Microwave Dielectric Behavior of Wet Soils Remote Sensing and Digital Image Processing VOLUME 8 Series Editor: Freek D. van der Meer, Department of Earth Systems Analysis, International Institute for Geo-Information Science and Earth Observation (ITC), Enschede, The Netherlands & Department of Geotechnology, Faculty of Civil Engineering and Geosciences, Technical University Delft, The Netherlands Editorial Advisory Board: Michael Abrams, NASA Jet Propulsion Laboratory, Pasadena, CA, U.S.A. Paul Curran, Department of Geography, University of Southampton, U.K. Arnold Dekker, CSIRO, Land and Water Division, Canberra, Australia Steven M. de Jong, Department of Physical Geography, Faculty of Geosciences, Utrecht University, The Netherlands Michael Schaepman, Centre for Geo-Information, Wageningen UR, The Netherlands The titles published in this series are listed at the end of this volume. MICROWAVE DIELECTRIC BEHAVIOR OF WET SOILS by JITENDRA BEHARI School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, India Springer Anamaya A.C.I.P. catalogue record for the book is available from the Library of Congress ISBN 1-4020-3271-4 (HB) ISBN 1-4020-3288-9 (e-book) Copublished by Springer 233 Spring Street, New York 10013, USA with Anamaya Publishers, New Delhi, India Sold and distributed in North, Central and South America by Springer 233 Spring Street, New York, USA In all the countries, except India, sold and distributed by Springer P.O. Box 322, 3300 AH Dordrecht, The Netherlands In India, sold and distributed by Anamaya Publishers F-230, Lado Sarai, New Delhi-110 030, India All rights reserved. Copyright © 2005 Anamaya Publishers, New Delhi, India No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. springeronline.com Printed in India. PREFACE Soil moisture plays an important role in the runoff process, and its evaluation can be used as a marker of flood risk or drought occurrence. The data is also required to validate the soil moisture component in a variety of soil types of hydrologic models before a possible flood event, and potentially improves the accuracy of their prediction. Keeping the above in view, enormous efforts have gone into measurement of soil moisture by in situ and remote sensing techniques using microwaves. The subject still remains an important point of investigation for it depends upon a number of parameters, for example, texture, topography and shows space and temporal variability. The advantages of both passive and active remote sensing techniques are discussed in detail. The book begins with the basics of soil physics and the soil moisture. Soil moisture measurement techniques presented are confined to microwave frequencies. A summary of theoretical models and a mix-up of experimental and theoretical details is included to offer a comparison. A special chapter is added on an upcoming technique of synthetic aperture radar. The book finally concludes with a summary of recent trends and techniques with a possible direction for future work. The book is primarily aimed to benefit postgraduates and researchers in the area of soil, agriculture physics and microwave remote sensing. The aim is to initiate beginners in the subject. The author will consider his efforts rewarded if it succeeds in such an endeavor. J. BEHARI LIST OF SYMBOLS A Equivalent area of ‘synthesized beam’ syn B System bandwidth C Heat capacity E Evaporation from the water surface eβ Measured sensitivity of the two layer medium e Emissivity of ground surface surface F(d) Visibility function G Gain of receiving antenna r G Gain of transmission antenna t h Roughness parameter K Boltzman’s constant k d Optical thickness e lε Sample length m Percentage of volumetric field capacity f m Gravimetric moisture content g m Mass of dry soil s m Mass of wet soil st m Transition moisture t m Volumetric water content (Soil moisture) v m Relative saturation vr m Saturated value vs P Total porosity of the soil P Pore volume filled with air a P Penetration depth d P Power transmitted by the sensor t R(θ) Surface roughness rg Reflectivity of the ground β r2 Correlation coefficient R Polarized surface reflectivity p R Distance between radar and target t T Temperature of the vegetation v T Average temperature of the atmosphere atm T Brightness temperature B V Soil reflectivity soil viii LIST OF SYMBOLS W Day length W0 Observed soil moisture W Wilting coefficient p W Transition point t Y Characteristic admittance of the