IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India, 2002 
AN ATTEMPT TO INCORPORATE THE EFFECT OF SOIL TEXTURE IN SOIL 
MOISTURE ESTIMATION USING MICROWAVE REMOTE SENSING 
Hari Shanker Srivastava 
Regional Remote Sensing Service Centre (ISRO), Dehradun —248 001 (India) 
E-mail: hari space? yahoo.com, Phone: +91-135-740628, Fax: +91-135-745439 
KEY WORDS: Soil texture, Field capacity, Wilting point, Bound water, Available water, Backscatter coefficient (0°) 
ABSTRACT: 
Texture is one of the most important physical properties of soil. It involves the size of individual particles and specially refers to the 
relative proportions of various-sized particles in a given soil. Hence the surface area of soil particles with in a soil type varies with 
the change in soil texture. Interaction of microwave signal in the soil medium is highly enhanced by the amount of free water in the 
soil medium. As the percentage of bound water and free water in a wet soil depends upon the surface area of soil particles in that soil 
medium, the backscatter coefficient (0°) from a wet soil changes with the soil texture. In this paper an attempt has been made to 
incorporate the effect of soil texture in soil moisture retrieval model using microwave remote sensing technique. 
1. INTRODUCTION 
The water content of soils is of fundamental importance to 
many hydrological, biological, meteorological and 
biogeochemical processes. Although the volume of water 
available within the shallow layer of the earth’s upper surface is 
small as compared to the total amount of water on global scale, 
it is this small amount of water that controls all the agricultural 
activities. Because of the importance of spatial and temporal 
information of soil moisture, soil moisture measurement 
techniques have been developed over the years. However, most 
of these techniques provide only point information. Since soil 
moisture is highly variable both spatially and temporally, point 
measurements cannot be extended over a large area. To bridge 
this gap, remote sensing methods are being exploited for the 
large area estimation of soil moisture. Among all the remote 
sensing techniques, microwave remote sensing is found to be 
the best tool for soil moisture estimation over a large area. 
Key factor behind the soil moisture estimation-using 
microwave is the large difference between dielectric constant of 
water (~80) and of dry soil (3 to 4) at microwave frequencies. 
Due to this reason radar backscattering coefficient (0?) is 
strongly related with soil moisture due to high dielectric 
constant of mixture of soil and water. Though radar 
backscattering coefficient is strongly related with soil moisture 
at the same time it is also sensitive for the other target 
properties like surface roughness, crop cover and soil texture 
(Henderson and Lewis, 1998; Ulaby et al., 1978,1979). Since 
for soil moisture estimation these parameters act as noise, it is 
necessary to reduce the effects of these parameters on to the 
soil moisture retrieval model. Attempts have already been made 
to incorporate the effects of surface roughness and crop cover 
in the soil moisture retrieval model (Srivastava, et. al., 2000, 
2002a, 2002b). In order to incorporate the effect of third noise 
parameter in soil moisture estimation, in this paper an attempt 
has been made to incorporate the effect of soil texture in soil 
moisture estimation using microwave remote sensing 
technique. 
2. EFFECT OF SOIL TEXTURE ON RADAR 
BACKSCATTER 
Texture involves the size of individual particles and specially 
refers to the relative proportions of various-sized particles in a 
given soil. Texture of soil is extremely important in 
determining how a soil can and should be used. Since the 
surface area of soil particles with in a soil type varies with the 
change in soil texture, it is convenient to classify a given soil by 
the weight-percent of the soil with in each specific size 
category (Sand, Silt and Clay). Since there is considerable 
difference in the diameters of these basic particles, the total 
surface area of soil particles with in a unit volume is largely 
determined by its texture. 
Wet soil is a heterogeneous mixture of soil, water and air 
pockets. In general water in wet soil can further be divided 
into bound water and free water. The bound water refers to 
water molecules located in the first few molecular layers 
surrounding the soil particles and therefore they are tightly held 
by the soil particles due to the effect of matric and osmotic 
forces. These forces significantly reduce the free energy of the 
adsorbed water molecules and therefore these water molecules 
are termed as bound water. In contrast to this, water molecules 
that are located several molecular layers away from soil 
particles are able to move with in the soil medium with relative 
ease and therefore termed as free water. 
The division of soil water in to bound water and free water is 
only an approximate description of water molecules in the soil 
medium although it is the percentage of free water and bound 
water present in a soil medium that largely determines the 
dielectric constant of a soil medium. This is due to the fact that 
when a bound water molecule interacts with an incident 
electromagnetic wave, it behaves as it does in the case of ice. 
Therefore, it exhibits a dielectric dispersion spectrum that is 
very different from that of free water. Ulaby, et al., (19862) 
studied the effect of soil texture on dielectric behavior of 
different soils. They observed that in case of change in soil 
texture, values of €’ soi (real part of complex dielectric constant) 
and €", (imagery part of complex dielectric constant) are 
different for all soil types (soil texture) even at the same value 
of soil moisture (except at zero moisture). 
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