RING 
es 
monitor spatial 
ing in the Little 
ve measurements 
moisture values 
his data set has 
Iry-down period. 
ween changes in 
rized by higher 
ower amounts of 
ral soil moisture 
cterized by low 
. This research 
nges in remotely 
The microwave 
spectrum offers 
ve measurements 
penetrate clouds, 
canopy (Engman 
sive microwave 
urements of the 
il at the longer 
A> 10 cm) to 
t in the surface 
the fact that the 
sths is a function 
ult of the large 
ter and dry soil. 
sctric constant is 
; less than 5, and 
om about 3.5 to 
| € from 0.95 to 
decrease in € is 
moisture and is 
| texture, surface 
r (Jackson, 1988; 
n the microwave 
nd their inherent 
tudied and well 
(Jackson, 1988; 
1993). Studies 
1easurements not 
relationship but 
have also helped to quantify the effects of 
various surface parameters such as soil texture, 
roughness and vegetation that distort and 
confound the basic relationship (Ulaby et al., 
1982; Theis et al., 1984; Wang, 1985; Jackson 
and Schmugge, 1991). A few studies have made 
temporal observations to map spatial variation in 
soil moisture (Engman et al., 1989; Wang et al., 
1989). While the surface soil moisture data can 
be used in mapping spatial pattern of rainfall 
(Ulaby et al., 1983) and predicting evaporation 
(Bernard et al. 1981), it has also been 
demonstrated that soil moisture information are 
promising in simulating water balance of a 
watershed (Engman et al., 1989). 
Therefore, microwave Sensors are 
capable of providing reliable spatial data that are 
invaluable to hydrologic research. Present day 
airborne and future space borne sensors have 
capabilities of providing multi-temporal soil 
moisture data sets for large areas from watershed 
to continental scales. The volume of such data 
sets will be enormous, and require powerful tools 
for handling and analysis. Geographical 
Information Systems (GIS) offer an appropriate 
technology to achieve this goal since they are 
designed and developed to efficiently capture, 
store, retrieve, manipulate and analyze spatially 
referenced data sets (Burrough, 1990). In this 
paper, a raster-based GIS has been employed for 
monitoring of spatial and temporal variability of 
surface soil moisture. The soil moisture data are 
derived from passive microwave remote sensing 
for the Little Washita watershed, Oklahoma, for 
the period between June 10-18, 1992. 
2. WASHITA'92 EXPERIMENT 
The Washita'92 was a research 
experiment carried out in the Little Washita 
watershed, near Chickasha, Oklahoma, during 
June 10-18, 1992, with a joint cooperation 
between National Aeronautics and Space 
Administration (NASA), Unites States 
Department of Agriculture (USDA) and various 
universities. The Little Washita watershed is a 
650 sq. km. drainage basin situated in the 
southern part of the Great Plains in southwest 
Oklahoma. The study site is of prime 
importance to USDA Agricultural Research 
Service and USDA Global Climate Change 
initiative, and has long term (more than 24 
years) hydrologic monitoring facility. Average 
annual rainfall of the region is about 640 mm. 
Land cover in the Little Washita watershed is 
dominated by the native grassland and winter 
wheat (Jackson and Schiebe, 1993). As part of 
the Washita'92 aircraft campaign, multi-temporal 
airborne microwave data were collected using 
45 
the Electronically Steered Thinned Array 
Radiometer (ESTAR). The ESTAR was mounted 
onboard NASA C-130 aircraft operated by 
NASA Ames Research Center. In addition to the 
aircraft measurements, a large number of 
ground soil moisture measurements (more than 
700 gravimetric samples per day) were carried 
out (Jackson and Schiebe, 1993). These ground 
truth data have been used to support and validate 
microwave remotely sensed data. The study area 
experienced heavy rainfall (more than 30 mm) 
on June 5, 1992, and mild rainfall continued till 
June 9, 1992. However, there was no rainfall in 
the watershed for the entire duration of the 
experiment. Therefore, the hydrologic 
conditions in the watershed were ideal, and it was 
possible to follow a drying pattern from very wet 
(about 30%) to dry (about 10%) over a period 
of ten days (figure 1). This forms an exceptional 
and valuable data set to study spatial and 
temporal variation of soil moisture. 
   
  
   
   
—e— Corn 
A 4s I —HB— Bare soil 
® 04 —e— Alfa Alfa 
Ë 354 —14— Rangeland 
" —^— Winter Wheat 
É 304 
= 25+ 
d 20 
o z 
8 
E 15+ 
2 10+ 
  
  
5 T ' ï T YT T T —1 
9 10 11 12 13 14 1$ 16 17 18 19 
June, 1992 
Figure 1. Illustration of the range of moisture 
conditions during the Washita'92 experiment. 
The study area experienced a clear dry-down 
from very wet to dry over a period of ten days. 
3. THE ESTAR SENSOR 
The ESTAR is a synthetic aperture, 
passive microwave radiometer which operates at 
L band (21 cm wavelength, or 1.4 GHz 
frequency). This band has been proved to be the 
most effective for measuring soil moisture 
(Schmugge et al., 1986). It is well established on 
both experimental and theoretical work that the 
soil moisture sampling depth is of the order of a 
few tenths of the wavelength (Schmugge et al., 
1986). For the 21 cm wavelength ESTAR, this 
translates to a depth of about 2-5 cm in the soil. 
The ESTAR instrument has been described in Le 
Vine et al. (1990) and (1992). It is a hybrid real 
and synthetic aperture radiometer which 
employs a real aperture to obtain resolution