The Manix Basin Area of the Mojave Desert has been used extensively  for the  cultivation of alfalfa with 

center-pivot sprinkler irrigation systems. Since 1972, 

a series of these fields has been abandoned. Data were 

collected using the Airborne Synthetic Aperture Radar 

and Airborne Visible/Infrared  Imaging Spectrometer instruments   in   the   summer of 1990. Polarimetric 

analysis of the AIRSAR data reveal changes  in  the morphology of the surfaces of the  abandoned  fields from  cultivation  patterns to patterns  resulting  from wind  erosion. Calculation of a  normalized difference vegetation  index (NDVI) based on the AVIRIS data suggests  that 

the abandoned   fields support more vegetation than the 

undisturbed  areas for the first few years of abandonment, 

but that the vegetation density on fields which have 

been abandoned for six or more years  is  lower  than   the  undisturbed   desert. Field observations confirm the remote sensing results

Farr, Tom G.

Ray, Terrill W.

van Zyl, Jakob J.

Caltech Authors - Main

MIT.2 Study of Land Degradation with Polarimetric SAR 
and Visi ble/Near-Infrared Imaging Spectroscopy 
Temll W. Ray Tom G. Farr and Jakob J. van Zyl 
Jet Propulsion Laboratory 
California Institute of Technology 
Division of Geological and Planetary Sciences 
California Institute of Technology 
Pasadena, California Pasadena, California 
Abstract 
The Manix Basin Area of the Mojave Desert has been 
used extensively for the cultivation of alfalfa with 
center-pivot sprinkler imgation systems. Since 1972, a 
series of these fields has been abandoned. Data were 
collected using the Airborne Synthetic Aperture Radar 
and Airborne Visible/Infrared Imaging Spectrometer 
instruments in the summer of 1990. Polarimetric 
analysis of the AIRSAR data reveal changes in the 
morphology of the surfaces of the abandoned fields 
from cultivation patterns to patterns resulting from 
wind erosion. Calculation of a normalized difference 
vegetation index (NDVI) based on the AVIRIS data 
suggests that the abandoned fields support more 
vegetation than the undisturbed areas for the first few 
years of abandonment, but that the vegetation density 
on fields which have been abandoned for six or more 
years is lower than the undisturbed desert. Field 
observations confirm the remote sensing results. 
Introduction 
Over the last 27 years, many areas of the Manix Lake 
Basin in the Mojave Desert have been cleared of the 
natural vegetation and leveled for the purpose of 
cultivating alfalfa using center pivot irrigation systems. 
Over the past 20 years a number of fields have been 
abandoned due to the high cost of electricity needed to 
pump the irrigation water [ll. This is a significant 
problem since, "The introduction of sprinkler irrigation 
systems, especially the center pivot systems, has 
enabled previously unsuitable rolling sandy lands to 
be cropped successfully. If and when those soils are 
abandoned, for reasons of economy or shortage of 
water, the United States will face an even greater wind 
erosion threat than it has had to cope with in the past." 
[21. The circumstances of similar general cultivation 
and varying dates of abandonment provide a semi- 
controlled experiment which allows the examination of 
changes resulting from the stress of human 
agricultural activity. 
The Manix Lake Basin is located in the Mojave Desert 
between the Calico Mountains on the west and Afton 
Canyon on the east along Interstate 15. The center of 
the study area is located at approximately 35O57" 
latitude and 116O35'W longitude. Beginning in the late 
Pleistocene, at least 350,000 years ago, this region 
consisted of a series of lakes [31. The present course of 
the Mojave River passes through the basin at the 
southern extreme of the study area. Away from the 
fields, the native vegetation consists of creosote bush 
with some smaller desert grasses with approximately 
37% plant cover. The soil is a poorly sorted sandy 
Figure 1. a) L-band total power image of the Manix Basin area. Agricultural fields are visible as 
circles or ellipses, which are bright if the field is active or dark if it is abandoned. Note the bright 
diameters visible in fields in the lower half of the image. b) Index map showing flight line, north, 
field symbols referred to in the text, and year of abandonment for each field. c) Normalized 
Difference Vegetation Index (NDVI) image generated using Landsat/TM bands synthesized from 
AVIRIS data. Note the dark plumes extending towards the upper right comer from some of the 
abandoned fields in the top part of the image. 
228 
I 
material with some pebbles, and in some places a 
poorly developed soil horizon is present. 
Data 
Data were acquired on June 28, 1990 using the 
Airbome Synthetic Aperture Radar (AIRSAR) system 
