Design of Triple‐Wire Time Domain Reflectometry Probes in Practice and Theory

Heimovaara, T.J.

doi:10.2136/sssaj1993.03615995005700060003x

Cited by 189 publications

(116 citation statements)

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“…Analytical precision is 0.4 • C. We monitored soil volumetric water content (VWC) using Time Domain Reflectometry (TDR) probes based on Topp's equation (Topp et al, 1980) calibrated in the close vicinity of our study site (Heimovaara, 1993;Garré et al, 2008;Beff et al, 2013).…”

Section: Monitoring Of Soil Co 2 Water and Temperaturementioning

confidence: 99%

Vertical partitioning and controlling factors of gradient-based soil carbon dioxide fluxes in two contrasted soil profiles along a loamy hillslope

2015

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Abstract. In this study we aim to elucidate the role of physical conditions and gas transfer mechanism along soil profiles in the decomposition and storage of soil organic carbon (OC) in subsoil layers. We use a qualitative approach showing the temporal evolution and the vertical profile description of CO 2 fluxes and abiotic variables. We assessed soil CO 2 fluxes throughout two contrasted soil profiles (i.e. summit and footslope positions) along a hillslope in the central loess belt of Belgium. We measured the time series of soil temperature, soil moisture and CO 2 concentration at different depths in the soil profiles for two periods of 6 months. We then calculated the CO 2 flux at different depths using Fick's diffusion law and horizon specific diffusivity coefficients. The calculated fluxes allowed assessing the contribution of different soil layers to surface CO 2 fluxes. We constrained the soil gas diffusivity coefficients using direct observations of soil surface CO 2 fluxes from chamber-based measurements and obtained a good prediction power of soil surface CO 2 fluxes with an R 2 of 92 %.We observed that the temporal evolution of soil CO 2 emissions at the summit position is mainly controlled by temperature. In contrast, at the footslope, we found that long periods of CO 2 accumulation in the subsoil alternates with short peaks of important CO 2 release. This was related to the high water filled pore space that limits the transfer of CO 2 along the soil profile at this slope position. Furthermore, the results show that approximately 90 to 95 % of the surface CO 2 fluxes originate from the first 10 cm of the soil profile at the footslope. This indicates that soil OC in this depositional context can be stabilized at depth, i.e. below 10 cm. This study highlights the need to consider soil physical properties and their dynamics when assessing and modeling soil CO 2 emissions. Finally, changes in the physical environment of depositional soils (e.g. longer dry periods) may affect the long-term stability of the large stock of easily decomposable OC that is currently stored in these environments.

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Section: Monitoring Of Soil Co 2 Water and Temperaturementioning

confidence: 99%

Vertical partitioning and controlling factors of gradient-based soil carbon dioxide fluxes in two contrasted soil profiles along a loamy hillslope

2015

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“…Triple TDR probes [Heimovaara, 1993] of 35 cm length were selected to be most suitable. Preliminary studies showed that the spatial variability in soil water contents was higher when sampling with shorter probes thereby requiring more grid measurements.…”

Section: Tdr Measurementsmentioning

confidence: 99%

Assessing rooting depths of an austrian pine stand by inverse modeling soil water content maps

