Agriculture Reference
In-Depth Information
CHAPTER FIVE
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
34
35
36
37
38
39
40
41
42
43
44
45
46
M
onitoring Drought Using
C
oarse-Resolution Polar-Orbiting
S
atellite Data
ASSAF ANYAMBA, COMPTON J. TUCKER,
[57],
Line
——
0.0
——
Norm
* PgEn
There are two distinct categories of remotely sensed data: satellite data
and aerial data or photographs. Unlike aerial photographs, satellite data
have been routinely available for most of the earth's land areas for more
than two decades and therefore are preferred for reliably monitoring global
vegetation conditions.
Satellite data are the result of reflectance, emission, and/or back scat-
tering of electromagnetic energy (figure 5.1) from earth objects (e.g., veg-
etation, soil, and water). The electromagnetic spectrum is very broad, and
only a limited range of wavelengths is suitable for earth resource monitor-
ing and applications. The gaseous composition (O
2
,O
3
,CO
2
,H
2
O, etc.)
of the atmosphere, along with particulates and aerosols, cause significant
absorption and scattering of electromagnetic energy over some regions of
the spectrum. This restricts remote sensing of the earth's surface to certain
“atmospheric windows,” or regions in which electromagnetic energy can
pass through the atmosphere with minimal interference. Some such win-
dows include visible, infrared, shortwave, thermal, and microwave ranges
of the spectrum.
The shortwave-infrared (SWIR) wavelengths are sensitive to moisture
content of vegetation, whereas the thermal-infrared region is useful for
monitoring and detecting plant canopy stress and for modeling latent and
sensible heat fluxes. Thermal remote sensing imagery is acquired both
during the day and night, and it measures the emitted energy from the
surface, which is related to surface temperatures and the emissivity of
surface materials.
This chapter focuses on the contribution of visible and infrared wave-
lengths to global drought monitoring, and chapter 6 discusses visible, in-
frared, and thermal wave contributions. Under microwave windows, the
satellite data can be divided into two categories: active microwave and
[57],
57
Search WWH ::
Custom Search