APPLICATION OF A GIS FOR SPATIAL AND TEMPORAL MONITORING 
OF REMOTELY SENSED SOIL MOISTURE 
N. M. Mattikalli, and E. T. Engman 
NASA/ Goddard Space Flight Center, Laboratory for Hydrospheric Processes 
Hydrological Sciences Branch, Code 974, Greenbelt, MD 20771 
T. J. Jackson 
USDA Agricultural Research Service, Hydrology Laboratory 
Beltsville, MD 20705 
ABSTRACT 
A raster-based Geographical Information System (GIS) has been employed to monitor spatial 
and temporal variability of surface soil moisture derived from microwave remote sensing in the Little 
Washita watershed, Oklahoma, for the period between June 10-18, 1992. Daily microwave measurements 
have been obtained across the watershed from airborne ESTAR instrument. Surface soil moisture values 
were derived from brightness temperatures and validated using field measurements. This data set has 
been georeferenced in the GRASS GIS to quantify soil moisture changes during the dry-down period. 
Analysis of soil moisture changes and digital soils data reveals a direct relationship between changes in 
soil moisture and soil texture. Areas identified by loam/ silt loam soils are characterized by higher 
changes of total soil moisture and those of sand/ fine sandy loam soils by remarkably lower amounts of 
change. This reveals an insight into estimation of soil hydraulic properties using temporal soil moisture 
data. Coarse grained sandy soils probably drained quickly while clayey soils, characterized by low 
hydraulic conductivity, retained water and later drained during the experiment. This research 
demonstrates that a GIS is a valuable tool to establish relationship between temporal changes in remotely 
sensed surface soil moisture and soil properties. 
1. INTRODUCTION 
Moisture content in the surface layers of 
the soil is of great importance to the hydrologic 
research for partitioning rainfall into runoff and 
infiltration components as well as separating 
incoming radiation into latent and sensible heat 
(Nielsen et al., 1994). However, few watershed 
scale studies may be found that employ soil 
moisture information. A need has been realized 
to develop a methodology to quantify the spatial 
variability of soil moisture in a watershed, and to 
establish criteria for delineating areas that can be 
treated as hydrologically uniform (Engman and 
Gurney, 1991). There is also a necessity for 
acquiring watershed scale soil moisture maps on 
a frequent basis to isolate regions with strong 
moisture gradients in both time and space 
(Jackson, 1988). Spatial and temporal soil 
moisture information can be used to monitor the 
effectiveness of precipitation as an input to a 
region and to provide information for 
hydrologic models. 
Microwave remote sensing holds a great 
potential for providing areal estimates of soil 
moisture because the primary physical property 
that affects the measurement is directly 
dependent on the amount of water present in the 
44 
soil (Schmugge et al., 1986). The microwave 
region of the electromagnetic spectrum offers 
the potential for truly quantitative measurements 
because of their capability to penetrate clouds, 
and to some extent, vegetation canopy (Engman 
and Gurney, 1991). The passive microwave 
remote sensing employs measurements of the 
thermal emission from the soil at the longer 
microwave wavelengths (A > 10 cm) to 
determine the moisture content in the surface 
layer of the soil. It relies on the fact that the 
emissivity (€) at these wavelengths is a function 
of moisture content as a result of the large 
dielectric contrast between water and dry soil. 
For water the real part of dielectric constant is 
about 80 while for dry soil it is less than 5, and 
therefore for soils it ranges from about 3.5 to 
20. This produces a change in € from 0.95 to 
less than 0.6 when wet. This decrease in € is 
approximately linear with soil moisture and is 
affected by factors such as soil texture, surface 
roughness, and vegetation cover (Jackson, 1988; 
Jackson and Schmugge, 1991). 
The relationship between the microwave 
emission of natural surfaces and their inherent 
moisture content has been studied and well 
documented in the literature (Jackson, 1988; 
Schmugge et al, 1992 and 1993). Studies 
involving truck and aircraft measurements not 
only demonstrated this basic relationship but 
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