Agriculture Reference
In-Depth Information
CHAPTER EIGHT
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
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A
ND WOLFGANG WAGNER
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Remote sensing can provide timely and economical monitoring of large
areas. It provides the ability to generate information on a variety of spa-
tial and temporal scales. Generally, remote sensing is divided into passive
and active depending on the sensor system. The majority of remote-sensing
studies concerned with drought monitoring have involved visible-infrared
sensor systems, which are passive and depend on the sun's illumination.
Radar (radio detection and ranging) is an active sensor system that trans-
mits energy in the microwave region of the electromagnetic spectrum and
measures the energy reflected back from the landscape target. The energy
reflected back is called backscatter. The attraction of radar over visible-
infrared remote sensing (chapters 5 and 6) is its independence from the sun,
enabling day/night operations, as well as its ability to penetrate cloud and
obtain data under most weather conditions. Thus, unlike visible-infrared
sensors, radar offers the opportunity to acquire uninterrupted information
relevant to drought such as soil moisture and vegetation stress.
Drought conditions manifest in multiple and complex ways. Accord-
ingly, a large number of drought indices have been defined to signal abnor-
mally dry conditions and their effects on crop growth, river flow, ground-
water, and so on (Tate and Gustard, 2000).
In the field of radar remote sensing, much work has been devoted to
developing algorithms to retrieve geophysical parameters such as soil mois-
ture, crop biomass, and vegetation water content. In principle, these param-
eters would be highly relevant for monitoring agricultural drought. How-
ever, despite the existence of a number of radar satellite systems, progress
in the use of radar in environmental monitoring, particularly in respect
to agriculture, has been slower than anticipated. This may be attributed
to the complex nature of radar interactions with agricultural targets and
the suboptimal configuration of the satellite sensors available in the 1990s
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