Satellite imagery of Tamarix (tamarisk) in Stinking Wash, Rio Blanco County, Colorado

Rick Moran - December 2006

ES 771, Remote Sensing, Emporia State University


Satellite imagery can be used to identify Tamarix, also called salt cedar, in the western US.  Stinking Wash, located in Rio Blanco County, Colorado has seen the introduction of Tamarix in the last 10 years.  Ikonos satellite imagery was used to map the extent of a small section of that wash.

Ikonos satellite

Satellite imagery was obtained from GeoEye's Ikonos satellite.  The Ikonos satellite orbits the Earth every 98 minutes at a 680 km altitude.  The satellite can produce 1 meter imagery every 3 days over the same area.   Multispectral 4 meter and a 1 meter panchromatic sensors are on the satellite. The panchromatic sensor covers .45 - .9 mm.  The multispectral bands are as follows:  band 1 is .45 -.52 mm; band 2 is .51 - .6 mm; band 3 is .63 - .7 mm; band 4 is .76 - .85 mm.  Imaged pixels are referenced into standard widely used coordinate systems.  The company provides a straight forward process at their website to select archived imagery for purchase.   Geographic coordinates are entered into an interactive application where low resolution images can be viewed before acquiring the detailed images.   Alternatively new images can be ordered on demand.

From GeoEye 290 km2 of imagery was acquired from their recent data archive.   The August 2006 images provided were the multispectral resample at 1 meter using the panchromatic imagery.  Imagery was received 3 days after purchase.  The imagery presented here represents 1/4 km2 of that dataset.  To meet license restrictions some of the image resolution has been decreased in this presentation.

Extensive studies of Tamarix across the western US have been on going through the use of NASA satellites .

Stinking Wash

Stinking Wash is located in Rio Blanco County, Colorado.   The wash is ~15 km long, 100 meters wide, and cuts 10 meters deep into the surrounding clay and shale landscape.  It has multiple smaller drainages into the wash.  The wash has a stream 50 to 200 cm wide and 10 to 50 cm deep that flows ~3 months out of the year.  The runoff eventually enters into the Colorado River ~300 miles down stream.  The area is classified as high plains desert.   Almost all the vegetation in the area is sage brush, grease wood, sparse grasses, and non-native vegetation.  The photo below shows the wash with sage and Tamarix.   The 3 cm of snow was bare dirt when the satellite image was taken.

Throughout the area non-native vegetation is easily identified in the infra-red, band 4.   This includes various species of trees that have been planted near residences, irrigated pastures, and Tamarix.   Tamarix is a small tree that is native to Eurasia.  It has spread into the Colorado River basin in every increasing density sinse introduced into North America.  About 10 years ago Tamarix entered into the confines of Stinking Wash and along some of the road by the wash.   The images to follow show the current location and extent that Tamarix has spread.

Below are images of near true color and false color only processed for an atmospheric correction.   The image with bands 1, 2, 3 was not adjusted to fully represent the true colors where vegetation is a lighter green and soils are slightly redder.  Sticking Wash runs NNW in this image with other linear features roads.


Several approaches were attempted to emphasize the presence of Tamarix using Clark Labs -IDRISI software.  In particular the various VEGINDEX types were tried.    Some of the vegetation indexes require a regression so the slope and intercept can be used as input.   A regression was made on a 160 km2 area of the data set with the intent to adjust for the soil reflectance.   Efforts at using other vegetation indexes were no more useful then a NDVI on this small area.

Field verification was done using a processed image to match selected individual pixels with the ground cover.   The area is dominated by dirt and sage brush.  In the thick active sage brush the NDVI was < .25 and mostly < .2.   Individual large Tamarix trees had a NDVI of .4 to .6.     Although at the time field investigation was done 3 cm of snow was on the ground and the Tamarix was defoliated, individual Tamarix trees could be matched without difficulty with the satellite image.  The 3 meter tall tree shown here is the lower red / yellow imaged below the photo.


A thick healthy patch of sage brush such as this has a NDVI in the .2 to .25 range.

The NDVI image displayed defines Tamarix well in red and yellow in the meandering north-south wash.  In a decade the Tamarix density in this section of the wash will likely be 4 times what it is today.  Large sections of the area only have isolated trees that will fill in.


The next step done was to create an ISOCLUSTER using bands 1, 2, and the NDVI as a 3rd band.   Other combinations of bands were not as effective at identifying Tamarix.  18 clusters were chosen which represent variations in soil texture, some very sparse grasses, sage health, and Tamarix.


Ultimately the 18 clusters were combined into 4.   By cross referencing the multiband 1, 2, and 3 with field work a high degree of confidence was developed for distinguishing mostly bare ground clusters from vegetation.   In this image the acreage breakdown is 25.5 acres of dirt, 23.1 sage, 1.2 either sage or Tamarix, and 2.3 acres of Tamarix.


60% of the ground cover over the entire 290 km2 are soils and without any noticeable color variation when viewed from ground level.  But yet they have noticeable contrasting coloration in the satellite image.  Most of the areas soils are from weathered Mancos shale's.   The preliminary investigation indicated a major factor for the color contrasts was soil texture.   In the Band 123 image smooth, especially packed down soils were light tan in color while a rough texture is light blue.   In addition during the warm summer months gypsum participates on some soil surfaces covering it with a thin white coating.   The photo's below show examples of the 2 different texture extremes.   That is gypsum and not snow on the surface in the middle of the photo.


An area near Stinking Wash with mostly bare soils shows color tones dominated by soil texture variations and to some extent terrain.



Tamarix can be readily identified from sage brush in Stinking Wash using satellite imagery.   Surface texture plays a significant role in the color tones of the images in this area.  Not shown in these images are other isolated watered shade trees, irrigated land, and rock outcrops which need taken into account when interpreting the images.


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