Field Estimation of Soil Water Content A Practical Guide to Methods, Instrumentation and Sensor Technology VIENNA, 2008 TRAINING COURSE SERIES30 TRAINING COURSE SERIES No. 30 Field Estimation of Soil Water Content A Practical Guide to Methods, Instrumentation and Sensor Technology INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 2008 The originating Section of this publication in the IAEA was: Soil and Water Management & Crop Nutrition Section International Atomic Energy Agency Wagramer Strasse 5 P.O. Box 100 A-1400 Vienna, Austria FIELD ESTIMATION OF SOIL WATER CONTENT: A PRACTICAL GUIDE TO METHODS, INSTRUMENTATION AND SENSOR TECHNOLOGY IAEA, VIENNA, 2005 IAEA-TCS-30 ISSN 1018–5518 © IAEA, 2008 Printed by the IAEA in Austria February 2008 FOREWORD During a period of five years, an international group of soil water instrumentation experts were contracted by the International Atomic Energy Agency to carry out a range of comparative assessments of soil water sensing methods under laboratory and field conditions. The detailed results of those studies are published elsewhere. Most of the devices examined worked well some of the time, but most also performed poorly in some circumstances. The group was also aware that the choice of a water measurement technology is often made for economic, convenience and other reasons, and that there was a need to be able to obtain the best results from any device used. The choice of a technology is sometimes not made by the ultimate user, or even if it is, the main constraint may be financial rather than technical. Thus, this guide is presented in a way that allows the user to obtain the best performance from any instrument, while also providing guidance as to which instruments perform best under given circumstances. That said, this expert group of the IAEA reached several important conclusions: (1) the field calibrated neutron moisture meter (NMM) remains the most accurate and precise method for soil profile water content determination in the field, and is the only indirect method capable of providing accurate soil water balance data for studies of crop water use, water use efficiency, irrigation efficiency and irrigation water use efficiency, with a minimum number of access tubes; (2) those electromagnetic sensors known as capacitance sensors exhibit much more variability in the field than either the NMM or direct soil water measurements, and they are not recommended for soil water balance studies for this reason (impractically large numbers of access tubes and sensors are required) and because they are rendered inaccurate by changes in soil bulk electrical conductivity (including temperature effects) that often occur in irrigated soils, particularly those containing appreciable amounts of clays with high ion exchange capacities, even when using soil specific calibrations; (3) all sensors must be field calibrated (factory calibrations were inaccurate in most soils studied) in order to obtain reasonable accuracy; (4) the one exception to conclusion (3) is conventional time domain reflectometry (TDR, with waveform capture and graphical analysis), which is accurate to ±0.02 m3 m–3 in most soils when using a calibration in travel time, effective frequency and bulk electrical conductivity (see Chapter 4); (5) with the possible exception of tensiometers and the granular matrix resistance sensors, none of the sensors studied is practical for on-farm irrigation scheduling; they are either too inaccurate (capacitance sensors) or too costly and difficult to use (TDR and NMM); (6) for research studies, only the NMM, conventional TDR and direct measurements have acceptable accuracy. In light of the intense commercial introduction of electromagnetic (EM) soil water sensors in the 1990s and to date, these conclusions are somewhat disappointing. However, the joint work of the expert group has resulted in numerous scientific publications detailing the problems with EM sensors, including the theoretical underpinnings of these problems, and sparked a special issue of the Vadose Zone Journal (Evett and Parkin, 2005) summarizing much of the fundamental work to date. Now that the problems are well understood, research and development of new sensor systems to overcome these problems can, and will, proceed to a satisfactory conclusion for both scientific studies and on-farm irrigation management. The IAEA officer responsible for this publication is Lee Kheng Heng of the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture. EDITORIAL NOTE The use of particular designations of countries or territories does not imply any judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and institutions or of the delimitation of their boundaries. The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA. CONTENTS CHAPTER 1. DIRECT AND SURROGATE MEASURES OF SOIL WATER CONTENT ................................................................................ 1 1.1. Purpose of this manual....................................................................................................1 1.2. Soil water measurement — Background ........................................................................ 1 1.3. The basics: How is soil water content described? .......................................................... 2 1.3.1. Calculation of water content of a volume of soil (e.g. the root zone) .................. 3 1.3.2. How much water can a soil hold?......................................................................... 3 1.4. Factors affecting direct measurement accuracy, precision and variability..................... 5 1.5. Surrogate measures of soil water content ....................................................................... 7 1.6. Factors affecting accuracy and variability of water contents derived from surrogate measures ....................................................................................................... 10 1.7. Accuracy, precision and the calibration process........................................................... 14 1.7.1. The manufacturer’s calibration........................................................................... 14 1.7.2. The calibration process....................................................................................... 15 1.7.3. Checking a calibration........................................................................................ 18 1.8. Summary....................................................................................................................... 20 References to Chapter 1........................................................................................................ 21 CHAPTER 2. GRAVIMETRIC AND VOLUMETRIC DIRECT MEASUREMENTS OF SOIL WATER CONTENT .............................................................................. 23 2.1. Equipment description.................................................................................................. 23 2.1.1. Manufacturers, instruments and parts references ............................................... 23 2.1.2. Measurement general principle .......................................................................... 24 2.1.3. Accessories and documents provided by the manufacturer ............................... 27 2.1.4. Software.............................................................................................................. 27 2.2. Taking measurements ................................................................................................... 27 2.2.1. Required equipment and procedures .................................................................. 27 2.2.2. Handling of data ................................................................................................. 34 2.2.3. “Hints and tricks” ............................................................................................... 35 References to Chapter 2........................................................................................................ 37 CHAPTER 3. NEUTRON MOISTURE METERS ................................................................. 39 3.1. Equipment description.................................................................................................. 39 3.1.1. Manufacturers, instruments and parts references ............................................... 40 3.1.2. Measurement general principle .......................................................................... 40 3.1.3. Safety.................................................................................................................. 43 3.1.4. Accessories and documents provided by the manufacturer ............................... 43 3.1.5. Software.............................................................................................................. 43 3.2. Field installation............................................................................................................ 44 3.2.1. Required equipment............................................................................................ 44 3.2.2. General procedure .............................................................................................. 45 3.2.3. “Hints and tricks” ..............................................................................................