| Andrea S. Laliberte, Douglas E. Johnson, Norman R. Harris and Grant M. Casady |
| Authors are graduate research assistant, professor, graduate research assistant and faculty research assistant, Department of Rangeland Resources, Oregon State University, 202 Strand Agricultural Hall, Corvallis, Ore. 97333. |
| Full text Article |
Abstract |
| Remote sensing and Geographic Information Systems (GIS) are common tools for time change analysis, however, in most cases satellite imagery or small-scale aerial photography is used. The increased resolution of large-scale aerial photos helps in identifying small features on the ground and is highly useful in the assessment of riparian areas. In this project, large-scale aerial photography from 1979 and 1998, GIS, Global Positioning Systems (GPS) and ground truthing were combined in a time change analysis of an eastern Oregon riparian area. The objectives of this study were: 1) to examine changes in stream morphology over 20 years, 2) to assess if changes were associated with management, topography or other factors, 3) to determine the feasibility of using large-scale aerial photography, GIS and GPS techniques as a tool for assessing change over time. The 2.5 km long study area, consisting of the stream and riparian area was separated into exclosures and grazed areas in 1978. Aerial photos from 1979 and 1998 (scale of 1:4000) were geo-referenced using 102 ground control points for the 41 ha study site. In addition, older aerial photography and previously collected survey data were available for this study. Stream features, such as islands and stream channel, were digitized using a GIS. Stream length, stream width and areas of change were identified for both years. The width of the stream was extracted automatically every 0.5 m along the 2.5 km long stream section, yielding 5070 width measurements. Although stream length remained the same over time, stream width decreased in both grazed and exclosed areas. The area of change (3.65 ha) was slightly larger than the area of no change (3.2 ha). Number of islands and island perimeter decreased, while the island area increased. Exclosures and grazed areas responded similarly, and it was concluded that the topography and stream dynamics had a greater impact than the grazing regime in this study. The use of large-scale aerial photography, GIS and GPS proved to be a powerful tool for detecting change and it is expected that these techniques will become more common in rangeland analysis in the future. |
| Key Words: Time change analysis, remote sensing, Geographic Information Systems, GIS, stream morphology, large-scale aerial photography |