probe t Z Characteristic impedance of transmission line 0 ρ Bulk density of soil b ε′ Real part of complex permittivity (dielectric constant) ε″ Imaginary part of complex permittivity (dielectric constant) ε′′ Conductivity loss σ ε′′ Total dielectric loss t ε Dielectric constant Γ Transmissivity of the vegetation layer t β Empirical constant φ Metric potential m δ Skin depth D ρ Particle density r τ Relaxation time ω Angular frequency (2π f) α Spread in relaxation ε Permittivity of free space 0 λ Wavelength µ Permeability of the medium r Γ Reflection coefficient σ Ionic conductivity (mos/cm) σ0 Backscattering coefficient σ Zero frequency electrical conductivity dc θ Soil moisture content θ Depth integrated vegetation/water content veg ϕ Phase constant γsur Surface scattering αβ γg Bistatic scattering coefficient of the ground α CONTENTS Preface v List of Symbols vii 1. Physical Properties of Soil 1 Introduction 1 Drainage 2 Physical Properties of Mineral Soils 3 Soil Texture Classes 3 Inorganic Constituents in Soils 5 Particle Density of Mineral Soils 6 Structure of Mineral Soils 6 Soil Organic Composition 6 Bulk Density 7 Pore Space 7 Soil Temperature 8 Thermal Infrared Surface Temperatures 8 Soil Water Characteristics 9 Water Retention 10 Parameters Related to Soil Moisture Retention Under Field Conditions 10 Computations of Soil Parameters 12 Soil Thermal Resistivity 13 Dry (Single-phase) Soils 13 Moist (Single-phase) Soils 13 Soil Air Characteristics 14 Clay and Humus 14 Surface Roughness 14 Roughness Estimation 15 Penetration Depth 16 Equivalent Soil Moisture 18 Emissivity of a Soil Layer 19 Microwave Brightness Temperature 20 Microwave Emission Model 20 x CONTENTS 2. Dielectric Behavior of Soil 22 Soil Moisture 22 Agriculture Applications 23 Soil Moisture Parameters 24 Soil Moisture Measurements 26 Visible and Infrared (0.3-3 µm) Electromagnetic Spectrum 26 Thermal Infrared Method 26 Microwaves 27 Physical Basis of Soil Moisture Measurement Using Microwaves 28 Dielectric Behavior of Wet Soils 28 Bound Water Layer 29 Bulk Density Effects 33 Moisture, Texture and Frequency Dependence 33 Soil Textural Composition 33 Salinity Dependence 35 Temperature Dependence 37 Special Class of Soil 38 Vertisol 38 Density Effects 38 Dielectric Behavior of Swelling Clay Soils 38 3. Measurement of Soil Water Content 41 Measuring Methods 41 Microwave Response to Soil Moisture 41 Importance of Dielectric Constant Measurements: Theoretical background 42 Importance of Microwaves in Soil Moisture Content 42 Microwave Based Experimental Techniques 43 Free Space Transmission Technique 43 Cavity Perturbation Method 44 Transmission Technique in Waveguides/Coaxial Lines 45 The Two Point Method 45 Network Analyzer in Reflection Measurements: Theoretical background 46 Reflection Techniques in Coaxial Lines/Waveguides 48 Single-Horn Reflectometry Method: Theory and Description 49 Height Adjusting Setup Description 50 Limitations 51 The Coaxial Cable Method 52 Limitations 54 Precautions 54 Capacitance Probe (CP) 55 Microstrip Ring Resonator 55 Time Domain Reflectometry (TDR) 56 CONTENTS xi Ground Penetrating Radar (GPR) 59 Measurements 60 Lysimeter 62 Comparison of Various Techniques 63 Conclusions 65 4. Microwave Remote Sensing Techniques in Soil Moisture Estimation 66 Introduction 66 Radiant Energy and Flux 67 Basis of Microwave Remote Sensing 67 Passive Microwave Remote Sensing 69 Microwave Radiometers 70 Active Microwave Remote Sensing 74 Principle 74 Scatterometers 74 S- and L-Band Microwave Radiometer (SLMR) System 75 Theoretical Basis 77 Description of Scatterometer 79 Applications of Scatterometric Studies 79 Crop Covered Soil 81 System Parameters Affecting Microwave Signature 81 Frequency 81 Incident Angle 82 Polarization 82 Combination of System Parameters 83 Target Parameters Influencing Microwave Signatures 83 Effect of Roughness and Soil Texture 83 Effect of Vegetation 84 Canopy Attenuation 86 Vegetation Cover and Its Effect on Soil Moisture 87 Fresnel Model 87 Active and Passive Microwave Remote Sensing: Comparison and a Relation 88 5. Dielectric Constant of Soil 92 Experimental Results 92 Prediction of e Using Calibration Procedures 92 Volumetric Soil Moisture Using Backscattering Data 93 Data Obtained Using Network Analyzer Technique 93 General Observations 97 Frequency 98 Finger Printing of Soil Texture with Moisture 99 In Situ Methods 103 Bound Water Problem in Relation to Experimental Data 103