbia [Coarse‐loamy, mixed, superactive, nonacid, thermic Oxyaquic Xerofluvents], Lincoln sandy loam (sandy, mixed, thermic Typic Ustifluvents), and a washed sand ‐ SRI30) was measured with the combined tensiometer‐coiled TDR probe (coil) as a function of the soil water content (θ) and soil water matric potential (h). The measured dielectric constant (εcoil) as a function of water content was empirically fitted with a third‐order polynomial equation, allowing estimation of θ(h)‐curves from the combined tensiometer‐coiled TDR probe measurements, with R2 values larger than 0.98. In addition, the mixing model approach, adapted for the tensiometer‐coiled TDR probe, was successful in explaining the functional form of the coiled TDR data with about 30% of the coiled‐TDR probe measurement explained by the bulk soil dielectric constant. This new TDR development provides in situ soil water retention data from simultaneous soil water matric potential and water content measurements within approximately the same small soil volume around the combined probe, but requires soil specific calibration because of slight desaturation of the porous cup of the tensiometer.

Physical controls on septic leachate movement in the vadose zone at the hillslope scale, Putnam County, New York, USA

AbstractThe fate and transport of contaminants in the vicinity of septic fields remains poorly understood in many hydrogeomorphological environments. We report hydrometric data from an intensive hillslope‐scale experiment conducted between 29 August and 11 November 1998 at a residential leach field in New York State. The objective of our study was to characterize water flux within the vadose zone, understand the physical controls on the flux, and predict how this ultimately will affect subsurface water quality. Soil‐water flux was calculated using matric potential measurements from a network of 25 tensiometer nests, each nest consisting of three tensiometers installed to depths of 10, 50 and 130 cm. Unsaturated hydraulic conductivity curves were derived at each depth from field‐determined time‐domain reflectometry–tensiometry moisture‐release curves and borehole permeametry measurements. Flownets indicated that a strong upward flux of soil water occurred between rainstorms. Following the onset of (typically convective) rainfall, low near‐surface matric potentials were rapidly converted to near‐saturated and saturated conditions, promoting steep vertical gradients through the near‐surface horizons of the hillslope. Lateral hydraulic gradients were typically 10 times smaller than the vertical gradients. Resultant flow vectors showed that the flux was predominantly vertical through the vadose zone, and that the flux response to precipitation was short‐lived. The flux response was controlled primarily by the shape of the unsaturated hydraulic conductivity curves, which indicated a rapid loss of conductivity below saturation. Thus, soil water had a very high residence time in the vadose zone. The absence of rapid wetting at 130 cm and the delayed and small phreatic zone response to rainfall indicated that water movement through macropores did not occur on this hillslope. These results are consistent with a Cl tracing experiment, which demonstrated that the tracer was retained in the vadose zone for several months after injection to the system. Copyright © 2002 John Wiley & Sons, Ltd.

Indirect estimation of soil thermal properties and water flux using heat pulse probe measurements: Geometry and dispersion effects

Traditionally, analytical solutions for heat transport in soils have been used in combination with heat pulse probe (HPP) measurements to estimate soil thermal properties. Although the analytical method has resulted in accurate estimation of soil thermal properties, we suggest that parameter estimation using inverse modeling (IM) provides new and unique opportunities for soil thermal characterization. Moreover, we show that the IM approach provides accurate estimation of soil water flux density in both unsaturated and saturated soil conditions for a wider range of water velocities than originally thought possible. Specifically, we show that accurate soil water velocity is obtained, simultaneously with soil thermal properties, if heat dispersion is included in the heat transport equation. The requirement for including heat dispersivity depends on the value of the newly defined dimensionless Keith Jirka Jan (KJJ) number, which is equal to the ratio of thermal dispersion to thermal conductivity. For example, when KJJ > 1, ignoring thermal dispersivity leads to errors in the water flux density which can exceed 10%. By including thermal dispersivity, water flow velocities were accurately determined for water flux densities ranging from 1.0 to >10 m d−1. We also demonstrate the general application of inverse modeling to estimate soil thermal properties and their functional dependence on volumetric water content in a separate numerical experiment. We suggest that inverse modeling of HPP temperature data may allow simultaneous estimation of soil water retention (when combined with matric potential measurements) and unsaturated hydraulic conductivity (through water flux estimation) from simple laboratory experiments.