operated by NASA's Jet Propulsion Laboratory. 
AIRSAR is a sidelooking radar system which operates 
at three wavelengths: C-Band (5.6 cm), L-Band (24 an) 
and P-Band (68 cm). The data acquired by this system 
records all four complex elements of the Stokes 
scattering matrix, allowing the mathematical synthesis 
of the response of the illuminated area for any 
combination of transmission and reception 
polarizations 141. 
The NASA/ JPL Airbome Visible/Infrared Imaging 
Spectrometer (AVIRIS) instrument was flown over the 
study area on July 23, 1990. AVIRIS provides data in 
224 spectral bands from 0.4 pm to 2.45 pm. These data 
were calibrated via the empirical line method [51, using 
field spectrometer data collected on July 24,1990. 
Analysis of AIRSAR Data 
Figure l a  (after [ll) is an L-band (24 cm) total power 
image of the study area. Figure l b  shows a sketch map 
of the area with the abandoned fields labeled with the 
date of abandonment and a number. The fields appear 
as circular or elliptical features which some of which 
are outlined by bright lines produced by metal fences. 
The abandoned fields are darker than the background 
desert, while active fields are slightly brighter and are 
crossed by bright diameters. 
The bright "spokes" can also be seen on abandoned 
field #1 (abandoned for 3 years and more dimly on 
abandoned field #2 (4 years). These patterns can be 
attributed to the circular planting rows 111 and the 
circular grooves produced by the wheels of the 
irrigation systems [6]. Their presence suggests that the 
human-produced circular patterns persist for 
approximately four years after abandonment. 
Figure 2, (after 111) shows the orientation angle of the 
maximum co-polarized response from the abandoned 
fields plotted against the number of years the field had 
been abandoned. There is a clear trend of departure of 
the maximum away from 900 (VV polarization). A 
surface with slight random isotropic roughness would 
have the greatest co-polarized return at W 
polarization [4]. Additionally, the ratio of co-polarized 
circular polarizations departs further from unity as the 
fields have lain abandoned for greater lengths of time. 
The undisturbed desert has the maximum co-polarized 
return at VV polarization and a circular co-polarization 
ratio of nearly unity. 
I *  
8 0  7 V w m  4hndon.d 
Figure 2. Plot showing the location of the maximum 
co-polarized response in the AIRSAR data for each of 
the abandoned fields plotted against the number of 
years abandoned. The dashed line is the location of the 
n m i " n  co-polarized response in the undisturbed 
desert (after [l]). 
These polarization effects require anisotropic 
roughness on the surface being illuminated. Wind 
ripples formed through processes of wind erosion 
could provide such anisotropy. A second-order small 
perturbation model has been used to demonstrate that 
a nearly sinusoidal topography can yield these 
polarization effects [l]. Field observations have 
revealed that wind ripples do occur on the abandoned 
fields, and that the fields showing the greatest co- 
polarization shifts are the fields most heavily covered 
by wind ripples [ 1 I .  
Analysis of AVIRIS Data 
Figure IC is a Normalized Difference Vegetation 
Index (NDVI) image calculated using the AVIRIS data. 
The Landsat Thematic Mapper band passes as reported 
in [7] were used to weight each AVIRIS band which 
could be detecting in each TM band. The synthesized 
Th4 bands 4 and 3 were used to produce the NDVI 
image. Figure 3 is a plot of the average NDVI values 
for each abandoned field plotted as a function of 
abandonment time. The trend in figure 3 seems to 
indicate that the vegetation density on an abandoned 
field is greater for the first three or four years, but there 
is a rapid decay of vegetation densities on the 
abandoned fields to values that are significantly less 
than those of the undisturbed desert. Field 
observations confirm that the recently abandoned 
fields are much more heavily vegetated than the old 
abandoned fields, and that the old abandoned fields 
are more barren than the surrounding desert. Figure 
IC also shows a set of "plumes" of low NDVI values 
extending downwind (ENE) of the older abandoned 
fields. These plumes may be the result of sand- 
blasting of the native desert plants caused by the 
increased wind erosion from the abandoned fields. 
I 
229 
NOM (June 1990) Conclusions and Future Work 
Figure 3. Plot of Normalized Difference Vegetation 
Index (NDVI) calculated using Landsat /TM bands 
synthesized from AVIRIS data against number of years 
abandoned. The variance bars are one standard 
deviation from the mean for each field. The solid line 
is the value for undisturbed desert, and the dotted 
lines are one standard deviation from the mean of the 
