Passive Microwave Remote Sensing of Soil Moisture and Regional Drought Monitoring - Monitoring and Predicting Agricultural Drought: A Global Study - page 89

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P assive Microwave Remote Sensing
o f Soil Moisture and Regional
D rought Monitoring
THOMAS J. JACKSON

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Mitigating the effects of drought can be improved through better informa-
tion on the current status, the prediction of occurrence, and the extent of
drought. Soil moisture can now be measured using a new generation of
microwave remote sensing satellites. These measurements can be used to
monitor drought conditions on a daily basis over the entire earth. The qual-
ity of these products will continue to improve over time as new sensors are
launched. These satellite products, combined with existing in situ observa-
tions and models, should be exploited in drought monitoring, assessment,
and prediction.
Measuring soil moisture on a routine basis has the potential to signifi-
cantly improve our understanding of climatic processes and strengthen our
ability to model and forecast these processes. Leese et al. (2001) concluded
that the optimal approach to monitoring soil moisture would be a combina-
tion of model-derived estimates using in situ and remotely sensed measure-
ments. In this regard, each method produces soil moisture values that are
both unique and complementary. This concept is essentially the process of
data assimilation described by Houser et al. (1998). In situ measurements
of soil moisture have been made in a few countries over the past 70 years
(Robock et al., 2000). However, due to cost and sensor limitations, there
are few soil moisture sensor systems available today, especially for auto-
mated measurements. A lack of routine observations of soil moisture has
led to the use of surrogate measurements (i.e., antecedent precipitation in-
dex) and modeled estimates, which limits the possibility of physically based
model validation and acceptance.
Current tools to predict drought, such as drought indices and Global
Climate Models (GCMs), do not include any direct observations of the
soil condition, which is critical for agriculture. Passive microwave remote
sensing instruments respond to the amount of moisture in the soil. Several

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