PAINTED ROCK RESERVOIR: 1993 WATER SURFACE AREA AND STORAGE CAPACITY ESTIMATE DERIVED FROM LANDSAT DATA CLASSIFICATION

The Painted Rock Reservoir, southwest of Phoenix, Arizona, had a storage capacity of about 2.5 million acre-ft in 1959, when dam closure was made. It was projected that the reservoir would lose about 200,000 acre-ft of its capacity to sedimentation over 50 years. When the flood of record occurred in 1993, however, it was feared that as much as 500,000 acre-ft of capacity had been lost, and an updated capacity estimate was needed. Because a proposed conventional reservoir survey turned out to be prohibitively expensive, it was decided to investigate the use of Landsat Thematic Mapper remotely sensed data, acquired at multiple reservoir levels, to obtain an updated capacity estimate at a more reasonable cost. Nineteen Landsat Thematic Mapper scenes from 1993 and 1995 were obtained, including reservoir elevations ranging from empty to 5 ft above spillway elevation. Water surface area was determined for each Landsat scene using computer classification of the digital imagery. These surface elevation records for the time of the Landsat data acquisitions and 1985 survey information, were used to generate an updated elevation vs. surface area curve for the reservoir, which in turn was used to compute an updated elevation vs. storage capacity curve. Investigation results indicate that the Painted Rock Reservoir lost approximately 157,000 acre-ft of storage capacity to sedimentation between 1953 and 1993, significantly less than the 500,000 acre-ft previously feared lost. This technique of using remotely sensed data to update area and capacity curves could be applied to other reservoirs, if (among other conditions) there is a record of reservoir elevation at the time of acquisition of the remotely sensed data, and if cloud-free data are available for the entire range of reservoir elevations from full to empty.