UPPER COLCA ARCHAEOLOGICAL RESEARCH PROJECT
Arequipa, Peru

SEASONALITY IN THE COLCA VALLEY

Precipitation in the Colca Valley is seasonal and it greatly influences the scheduling of irrigation agriculture and grazing activities in the region (Guillet and Mitchell 1991). The Colca is located at the edge of the tropics at a latitude of 15.6° South, and is in a semi-arid environment. Two ASTER satellite images are presented below that demonstrate the seasonality of precipitation in a part of the Colca Valley where the river is at an altitude of approximately 3600 meters above sea level (masl) or about 11,800'. 

About this imagery
These ASTER images are composed of Very Near Infra Red (VNIR) bands. In this data the band 3 (displayed as red) is of a slightly longer wavelength than the visual red band and therefore highly photosynthesizing plants appear as a brilliant red in these images. More information about ASTER is available online.

Sept 28, 2000 scene
The dry winter season was nearly over when the first image was recorded on Sept 28, 2000 by the ASTER sensor. Note that the well-watered marshlands on eastern side of this subset are at over 4500 masl but are quite lush.

May 1, 2001 scene
The second scene is from May 1 which is well after the end of the rainy season and is the austral autumn. Higher elevation crops that grow more slowly may not yet been harvested. The vegetation in non-irrigated areas is considerably more vibrant than in the September scene.

Click here to view a scanned 1:100,000 scale topographic map of the same coverage region (140k file).

A procedure called Normalized Difference Vegetation Index (NDVI) can be used to quantify photosynthesis and vegetative mass with some multispectral imagery. The NDVI method was used with bands 2 and 3n of the Sept 28 2000 ASTER scene in Erdas Imagine 8.4. A modified version of the Landsat TM NDVI script was used in the calculation, the function is as follows:
ASTER NDVI = (B3n — B2) / (B3n + B2)

In the resulting image the vegetation with high photosynthesis rates, which will absorbs most of the visual band light, is displayed as dark green. The sparse, dry, or unhealthy vegetation that fails to absorb visual spectra light, and therefore reflects it back at the satellite sensor, is displayed as dark brown.

Click image to view higher resolution PDF file.

The higher altitude marshlands and irrigated grazing areas stand out in the NDVI image as having high values (Dark Green), as do the irrigated and yet unharvested crops in the valley bottom. The high altitude puna grassland is relatively low productivity vegetation. The lowest NDVI values are derived from the rhyolitic boulder fields on the 5000m high Ancachita volcano, on the right side of the image. These rocky slopes have extremely low vegetative growth and therefore the Near IR and Visual bands reflect back in roughly equal amounts.  

 

ASTER visual band spectral information

ASTER Band	Visual Color	Spectral wavelength
Band 1 (Visual)	Blue	0.52 - 0.60 mm
Band 2  (Visual)	Green	0.63 - 0.69 mm
Band 3 (Visual - Near IR)	Red	0.78 - 0.86 mm

 

Guillet, David W., & Mitchell, William (editors)

1991. Irrigation at high altitudes: the social organization of irrigation in the Andes. American Anthropological Association, Washington, D.C.

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