Determining irrigation canal seepage with electrical resistivity

Procedures were developed and tested for quantifying seepage losses in unlined irrigation canals for reaches on the order of 100 ft (30 m). The procedure is appropriate for trapezoidal canal sections underlain by clay with a layer of more permeable material at depth. These conditions prevail throughout much of the Central Nebraska Public Power and irrigation District (Central) canal system. The procedure uses electrical resistivity (ER) measurements while canals are in service to determine the resistivity of the underlying clay layer. ER data were correlated to canal depth and then to seepage rate. Seepage rates were determined using seepage meters. Field test sites showed seepage rates from 0.07 to 0.62 (ft(3) . s) (ft width . mi) [0.0041 to 0.036 (m(3) . s) (m width . km)]. Accuracy is approximately +/-20%, comparable to that achieved using stream-gauge methods, which can achieve a +/-5% error of total canal discharge. The ER approach, however, can easily pinpoint seepage zones more precisely than accomplished using stream gauging and will allow Central to reduce the length of canal lining projects. The methods outlined in this paper are simple, relatively quick, and can provide seepage estimates for short reaches in extensive conveyance systems. Such information will be valuable to managers as they make decisions regarding water conservation and ground-water management and where to invest limited resources.