Substantial advances in the measurement of water content and bulk soil electrical conductivity (EC) using time domain reflectometry (TDR) have been made in the last two decades. The key to TDR's success is its ability to accurately measure the permittivity of a material and the fact that there is a good relationship between the permittivity of a material and its water content. A further advantage is the ability to estimate water content and measure bulk soil EC simultaneously using TDR. The aim of this review is to summarize and examine advances that have been made in terms of measuring permittivity and bulk EC. The review examines issues such as the effective frequency of the TDR measurement and waveform analysis in dispersive dielectrics. The growing importance of both waveform simulation and inverse analysis of waveforms is highlighted. Such methods hold great potential for obtaining far more information from TDR waveform analysis. Probe design is considered in some detail and practical guidance is given for probe construction. The importance of TDR measurement sampling volume is considered and the relative energy storage density is modeled for a range of probe designs. Tables are provided that compare some of the different aspects of commercial TDR equipment, and the units are discussed in terms of their performance and their advantages and disadvantages. It is hoped that the review will provide an informative guide to the more technical aspects of permittivity and EC measurement using TDR for the novice and expert alike.

A Review of Advances in Dielectric and Electrical Conductivity Measurement in Soils Using Time Domain Reflectometry

Measurement of soil water content and electrical conductivity by time domain reflectometry: a review

Northern peatlands provide important global and regional ecosystem services (carbon storage, water storage, and biodiversity). However, these ecosystems face increases in the severity, areal extent and frequency of climate-mediated (e.g. wildfire and drought) and land-use change (e.g. drainage, flooding and mining) disturbances that are placing the future security of these critical ecosystem services in doubt. Here, we provide the first detailed synthesis of autogenic hydrological feedbacks that operate within northern peatlands to regulate their response to changes in seasonal water deficit and varying disturbances. We review, synthesize and critique the current process-based understanding and qualitatively assess the relative strengths of these feedbacks for different peatland types within different climate regions. We suggest that understanding the role of hydrological feedbacks in regulating changes in precipitation and temperature are essential for understanding the resistance, resilience and vulnerability of northern peatlands to a changing climate. Finally, we propose that these hydrological feedbacks also represent the foundation of developing an ecohydrological understanding of coupled hydrological, biogeochemical and ecological feedbacks. Copyright © 2014 John Wiley & Sons, Ltd.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/eco.1493

Hydrological feedbacks in northern peatlands

https://cyberleninka.org/article/n/581859.pdf

Structure of peat soils and implications for water storage, flow and solute transport: A review update for geochemists

Quantifying preferential flow in soils: A review of different techniques

Methods for Analyzing Constant-Head Well Permeameter Data

The Guelph Permeameter (GP) method for simultaneous, in situ measurement in the vadose zone of field-saturated hydraulic conductivity (KfS), sorptivity (S) and the conductivity-pressure head relationship [K(Ψ)] is described and discussed.

The method involves measuring the steady-state liquid recharge, Q, necessary to maintain a constant depth of water, H, in an uncased, cylindrical well of radius, a, finished above the water table. An “in-hole” Mariotte bottle device is used to maintain H and to measure Q.

Step-by-step procedures with example calculations are given for obtaining KfS, S and K(Ψ). Techniques for assessing the results are also given.

Although detailed field verification of the GP method is still incomplete, the studies conducted so far are quite encouraging.

Theoretical and practical advantages afforded by the GP method make it a desirable and cost-effective means for in situ measurement of Kfs, S and K(Ψ), which are three of the most important parameters governing the flow of water and other wetting liquids in the vadose zone.

A Method for Simultaneous In Situ Measurement in the Vadose Zone of Field-Saturated Hydraulic Conductivity, Sorptivity and the Conductivity-Pressure Head Relationship

Simplified measurements of the field-saturated hydraulic conductivity, K fs , require short duration experiments, small water volumes, and easily transportable equipment. A simplified falling-head (SFH) technique for the rapid determination of K fs has been developed and tested. The technique consists in applying a small volume of water on a soil surface, confined by a ring inserted a short distance into the soil, and then measuring the time from the application of water to the instant at which the surface area is no longer covered by water. A measurement of the initial and field-saturated soil water contents, and an estimate of the α* parameter of the Gardner's exponential model are then used to calculate K fs using a simple solution that includes gravity. The K fs of both repacked and undisturbed soil cores was determined in the laboratory by the SFH and the early time constant-head (ECH) techniques. The SFH and (constant-head) pressure infiltrometer (PI) techniques were then compared in the field. The maximum discrepancy between the mean K fs results obtained within an experiment was of a factor of approximately two. This difference is negligible in most practical applications and it was concluded that the SFH technique compared favorably with the ECH technique in the laboratory and to the PI technique in the field. The SFH technique appears promising for determining K fs in a relatively short period of time without the need for extensive instrumentation or analytical methodology, and therefore it appears suitable for detailed field measurements over large areas.

A simplified falling-head technique for rapid determination of field-saturated hydraulic conductivity

Infiltration from Constant-Head Well Permeameters and Infiltrometers

Sphagnum mosses (Sphagnum L.) are the primary pear-forming plant in northern peadattds and rely on capillary transport of water to facilitate physiological processes. The unsaturated hydraulic conductivity of the living, undecomposed, and poorly decomposed mosses is needed to estimate and model water flux to their growing upper layer. This study describes a new apparatus to measure this in the highly porous (∼90%) hummock profile where the pore sizes are large and the mosses delicate, in which established methods do not work. Independent tension disks controlled the pressure hcad (ψ, between 0 and -35 cm of water) and the pressure gradient and thus flow. The uppermost 5-cm layer of moss had a saturated hydraulic conductivity of 1800 μm s -1 , and decreased when unsaturated (ψ= -25 cm of water) to 0.03 μ s -1 . Moss 25 cm below the surface had equivalent values of 230 and 11.0 μm s -1 at moisture contents of 0.18 to 0.22 m 3 m -3 . The The soil water retention model RETC provided a good fit for both hydraulic conductivity and water retention when fitted simultaneously, but did not perform well to predict hydraulic conductivity from water retention data alone.

https://www.gret-perg.ulaval.ca/uploads/tx_centrerecherche/Price_etal_SSSAJ2008.pdf

A Method to Determine Unsaturated Hydraulic Conductivity in Living and Undecomposed Sphagnum Moss

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