desert. 
It is possible to perform spectral unmixing operation 
on the AVIRIS data and calculate the relative 
abundances in each pixel of each of a set of defined 
endmembers. For this study, two endmembers were 
defined: an average spectrum from field #1, 
abandoned in 1987, was defined as "recently 
abandoned field, and an average spectrum of field #8, 
abandoned in 1973, was defined as "long abandoned 
field". These were the only endmembers used in this 
unmixing. Figure 4 shows the abundance of the 
"recently abandoned field endmember as a function of 
time abandoned. The fact that these values lie nearly 
entirely between 0 and lo00 (lo00 indicating 100% 
relative abundance), suggests that this two endmember 
linear mixing model is well suited to describing the 
spectral characteristics of the abandoned fields. 
L 
* *  
0 I . ' ' ' ; ! .  10 1s 
Y.W. *DoMon.d 
Figure 4. Plot of abundance of "recently abandoned 
field" endmember against number of years of 
abandonment for all abandoned fields. 230 
This study demonstrates that both radar and imaging 
spectroscopy can be useful in investigating human- 
induced land degradation. Polarimetric SAR is capable 
of detecting the presence of certain kinds of features 
below the pixel resolution level if such features cover a 
sufficiently large area. Both data sets and field 
observations seem to show that the abandoned fields 
are not returning to their original predisturbance state 
on timescales of a few decades. 
In addition to the AIRSAR and AVIRIS data, several 
other datasets are being analyzed. Two Landsat TM 
images, one from 1984 and one from 1985, along with a 
sequence of Landsat MSS images stretching from 1973 
through 1988 will be used to look at changes in NDVI 
and spectral signatures of the abandoned fields. Data 
has also been acquired over this area using the Russian 
RESOURCE satellite system, which provides three 
bands of data in the visible and near-infrared (0.5-0.6 
pm, 0.6-0.7 pm, and 0.8-0.9 pm) at a pixel resolution of 
30 meters. An NDVI calculated from the RESOURCE 
data shows the same general pattem as that based on 
the AVIRIS data although the RESOURCE data seems 
to be somewhat noisy. 
Supplementing and extending the remote sensing 
observations have been several trips to the field area to 
observe the state of the abandoned fields and the 
surrounding desert from the ground. The field 
observations have included: Sampling of the ecological 
communities present in the surrounding desert and on 
the abandoned fields, collection of soil samples for 
laboratory analysis, and in situ measurements of soil 
properties. These field observations will be used as 
ground truth for the remotely sensed data and also to 
further understand the processes acting on the 
abandoned fields. 
Plans are being made to extend this study to include 
other regions where agriculture is being performed in 
marginal arid to semi-arid environments. One area 
being looked at as a potential target area is the 
Sandhills region of NE Colorado and Nebraska. The 
ultimate focus of this project is to develop an 
understanding of land degradation which would 
enable monitoring with remote sensing techniques on 
regional and global scales. 
References 
[l] T. W. Ray, T. G Farr and J. J. van Zyl, "Detection of 
Land Degradation with Polarimetric SAR," Geophysical 
Research Letters, Vo1.19, pp. 1587-1590, August 3,1992. 
[2] H. E. Dregne, Desertifiuztion of Arid Lands, Harwood 
Academic Publishers, 1983. 
[3] G. T. Jefferson, "Brief history of Pleistocene Lake 
Manix, in Manix Lake and the Manix Fault, Quarterly San 
Bernardino Co. Museum Assoc, Vol. 29, pp.7-14,1982. 
[4] H. A. Zebker, J. J. van Zyl and D. N. Held, "Imaging 
radar polarimetry from wave synthesis," Journal of 
Geophysical Research, Vol. 92, pp. 683-701,1987. 
[5] J. E. Conel, R. 0. Green, G. Vane, C. J. Bruegge, R. E. 
Alley and B. J. Curtiss, "Airborne Imaging 
Spectrometer-2: Radiometric spectral characteristics 
and comparison of ways to compensate for the 
atmosphere," in Imaging Spectroscopy 11, Proceedings of 
SPIE-Society of Photo-Optical Instrumentation Engineers, 
Vol. 834, pp. 140-157,1987 
[6] P. C. Dubois, E. Rignot and J. J. van Zyl, "Direction 
Angle Sensitivity of Agricultural Field Backscatter with 
AIRSAR Data," Proceedings IGARSS '92, pp. 1680- 
1682,1992. 
[7] B. Markham and J. L. Barker, "Spectral 
Characteristics of Landsat Thematic Mapper Sensors," 
Int. Journal of Remote Sensing, Vol. 6, pp. 697-716, 
1985. 
23 1 
I 


Study Of Land Degradation With Polarimetric SAR And Visible/near-infrared Imaging Spectroscopy

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Study Of Land Degradation With Polarimetric SAR And Visible/near-infrared Imaging Spectroscopy

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