LONG-TERM USE OF SALINE WATER FOR IRRIGATION

Use of saline drainage water in irrigated agriculture, as a means of i ts disposal, was evaluated on a 60 ha site on the west side of the San Joaquin Valley. In the drip irrigation treatments, 50 to 59% of the i rrigation water applied during the six-year rotation was saline with a n EC(w) ranging from 7 to 8 dS/m, and containing 5 to 7 mg/L boron and 220 to 310 mug/L total selenium. Low salinity water with an EC(w) of 0.4 to 0.5 dS/m and B almost-equal-to 0.4 mg/l was used to irrigate th e furrow plots from 1982 to 1985 after which a blend of good quality w ater and saline drainage water was used. A six-year rotation of cotton , cotton, cotton, wheat, sugar beet and cotton was used. While the cot ton and sugar beet yields were not affected during the initial six yea rs, the levels of boron (B) in the soil became quite high and were acc umulated in plant tissue to near toxic levels. During the six year per iod, for treatments surface irrigated with saline drainage water or a blend of saline and low salinity water, the B concentration in the soi l increased throughout the 1.5 m soil profile while the electrical con ductivity (EC(e)) increased primarily in the upper 1 m of the profile. Increases in soil EC(e) during the entire rotation occurred on plots where minimal leaching was practiced. Potential problems with germinat ion and seedling establishment associated with increased surface soil salinity were avoided by leaching with rainfall and low-salinity pre-p lant irrigations of 150 mm or more. Accumulation of boron and selenium poses a major threat to the sustainability of agriculture if drainage volumes are to be reduced by using drainage water for irrigation. Thi s is particularly true in areas where toxic materials (salt, boron, ot her toxic minor elements) cannot be removed from the irrigated area. C ontinual storage within the root zone of the cropped soil is not susta inable.