Abstract
A 30-mi-long upstream reach of the unlined irrigation Outside Canal in the San Joaquin Valley, California, is affected by subsidence caused by irrigation overdraft of the confined aquifer system. This subsidence interferes with normal operation and maintenance of the canal. Good-quality leveling data along the canal are insufficient for predictions of its ultimate subsidence.
Ratios between past subsidence along the Outside Canal and the nearby, better-surveyed Federal Delta-Mendota Canal were graphically obtained for five pairs of points along the canals using: 1) past Federal levelings along the Delta-Mendota Canal and irrigation district levelings along the Outside Canal, and 2) selected subsidence contour maps. These ratios and previously calculated amounts of ultimate subsidence along the Delta-Mendota Canal were used for estimates of ultimate subsidence (after 1966) along the Outside Canal. Estimates based on district leveling of the canal yield ultimate values ranging from 0.1 to 1.3 ft, and estimates based on subsidence contour maps yield similar values ranging from 0.6 to 1.9 ft.
