Environmental Engineering Reference
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Fig. 20.3
Relationship between canopy water content (
CWC
) and the Normalized Difference
Infrared Index (
NDII
) from different data sets (Hunt et al.
2011b
). The Soil Moisture Experiment
2004 (
SMEX04
) was conducted in Arizona, USA, and Sonora, Mexico (Yilmaz et al.
2008a
), and
the Soil Moisture Experiment 2005 (
SMEX05
) was conducted in central Iowa (Yilmaz et al.
2008b
).
Grassland data sets were kindly provided by (Ceccato et al.
2002b
) and (Davidson et al.
2006
)
WindSat is a multifrequency polarimetric passive microwave spaceborne radi-
ometer built by the US Naval Research Laboratory for measuring wind speed and
direction over the oceans. Li et al. (
2010
) developed a biophysically based algo-
rithm for estimating VWC and soil moisture content for WindSat. Hunt et al.
(
2011b
) upscaled the NDII-VWC relationship using MODIS to the 50-km diameter
WindSat footprint for the 2003-2005 growing seasons. The estimated VWC from
MODIS and WindSat were linearly related, but the WindSat VWC was about twice
the MODIS VWC (Fig.
20.4b
). The lowest frequency on Windsat was 6.9 GHz
(C-band), which had strong radio frequency interference, so the lowest frequency
used for the WindSat retrievals was 10.7 GHz (Li et al.
2004
). Much lower
frequencies, such as 1.26-1.41 GHz (L-band), are considerably more sensitive to
soil and vegetation water content and thus are being used for the Soil Moisture
Active Passive (SMAP) mission (Entekhabi et al.
2010
). The merger of the optical
and microwave domains for remote sensing of liquid water in vegetation and soil
will enable new and better methodologies for the study of water stress and drought.
20.7 Detection of Water Stress
Plant water stress is a general term that encompasses different phenomena at
different temporal scales, from short-term leaf wilting to long-term drought
(Jones
2007
; Passioura
2007
). Low water content in the soil causes plants to close
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