Deficit Irrigation for Potatoes in the Pacific Northwest.

Potato is very sensitive to water stress, which results in significant reductions in tuber yield and quality. The severity of response depends on: crop growth stage, as well as the degree of water deficit. In the dry, low-rainfall (<200 mm/year) region in Columbia Basin along eastern Washington and Oregon, the current practice is to irrigate potatoes to replenish full evapotranspiration (ET). As a result, up to 800 mm of water is applied during the growing season in this region. Increasing and rather competitive pressures for available water resources necessitate exploring water-saving techniques without sacrificing yield and/or quality, thus, the need for this study. This study was conducted in Columbia Basin region in eastern Washington, using two cultivars (‘Ranger Russet’ and ‘Umatilla Russet’). Two irrigation treatments, irrigation to replenish full ET and irrigation to replenish 80% of ET (deficit irrigation), were evaluated after row closure. During 2006, both cultivars showed similar yield decline (7.0-7.5%) under deficit irrigation as compared to that with full ET irrigation. The corresponding tuber yield declines were 7.3 and 10.4 percent, respectively for ‘Ranger Russet’ and ‘Umatilla Russet’ cultivars in 2007. The tuber yield reduction in deficit irrigation was mainly associated with reduction in large size tubers, i.e. >0.227 kg/tuber size. Tuber specific gravity also decreased significantly with deficit irrigation as compared to that with full irrigation. Real-time, continuous monitoring of soil-water content, within and below the root zone was done using capacitance probes. Irrigation set points were developed using depth integrated soil-water content for the root zone of 0-60 cm depth soil profile. Soil-water content in relation to irrigation set points were discussed for the entire growing period, across both irrigation regime treatments.