Time-lapse remote sensing for rapid measurement of changing landforms

ABSTRACT:

Gullies with large reliefs, 15 to 20 feet, were accurately mapped with timelapse aerial photographs. Contour maps (0.5-foot) of the gully area provided a general study of changes in the valley floor, headcut, and gully erosion by overlaying successive topographic maps. Plotting individual cross-section points achieved the desired accuracy of ± 0.05-feet for surveys on bare ground. Sheet and rill erosion also were determined to the same accuracy with low-altitude, time-lapse aerial photographs. Soil movement was determined for each stereomodel and algebraically summed. Net, 9-year soil erosion was 130 tons/acre, resulting in average annual erosion of 14 tons/acre. Average annual sediment delivery ratio of 53% was determined using the 7.7-ton-per-acre average annual sediment yield for the 75-acre watershed. Universal soil loss equation calculations predicted an annual soil loss of 37 tons/acre, resulting in a sediment delivery ratio of 21%. The sediment delivery of 53% for the photogrammetric study compared to the 21% computed with the USLE emphasizes the need for continued study of sediment delivery.