Musters¹,

Bouten²

1999

Water Resources Research

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Abstract. Rooting depths in a forested stand (0.11 ha) were estimated indirectly by inverse modeling maps of soil water contents from time domain reflectometry (TDR) measurements at 150 points. These maps were described with a calibrated onedimensional soil water flow model, with specific values for the rooting depth, van Genuchten's [1980] a parameter, and throughfall fraction at each point. At about one third of the 150 points, modeled rooting depths did not fall within the a priori likely range of rooting depths; these values were discarded when kriging the point estimates to a map of rooting depths. The resulting rooting depths range from 0.8 m to 3.5 m within the stand in good agreement with root observations (R 2 = 0.85). Several factors contributed positively to this good match, such as the fairly uniform soil hydraulic properties and the high sensitivity of soil water dynamics to root water uptake. Overall, the results demonstrate the suitability of soil water content maps based on TDR measurements to quantify spatial variability in soil water dynamics. IntroductionRoot water uptake in ecosystems is often studied with soilvegetation-atmosphere (SVAT) models that integrate the simulation of the different elements of the hydrological cycle. SVAT models offer a tool to estimate plant water uptake and its distribution over the soil as plant uptake fluxes cannot be measured directly. From a hydrological perspective, water fluxes to the atmosphere and groundwater are largely controlled by root systems of trees Schaap et al., 1995]. Rooting depths are therefore key parameters in SVAT models but are difficult and tedious to collect by classical methods such as root excavation and soil coring [Atkinson, 1991]. As a consequence, rooting depths are often only known by estimation; hence results of SVAT models are difficult to interpret.Potentially, root distribution parameters like the rooting depth can be estimated by inverse modeling the soil water dynamics. This indirect estimation is preferable to conventional root observation methods, mainly because root information is not directly related to root activity [Gardner, 1991]. Inverse modeling will succeed only if root water uptake significantly influences the soil water dynamics; its use is therefore restricted to shallow root systems (<4 m) and to dry periods when sensitivity of soil water dynamics to root water uptake is largest. Also, root parameters cannot be estimated indirectly without accurately describing the soil water fluxes. Thus inverse modeling requires a calibrated soil water flow model and consequently a monitoring program to provide the necessary data for calibrating a soil water flow model. In addition, roots never grow uniformly through a soil, and root parameters should therefore be studied on a relatively large scale (>100 The use of indirect methods to estimate root parameters has for long time been frustrated by the inability to measure The aim of this study was to estimate rooting depths in a forested pine stand by inverse modeling the soil ...

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“…To reduce conductive losses and improve the quality of the TDR waveforms and their analysis by using the double-reflection procedure (Heimovaara, 1993), probe rods were insulated with polyolefin heat-shrink tubing having a wall thickness of 0.5 mm. Figure 2 shows the effect of this type of coating.…”

Section: Tdr-based Water Level Sensing Set-up: Probe Design and Wavefmentioning

confidence: 99%

TDR application for automated water level measurement from Mariotte reservoirs in tension disc infiltrometers

Moret

López

Arrúe

2004

Journal of Hydrology

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This paper describes the use of an automated method of measuring water level changes in Mariotte-type reservoirs via time domain reflectometry (TDR) and demonstrates its field application for measurements of soil hydraulic properties. The method is based on the assumption that the travel time of a TDR pulse propagating along a transmission line immersed in an air-water medium is the summation of the pulse travel times in the air and water phases. A TDR cable tester generates a pulse that propagates through a three-rod probe traversing the centre of the Mariotte reservoir from top to bottom. The reflection of the pulse is automatically transferred to a computer for waveform analysis with the water level being a simple function of probe length and the air, water, and air-water medium dielectric constants as measured by the cable tester. Water level measurements obtained with the TDR technique showed close agreement with those obtained using visual and pressure transducer procedures. The application of this TDR method as an alternative to more traditional methods was demonstrated in a field experiment using a tension disc infiltrometer. The TDR approach allows for automated water level measurements and is simple, accurate and easy to implement. Moreover, it allows for simultaneous TDR measurements of both water flow and volumetric water content of soil below the infiltrometer disc.

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Design of Triple‐Wire Time Domain Reflectometry Probes in Practice and Theory

Cited by 189 publications

References 0 publications

Vertical partitioning and controlling factors of gradient-based soil carbon dioxide fluxes in two contrasted soil profiles along a loamy hillslope

Vertical partitioning and controlling factors of gradient-based soil carbon dioxide fluxes in two contrasted soil profiles along a loamy hillslope

Assessing rooting depths of an austrian pine stand by inverse modeling soil water content maps

TDR application for automated water level measurement from Mariotte reservoirs in tension disc infiltrometers

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