Soil Water Measurement: A Practical Handbook

This book will aim to be a practical guide to all aspects of measuring water in field soils and unsaturated rocks. It will cover all the most common methods of measuring and estimating soil water content, potential, hydraulic conductivity, solute content and relationships between them, with a strong emphasis on in situ measurements in the field. The book will contain thorough descriptions of the principles underlying each method, accessible to anyone with a university entrance–level scientific background, as well as practical guidance on the use of each method. Some case studies of projects using the various techniques will be included to illustrate the ways in which different measurements can be combined to produce useful information.

The book is aimed at providing a bridge between soil physics text books and monographs on different experimental methods. There are many examples of the former, which are good at describing the principles of water interactions with soil and rock, but mostly contain only a cursory description of measurement methods, particularly as they are applied in the field. There are rather fewer examples of the latter. They fall into two categories. Compendia of methodologies for measuring aspects of soil water and monographs on specific techniques. The compendia consist of a series of chapters, each written by different authors, are authoritative, but are generally difficult to use in practical situations because of too much detail, a high academic level, lack of practical guidance and uneven approach. Monographs dealing with specific techniques are generally exhaustive, but have very specific scope. No monographs exist for some common techniques. Since it is rare for any investigation to require only one type of measurement, it is necessary to consult several different monographs to obtain the required information.

Brief overview of what is meant by soil water, why it is important to measure it and how different forms may respond differently to various measuring methods.
2 How does water in soil interact with the soil matrix, air, roots, gravity and other substances present?.

Summary of important concepts of soil physics to provide a background to the rest of the book.
3 What do we need to measure?.

Continuation of the above, highlighting different variables that may be measured.
4 Spatial variability.

Health warning on the limitations imposed by spatial variability in field soils and consequences for devising an appropriate measurement strategy.
II WATER CONTENT
5 Definitions.

Different ways to express water content and appropriate units.
6 Gravimetric Method.

Description and limitations of the reference method for measurement of water content.
7 Neutron Probe.

Description of the principles of the method and practicalities including access tube installation, calibration, ground protection, sampling strategies, data handling and interpretation and radiological protection. Also example data.
8 Dielectrics.

Overview of dielectric theory (this might be better placed in an appendix) and application to devices dependent on this – TDR, capacitance, bridge and theta probes with brief description of ground penetrating radar. Practical information similar to that for neutron probes.
9 Heat Dissipation.

Not a very common technique, but I intend to cover it for completeness.
10 Other methods.

Resistivity, NMR, optical, and other techniques not frequently encountered.
III WATER POTENTIAL
11 Definitions.

Concepts of soil water potential explained, together with its significance to water movement and plant growth. Units of measurement.
12 Tensiometers.

Principles and practice of making and using tensiometers – Bourdon gauge, manometer, pressure transducer and puncture types described.
13 Indirect methods.

This will cover resistance blocks, heat dissipation blocks, equitensiometers and filter papers.
14 Beyond –1 bar.

The principles and practice of using soil psychrometers, osmotic potential and low–potential pressure transducer devices.
IV WATER CONTENT – POTENTIAL RELATIONS
15 Air entry, hysteresis & other pitfalls.

Relationships between water content and water potential explained.
16 Pressure plates & membranes.

Laboratory methods for deriving the relationships.
17 In–situ methods.

Use of field observations to derive water release characteristics, together with some of the limitations.
18 Predictive and other methods.

Use of grain–size distributions to predict water release characteristics. Van Genuchten and similar relationships described.
V HYDRAULIC CONDUCTIVITY
19 Definitions & principles.

Darcy’s Law, units and some practical considerations.
20 Saturated hydraulic conductivity.

Methods to measure saturated conductivity in laboratory and the field.
21 Unsaturated hydraulic conductivity – Small scale methods.

Guelph permeameter, tension infiltrometer and other methods. Crust and other column methods.
22 Unsaturated hydraulic conductivity – Large scale methods.

Steady state infiltration, instantaneous profile, unit gradient and back calculation from water balance measurements described.
23 Inversion and predictive methods.

Use of grain size distribution and water release curves (e.g. Mualem’s relationship). Inversion techniques from water content and/or water potential observations with time.
24 Dual porosity media.

Problems and peculiarities when dealing with aggregated and structured soils, macropores and fractured porous rocks.
VI SOLUTES
25 Principles and pitfalls.

Definitions and discussion of adsorption and representivity.
26 Sampling methods.

Obtaining samples of soil solution using dilution techniques, suction and other samplers.
27 Electrical conductivity measurement.

Surrogate methods via electrical conductivity measurement using Wenner arrays and modified dielectric methods.
VII MONITORING WATER AND SOLUTE MOVEMENT
28 Zero Flux Plane Method.

Description of the theory and practice of calculating water balances with this elegant technique.
29 Darcy’s Law Solution.

Practice and problems in using hydraulic conductivity measurements to derive water flux.
30 Adsorption and anion exclusion.

The limitations of estimating solute flux from a product of concentration and water flux.
31 Modelling Methods.

Use of numerical models to calculate soil water dynamics. There are complete books on this subject and this will give only a flavour of the topic.