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Christiaan Dirksen:

Soil Physics Measurements

1999. 154 pages, 58 figures, 7 tables, Catena ISBN 978-3-923381-43-2, US-ISBN 978-1-59326-257-0, 17x24cm, 410 g
Language: English

(GeoEcology paperback)

ISBN 978-3-510-65386-7, paperback, price: 35.00 €

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Keywords

soilwatertensiometryhydraulicmeasurement

Contents

Synopsis top ↑

Soil Physics deals with the analysis and quantification ofthe physical properties and processes in the upper layer of the earth's crust, with major emphasis and activity on transport and accumulation of water and solutes in the (water) unsaturated zone. There is a relative abundance of textbooks dealing with the theory and application of Soil Physics, but there are very few books that give detailed descriptions of soil physics measurements and step by step instructions for exercises that are suitable for teaching. This book is intended to help fill this gap for measurements related to water transport in unsaturated soil.
It is impossible to cover all the soil water measurements that are presently in use. This volume gives a representative cross section of the available types of methods. As such, it reflects the present status of the practical "Soil Physics Measurements" (SPM) that was initiated at Wageningen University about fifteen years ago. This advanced practical is required for undergraduates in the Soil, Water and Atmosphere study program, but is also taken regularly by undergraduates in related disciplines and graduate students from abroad. Through the years, new measuring methods and techniques have been incorporated. A prime example is soil water content measurements. Whereas in the past gamma ray attenuation and neutron thermalisation have been used, presently water contents are measured, beside the standard gravimetrical method, mainly by time domain reflectometry. The contents of the SPM practical is covered in chapters on soil water content, tensiometry, steady hydraulic conductivity measurements, instantaneous profile method, and sorptivity and diffusivity measurements. The final chapter presents a frame work for evaluating direct and indirect methods for determining soil hydraulic conductivity functions.
More elementary experimental operations such as retrieving "undisturbed" core samples, gravimetric determination of soil water content, volume fractions, bulk density, and soil water retention characteristic, as well as measurements of hydraulic conductivity, infiltration, capillary rise, and sorptivity under saturated conditions are taught in an introductory practical. To make this book complete in itself, a review of the basic concepts of Soil Physics and detailed descriptions of these elementary experimental operations are presented in the second chapter. An introductory chapter briefly describes the hydrology and hydraulic properties of the unsaturated zone and summarizes the contents of each of the chapters. It also offers organizational details ofthe SPM practical for teachers who may want to set up a similar practical.
The general pattern of the chapters consists of theory, review of available methods, selection of one or more methods, practical aspects, evaluation, and step by step instructions for exercises. They are written from the experimentalist's point of view. Only the easily understood head equivalents of soil water potentials are used and the mathematics is kept to a minimum. Students and professionals in soil science, hydrology, and other earth sciences with little knowledge of calculus should be able to understand the subject matter and carry out the exercises. Students participating in the practical are expected to have mastered the subject matter treated in the book "Elements of Soil Physics" (Koorevaar et al., 1983), but only parts of the third and fifth chapter of this book are really needed.

