Reduced-runoff irrigation of Sudan grass hay, Imperial Valley, California

Sudan grass is a moderately salt-tolerant annual that is capable of substantial osmotic adjustment under high soil salinity conditions, but little is documented about its actual water use and yield under saline conditions. We estimate water use and evaluate the effects of "reduced-runoff" irrigation on soil salinity associated with Sudan grass hay production during a three-year field study (1996-98) in the Imperial Valley, California. The reduced-runoff irrigation method relies on the application of a simplified volume-balance surface irrigation model, and can result in negligible surface runoff; however, its use may have adverse impacts on soil salinity. Despite an anticipated salinity-induced yield reduction of about 15% associated with an average soil salinity of 6 dS/m (0-0.6 m depth), use of the reduced-runoff method resulted in satisfactory crop yields, practically no tailwater runoff, and a slight decrease from the initial average profile soil salinity. The average applied water depth and estimated consumptive use (ET,) during the project were 1,019 and 935 mm, respectively, resulting in an average hay yield of 14.4 Mg/ha versus the 1996-98 county average of 12.6 Mg/ha. The project average ETc/ET0 and yield/ETc ratios of 0.73 and 15.5 kg/ha . mm, respectively, were approximately 15% less than those estimated from water-use-efficiency studies, probably as a result of salinity-induced hay yield reduction.