At the watershed scale, soil moisture is the major control for rainfall–runoff response, especially where saturation excess runoff processes dominate. From the ecological point of view, the pools of soil moisture are fundamental ecosystem resources providing the transpirable water for plants. In drylands particularly, soil moisture is one of the major controls on the structure, function, and diversity in ecosystems. In terms of the global hydrological cycle, the overall quantity of soil moisture is small, ∼0.05%; however, its importance to the global energy balance and the distribution of precipitation far outweighs its physical amount. In soils it governs microbial activity that affects important biogeochemical processes such as nitrification and CO2 production via respiration. During the past 20 years, technology has advanced considerably, with the development of different electrical sensors for determining soil moisture at a point. However, modeling of watersheds requires areal averages. As a result, point measurements and modeling grid cell data requirements are generally incommensurate. We review advances in sensor technology, particularly emerging geophysical methods and distributed sensors, aimed at bridging this gap. We consider some of the data analysis methods for upscaling from a point to give an areal average. Finally, we conclude by offering a vision for future research, listing many of the current scientific and technical challenges.

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Soil Moisture Measurement for Ecological and Hydrological Watershed-Scale Observatories: A Review

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

. The holistic study of soils requires an interdisciplinary approach involving biologists, chemists, geologists, and physicists, amongst others, something that has been true from the earliest days of the field. In more recent years this list has grown to include anthropologists, economists, engineers, medical professionals, military professionals, sociologists, and even artists. This approach has been strengthened and reinforced as current research continues to use experts trained in both soil science and related fields and by the wide array of issues impacting the world that require an in-depth understanding of soils. Of fundamental importance amongst these issues are biodiversity, biofuels/energy security, climate change, ecosystem services, food security, human health, land degradation, and water security, each representing a critical challenge for research. In order to establish a benchmark for the type of research that we seek to publish in each issue of SOIL, we have outlined the interdisciplinary nature of soil science research we are looking for. This includes a focus on the myriad ways soil science can be used to expand investigation into a more holistic and therefore richer approach to soil research. In addition, a selection of invited review papers are published in this first issue of SOIL that address the study of soils and the ways in which soil investigations are essential to other related fields. We hope that both this editorial and the papers in the first issue will serve as examples of the kinds of topics we would like to see published in SOIL and will stimulate excitement among our readers and authors to participate in this new venture.

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The interdisciplinary nature of SOIL

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

Quantification of compaction effects on soil physical properties and crop growth

SUMMARY The paper is addressed to soil scientists who use Time-Domain Reflectometry (TDR) technology to measure soil moisture. The practical aspects of the measurement calibration are discussed, and an empirical approach to establishing the existence of a universal calibration function is presented. Samples of 11 mineral soil horizons and seven organic soil horizons with different chemical and physical properties (including magnetic properties) were selected with the aim of determining their dielectric constant-volumetric water content relationship as calibration functions for TDR soil moisture measurements. These samples were supplemented by other, soil-like, capillary-porous reference materials (montmorillonite, glass beads, washed sand and a sand from a C horizon). The study showed that a unique calibration function for mineral soils and another distinct calibration function for organic soils can be established.

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Empirical evaluation of the relationship between soil dielectric constant and volumetric water content as the basis for calibrating soil moisture measurements by TDR

Summary
The influence of soiľs solid phase on the dielectric constant of the soil over a range of moisture contents has been studied. Samples of soil, soil-like, and also other porous materials were analysed using Time Domain Reflectometry (TDR) to determine the contribution of bulk density and porosity to the function that relates dielectric constant to water content.

The study showed that bulk density, and thus also porosity, substantially affects the relation between dielectric constant and water content. Two equivalent, empirical, normalized conversion functions were found, one accounting for bulk density and the other for porosity. Each of them reduced the root mean square error of the dielectric TDR determinations of moisture to 0.03, regardless of the materialľs bulk density and porosity.

Improving the calibration of dielectric TDR soil moisture determination taking into account the solid soil

The principle of operation of a simple, manually controlled Time Domain Reflectometer (TDR meter) for soil moisture measurements, which operates with a needle pulse of 300 ps rise-time, is described. A block diagram and constructions are also given. Construction of a switchless multiple sensor probe, having an inherent delay reference, is presented. Results of measurements of the soil dielectric constant as related to water content, for soils having different bulk densities, textures and humus content show a high correlation. The results agree closely with other investigators measurements with different, more expensive, TDR instruments. The general principle of microprocessor-controlled TDR operated soil moisture meter is considered.

A manually controlled TDR soil moisture meter operating with 300 ps rise-time needle pulse

Summary
In the range of volumetric water content, θ, from about 0.12 cm3 cm–3 to saturation the relation between bulk electrical conductivity, Cb, and bulk electrical permittivity, e, of mineral soils was observed to be linear. The partial derivative ∂Cb/∂e appeared independent of the moisture content and directly proportional to soil salinity. We found that the variable Xs = ∂Cb/∂e determined from in situ measurements of Cb(θ > 0.2) and e(θ > 0.2) can be considered as an index of soil salinity, and we call it the ‘salinity index’. Knowing the index and sand content for a given soil we could calculate the electrical conductivity of the soil water, Cw, which is a widely accepted measure of soil salinity. The two variables from which the salinity index can be calculated, i.e. Cb and e, can be read simultaneously from the same sensor by time-domain reflectometry.

Quantities and symbols

a constant /dS m–1

b constant

c constant /dS m–1

Cb electrical conductivity of bulk soil /dS m–1

Cb′ constant equal to 0.08 dS m–1

Cs electrical conductivity of a solution used to moisten soil samples /dS m–1

Cw electrical conductivity of soil water defined as the soil salinity /dS m–1

Cwref reference salinity (that truly existing) resulting from the procedure of moistening samples, expressed as Cs + Cr/dS m–1

Cr baseline value of Cs due to residual soluble salts present in the soil /dS m–1

d constant

D dry soil bulk density /g cm–3

l slope

r ratio

S sand content /% by weight

t time /s

Xs salinity index /dS m–1

Xsi initial salinity index when distilled water is used to moisten soil samples /dS m–1

Y a moisture-independent salinity-dependent variable /dS m–1

z coordinate along direction of flow of the soil solution

e′ constant equal to 6.2

e relative bulk electrical permittivity (dielectric constant) of the soil

θ volumetric water content determined thermogravimetrically using oven-drying /cm3 cm–3

Evaluating soil salinity status from bulk electrical conductivity and permittivity

Construction and operating characteristics of a TDR soil moisture miniprobe, that can be inserted through the wall of a metal cylinder, are shown. A laboratory stand for monitoring unsaturated water flow in undisturbed soil cores, involving a set of TDR miniprobes in combination with a set of minitensiometers, is described. This stand was used for frequent readings of instantaneous profiles of moisture and matric potential during transition from saturated to air dry state, in order to compute the unsaturated soil water characteristics. Some methodical aspects of the applied data processing algorithm are outlined.

Application of time-domain reflectometry (TDR) soil moisture miniprobe for the determination of unsaturated soil water characteristics from undisturbed soil cores

M. A. Malicki

Papers

Empirical evaluation of the relationship between soil dielectric constant and volumetric water content as the basis for calibrating soil moisture measurements by TDR

Improving the calibration of dielectric TDR soil moisture determination taking into account the solid soil

A manually controlled TDR soil moisture meter operating with 300 ps rise-time needle pulse

Evaluating soil salinity status from bulk electrical conductivity and permittivity

Application of time-domain reflectometry (TDR) soil moisture miniprobe for the determination of unsaturated soil water characteristics from undisturbed soil cores