Full text: Mapping without the sun

112 
SOIL MOISTURE RETRIEVAL COMBINING OPTICAL AND RADAR DATA 
DURING SMEX02 
Chen Quan a-b ' *, Li Zhen a ,Tian Bangsen d,b 
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' State Key Laboratory of Remote Sensing Science, Jointly Sponsored by the Institute of Remote 
Sensing Applications of Chinese Academy of Sciences and Beijing Normal University, 
b Graduate School of Chinese Academy of Sciences 
Commission VII, WG VII/6 & VII/7 & II/l 
KEY WORDS: Soil Moisture, Optical. Radar, PALS, SMEX02, NDVI, NDWI 
ABSTRACT: 
Soil moisture is a key parameter in numerous environmental studies, including hydrology, meteorology, and agriculture. Microwave 
remote sensing is a promising way for the estimation of surface soil moisture, which has been known by scientists for decades of 
years. Unfortunately, soil moisture retrieval from dense vegetation area is still a troublesome nowadays. In this paper, the PALS 
(Passive and Active L and S band System) radar data acquired during SMEX02 (The Soil Moisture Experiments in 2002) are used 
for the estimation of soil moisture. Firstly, VWC (Vegetation Water Content, kg/m 2 ) estimation is compared by using NDVI (The 
Normalized Difference Vegetation Index) and NDWI (The Normalised Difference Water Index), the result showed that VWC from 
NDWI has high accuracy than from NDVI. Then, canopy transmittivity was retrieved using optical thickness, which is obtained by 
VWC and a constant factor ‘b’. Then, under some assumptions, backscatter from underlayer soil is obtained after volume and 
interaction scatter is removed from totally backscatter. Finally, soil moisture change between two acquisitions is retrieved using a 
simple bare-soil scatter model, which is simplified from AIEM (Advanced Integral Equation Model) model. The retrievals are 
compared with in-situ GSM (Gravimetric Soil Moisture) in SMEX02, the results showed that the algorithm can be used to estimate 
soil moisture at vegetation area, but the retrieval is relatively poor when the vegetation is dense, such as in com field. Therefore, 
some improvements need to be done in the future at high vegetation area. 
1. INTRODUCTION 
1.1 Importance of Soil Moisture 
Soil moisture is an important parameter in atmospheric, 
hydrological, and dynamic global vegetation and carbon 
dynamics models. It plays an important role in the interactions 
between the land surface and the atmosphere, as well as the 
partitioning of precipitation into runoff and ground water 
storage 11 1 Although objectives are different for each class of 
model, soil moisture plays an important role in each discipline 121 . 
In atmospheric models, the principle interest in soil water is its 
impact on evaporation and sensible/latent heat partitioning at 
scales that are appropriate for simulating the forcing of 
atmospheric processes. In hydrological applications the focus is 
generally on water balance components—infiltration, surface 
runoff, evaporation, deep percolation, and changes in water 
content of the vadose zone. In dynamic global vegetation 
models the principal interest is in quantifying the distribution of 
vegetation with implications for the terrestrial component of the 
current global C0 2 budget. An analysis of end-user 
requirements found that a volumetric water content accuracy of 
0.04 m 3 m' 3 or 4% in absolute terms was realistic for 
atmospheric modeling applications, though this may prove 
inadequate for modeling soil profiles down to the root zone 121 . 
Therefore, soil moisture provides a link between the earth’s 
surface and atmosphere through its effect on surface energy and 
moisture fluxes. The spatial and temporal dynamics of soil 
* Corresponding author: cq801915@hotmail.com 
moisture are important parameters for various processes in the 
soil-vegetation-atmosphere-interface. 1 ' 1 
1.2 Radar Soil Moisture Estimation 
Radar is one of the newer instruments for studying Earth’s 
environment. Although radar imaging was developed initially 
for military applications, at present it has become a very 
important tool in remote sensing. The reasons for this have been 
their all-weather ability and their dependence on the 
information of specific physical properties of the target. 
The measurement of soil moisture using microwave sensor 
(Radar and radiometer) is based on the large contrast between 
the dielectric properties of dry soil («2.5) and water («80). 
As the soil is moistened, its dielectric constant varies from 
approximately 2.5 when dry to about 25-30 under saturated 
conditions. This translates to an increase in power reflected by 
almost 8dB for wet soil compared to dry soil. The range of 
wavelength of the microwave bands is from 1mm (short) to lm 
(long). Long wavelength microwave bands offer the greatest 
potential for remote sensing of moisture due to their capability 
to penetrate vegetation cover. 
Currently, several theoretical and empirical models have been 
set up to relate soil moisture to radar backscatter of bare soil. 
Theoretical models are usually quite complex and difficult to be 
used in inversion process. Empirical models are usually 
statistical relationships between in-situ samples and truck-borne 
scatterometer observation, and can be used in estimating soil 
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