International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
4. RESULTS AND DISCUSSION
4.1 Fourier Analysis for Crop Rotations Detection
Power densities for the first four harmonics in Yellow-Huai-
Hai Rivers’ Region and Loess Plateau Region were compared
with each other (Fig.2). The comparison showed that there is
an obvious difference in proportions of the first four harmonics
between these two regions, which means the quite different
crop rotations patterns in these two regions. In Loess Plateau
Region the power in the first harmonic is much greater than the
power in successive harmonics, so unimodal pattern, in other
words, single growing season and period of peak greenness is
the main pattern for crops planted there and single crop
rotations is predominantly located in that area. In Yellow-Huai-
Hai Rivers’ Region the situations is more complicated because
the amplitude value of the third harmonic is higher, which
means the possibility of much more cropland using in
multimodal pattern resulting from muilti-cultivation practices.
J
Power Density
HT HH
Border! BlOrder2 OOrder3 OOrderd
Figure 2. The power density of the first four frequencies in
Loess Plateau Region (HT) and Yellow-Huai-Hai
Rivers’ Region (HH)
The result of the classification (Fig.3) shows that single and
double cropped non paddy rice is the main crop rotations types
in the study areas. About 39.1% of cropland is single cropped
and 60.9% is double cropped. Single crop rotations are
predominantly located in the Loess Plateau Region (70.5%),
while double crop rotations occurs mainly in Yellow-Huai-Hai
River’ Region (90.1%).
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Figure 3. The distribution of single and double crop rotations in
Loess Plateau Region and Yellow-Huai-Hai Rivers'
Region. 11. rice, 12. others, 21. rice/others, 22.
others/others. Here others means non-paddy rice
including wheat, maize, rapeseed, soybean, etc.
As shown in Fig. 3, in the Loess Plateau Region nearly 82.1%
of the cropland is singled cropped but most of the cropland in
Yellow-Huai-Hai River’ Region is planted in double cropped
ways, and only 17.3% of the cropland is planted in single
rotations. The obvious difference of crop rotations pattern
between Yellow-Huai-Hai Rivers’ Region and Loess Plateau
Region derived from classification is consist with the result of
the power densities comparison for the first four harmonics.
Crops planted in single crop rotations possess a single distinct
growing season, attaining peak greenness during midsummer.
This is manifested in the curve constructed from the first four
harmonics but mainly depended on the first harmonic (Fig. 4).
Since high amplitude values for a given harmonic indicate a
high level of variance in temporal NDVI, and the harmonic in
which that variance occurs indicates the periodicity of the
event ((Jakubauskas, 2002)), the amplitude value in the first
harmonic is much higher than the others, and as shown in
Table.1, the amplitude values in successive harmonics are in
descend order means a unimodal temporal NDVI pattern, in
other words, single cropped pattern. The phase angle for the
first harmonic (21.363, the beginning of August) (Table. 1) is
almost the same as the time for the peak greenness of crops
planted in single cropped ways and keeps an agreement with
the crop phonological calendar.
Compared with the single crop rotations, curves for double
crop rotations constructed from first four harmonics also
exhibit the characteristics of double crop rotations (Fig.5),
which is the two peak greenness occurred in temporal NDVI
curve of one year, and differ from the curve for single crop
rotations in that they depend on not only the first harmonic but
also the second and third harmonics due to cultivating twice in
a year. The high amplitude value in the third harmonic
indicates the high variability of NDVI within four months. The
phase of the third harmonic keeps an agreement with the time
of peak greenness for the crops planted twice a year. In our
study areas, winter wheat and summer maize is the main crop
rotation. Winter wheat is planted in the fall, sprouts, and goes
dormant over winter. In the spring, the wheat greens up and is
harvested by May, followed by planting summer maize or
cotton, etc. The springtime peak of the curve constructed with
first four harmonics emphasizes the strength of winter wheat
growing and corresponding time for the summertime peak is
consistent with the phonology of summer maize.
Nd. Order Amplitude Phase % Var Cum
Single 0 153.460
1 6.267 21.363 0.69 0.69
2 1.978 7.184 0.10 0.79
3 1.457 8.091 0.06 0.85
4 0.894 4.459 0.02 0.87
Double 0 162.762
1 5.658 17.645 0.34 034
2 2.954 7.067 0.09 0.43
3 5.536 10.416 0.32 0.75
4 2.923 3.398 0.09 0.84
Table |. Amplitudes, phases and variances of the first four
harmonics for single crop rotations and double crop
rotations in Loess Plateau Region and Yellow-Huai-
Hai Rivers’ Region.
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