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A Guide to the Practical Use of Aerial Color-infrared Photography in Agriculture

Agricultural Applications of Color-infrared Film

Crop Stresses

CIR aerial photography has attracted the most attention as a means of interpreting crop stress. As noted earlier, it is possible under certain conditions to detect some types of crop stresses on CIR film, sometimes before the stress is visible to the naked eye. Considerable scientific discussion has been devoted to the length of this "pre-visual" interpretation period; some claims even exceed a week. But such claims seem dubious at best, and a much more realistic estimate would be a few to several hours. Hence, the "turn-around time" between the photographic exposure and the viewing of the images is critical. For practical use, one needs to be able to view the photography almost immediately, in "near real-time". This timeliness is one of the biggest problems when attempting to use aerial CIR images in to discern crop stress.

In other situations, the plant stress might be visible to the viewer as he stands in the field, but the CIR photo can still contribute to good crop management by enhancing the interpretation (by allowing a view of near-infrared plant reflectance) and/or allowing one to accurately delineate the affected area on the aerial photograph. With regard to the latter, it is then relatively easy to map out the specific area of a field that is affected by the particular problem.

Crops can be stressed by numerous environmental pressures. Lack of moisture, probably the most apparent and common vegetative stress associated with cropland agriculture leads, as noted earlier, to the collapse of the cell walls and a corresponding decrease in the amount of near-infrared reflectance. Thus, drought conditions desecrate dryland crops, the result of which can be detected on CIR film (figs. 2 and 9).

The "Young" Leaf The "Mature" Leaf The "Old" Leaf
Fig. 2 - Generalized spectral-reflectance characteristics of young, mature, and old vegetation.

Fig. 9 - Soybean irrigation experiment at the NU South Central Research and Extension Center, Clay Center, Nebraska. Date of photograph is Aug. 15, 1983

Insects often attack plant tissues, leading either to a reduction in the density of leaf cover or a decrease in a plat's ability to transport water and nutrients. In both cases, the level of near-infrared reflection is reduced, and the damaged areas generally contrast sharply with areas of normal vegetation on the CIR imagery.

By studying sequential CIR photos, the diffusion of an infection or pest can be detected and monitored with ease. Perhaps one photo taken every two or three days might provide information about the dynamics and behavior of a particular disease or insect. Such analysis could lead to the development of preventative measures or procedures. The aerial photos could also be used to guide treatment within the field before the entire crop is lost, again, if they can be obtained and processed in a timely manner.

One of the easiest management errors to detect with CIR film is uneven fertilizer application. Portions of the field that have received proper amounts of nutritional supplements contrast sharply with those that have not (fig. 10). Of course, excess fertilizer will severely damage or kill a crop, these areas are also apparent on CIR images.

Fig. 10 - Uneven fertilizer application in a pasture. The darker stripes are areas where the fertilizer has stimulated good growth of the rangeland.

The use of CIR aerial photographs to evaluate crop damage due to sever weather is a application that is increasing. Some insurance companies have begun to investigate the use of CIR images in computing their weather-related financial adjustments to farmers. Damaged portions of a field will possess a CIR signature that varies from that of the unaffected areas (fig. 11).

Fig. 11 - Lodging of wheat due to high winds. The light tones delineate areas of damage.