Table of Contents top ↑

1 Introduction 1
1.1 Unsaturated zone 1
1.1.1 Hydrology 1
1.1.2 Soil hydraulic properties 2
1.2 Chapter contents 3
1.3 Soil physics measurements practical 5
1.3.1 Time schedule 5
1.3.2 Report 9
2 Basic concepts of soil physics 10
2.1 Composition of unsaturated soil 10
2.2 Soil water potentials 11
2.3 Soil water retention characteristic 12
2.4 Soil water transport 14
2.4.1 Hydraulic conductivity 14
2.4.2 Continuity equation 14
2.4.3 Hydraulic diffusivity 15
2.4.4 Infiltration, capillary rise and absorption 15
2.4.5 Sorptivity 16
Exercises 18
A. Core soil samples 18
Positioning 18
Retrieval 19
Trimming 20
B. Soil water retention characteristic 21
Hanging water column 21
Pressure membrane apparatus 22
3 Soil water content 24
3.1 Time domain reflectrometry 25
3.1.1 Permittivity 25
3.1.1.1 Frequency dependence 26
3.1.2 Water content 26
3.1.2.1 Operating principle 26
3.1.2.2 Wave form analysis 28
3.1.2.3 Calibration 29
3.1.2.4 Mixture models 30
3.1.2.5 Sensor length 32
3.1.2.6 Relative contributions 33
3.1.3 Bulk electrical conductivity 34
3.1.3.1 Determination of sensor constant 36
3.1.3.2 Temperature correction 3 7
3.1.3.3 Electrical conductivity of soil solution 37
3.1.4 TDR instruments 38
3.2 Frequency domain measurements 39
3.2.1 20-Mhz FD sensor 39
3.2.2 Water content 40
3.2.3 Bulk electrical conductivity 41
3.3 Neutron probe 42
3.3.1 Neutron thermalisation 42
3.3.2 Operating principle 43
3.3.3 Standard count rates 44
3.3.4 Calibration 45
3.3.5 Access tubes 46
3.3.6 Radiation hazards 47
Exercises 49
A. Tektronix 1502B TDR cable tester 49
Basic manual operations 49
Determination of diero 49
Measurements in water and air 50
Determination of rod length and correction of dzero 51
Soil water content calibration 51
Semi-automatic measurements 52
Bulk electrical conductivity 55
B Frequency domain measurements 56
4 Tensiometry 57
4.1 Hydrostatics of pressure measurements 57
4.1.1 Soil water potentials 57
4.1.2 Piezometers versus tensiometers 57
4.1.3 Water manometer 58
4.1.4 Mercury manometer 59
4.1.5 Pressure sensors 60
4.1.6 Septum tensiometer 61
4.1.7 Survey tensiometers 63
4.2 Operational aspects 63
4.2.1 Slope and zero-offset 64
4.2.2 Sensitivity and precision 64
4.2.3 Cup conductance 66
4.2.4 Response time 66
4.2.5 Tensiometer range 67
4.2.6 Osmotic tensiometers 67
4.2.7 Temperature effects 68
4.2.8 Spatial resolution 70
4.2.9 Automation 70
4.2.10 Self-made tensiometers 71
4.2.11 Commercial tensiometers 71
4.2.12 Air sources 72
4.2.13 Flushing 73
5 Steady hydraulic conductivity measurements 76
5.1 Accuracy enhancing features 76
5.1.1 Darcy's law 76
5.1.2 Steady state 76
5.1.3 Identifying parameter 77
5.1.4 Gravitational flow 77
5.1.5 Effective hydraulic head gradient 78
5.2 Saturated hydraulic conductivity 78
5.2.1 Experimental setup 78
5.2.2 Effect of macropores 79
5.3 Hydraulic conductivity at small tensions 80
5.3.1 Experimental setup 80
5.4 Small flux densities 82
5.4.1 Water application devices 82
5.4.1.1 Sprinkling infiltrometer 82
5.4.1.2 Cement crust 83
5.4.1.3 Needle reservoir with pulse pump 83
5.4.2 Automated water spray system 84
5.4.2.1 Spray Compartment 84
5.4.2.2 Regulating apparatus 85
5.4.2.3 Water flux densities 88
5.4.2.4 Experimental setup 89
5.4.2.5 Experimental results 90
5.4.2.6 Discussion 91
5.4.2.7 In situ measurements 92 Exercises 93
A. Hydraulic conductivity at small tensions 93
B. Automated water spray system 94
6 Instantaneous Profile method 96
6.1 Theory 97
6.2 Draining soil column 99
6.3 Calculation procedure 100
6.4 Results 104
6.5 General flow system 104
6.6 Zero-flux-plane calculation procedure 105
6.7 Discussion 105
6.7.1 One-dimensionality 105
6.7.2 Tensiometer placement 106
6.7.3 Measuring frequency 106
6.7.4 Water content measurements 106
6.7.5 Accuracy 107
6.7.6 Range of application 107
6.8 Simulations 108
6.8.1 Soil hydraulic properties 108
6.8.2 Simulation results 108
Exercise 111
Instantaneous Profile method 111
Experimental setup and operations 111
Water contents 111
Hydraulic heads 112
Calculations 113
7 Sorptivity and diffusivity measurements 114
7.1 Available methods 114
7.1.1 Sorptivity measurements 114
7.1.2 Diffusivity - Boltzmann transform methods 116
7.1.3 Combination methods 117
7.2 Flux-controlled sorptivity method 117
7.2.1 Potential-control 119
7.2.2 Flux-control 119
7.2.3 Syringe pump 120
7.2.4 Stepping motor 121
7.2.5 Soil column apparatus 122
7.2.6 Evaluation 125
7.3 Tension disc infiltrometer 125
7.3.1 Theory 125
7.3.2 Experimental apparatus 127
7.3.3 Discussion 128 Exercises 129
A. Flux-controlled sorptivity method 129
General remarks 129
Experimental operations 129
Calculations 131
Example output of computer program 132
B. Tension disc infiltrometer 134
8 Soil hydraulic conductivity determinations 135
8.1 Types of methods 135
8.2 Selection criteria 136
8.2.1 Accuracy 137
8.2.2 Range of application 138
8.2.3 Evaluation of sorptivity method 139
8.3 Laboratory versus field methods 140
8.3.1 Working conditions 140
8.3.2 Sampling techniques 140
8.3.3 Sample representativeness 143
8.4 Standard methods 143
8.5 Derivation from water retention characteristic 144
8.5.1 Van Genuchten - Mualem equations 145
8.6 Parameter optimization 147
References 150