REPEAT PHOTOGRAPHY AND CHANGES IN GEOMORPHOLOGY AND VEGETATION IN NATIONAL PARKS OF THE SOUTHWESTERN UNITED STATES

Repeat photography has been used for 44 years in the southwestern United States to document changes in landforms and the amount and longevity of perennial vegetation. The Desert Laboratory Collection of Repeat Photography contains 6,462 camera stations representing 9,138 photograph matches. The earliest photographs are from 1863, and about 20% were taken between 1880 and 1910. The stations are mostly in Arizona but also are in California, Nevada, Utah, New Mexico, and the Mexican states of Sonora and Baja California. A total of 2,470 stations are in U.S. national parks, notably Grand Canyon (1,635), Canyonlands (240), Zion (136), and Saguaro (108). Although unintentional matches were made as early as 1911, Hastings and Turner started systematic repeat photography in the region in 1960. The Desert Laboratory Collection consists primarily of 4 by 5 in. film negatives and positives, which are stored archivally. A digital database allows ready access, but technology migration issues prevent digital media from being archival because preservation cannot be guaranteed for longer than several decades. Recorded metadata include directions to the camera station, camera orientation and settings, geospatial data, and notes concerning interpretable features in the view. Repeat photography provides a monitoring technique that fills a unique niche. Unlike typical satellite-platform remote sensing or aerial photography, repeat photography provides a long-time series for evaluating landscape change as well as yielding data on species-specific changes at camera stations; this technique is especially useful for quantifying changes in trees, shrubs, and columnar cacti. Count statistics generally are more valuable than data collected using image analysis owing to insurmountable problems with perspective and view blockage. For example, count statistics on the shrub Ephedra sp. yield undisturbed mortality rates of 18% and 17% per century in Grand Canyon and Canyonlands NPs, respectively. Probability of debris-flow occurrence in Grand Canyon is estimated using logistic regression on categorical data developed from repeat photography combined with morphometric and lithologic data. Recent advances in geospatial modeling may provide useful analytical tools for more efficient quantification of change in expansive views.