Effects of Converting from Sprinkler to Drip Irrigation on Water Conservation and the Performance of Merlot Grown on a Loamy Sand

Effects of converting from overhead sprinkler to drip irrigation on the growth, leaf gas exchange, and fruit production of Merlot grapevines with and without cluster thinning were determined over four years. Drip or sprinkler irrigation was applied to the loamy sand soil when in-row soil moisture was depleted to <8%. Irrigation frequency averaged 50% higher (27 compared with 18 times per year) under drip than sprinkler irrigation, but 64% less water (574 compared with 1580 L per vine per yr) was applied on average under drip. Maps of moisture in the soil profile revealed differences in moisture distribution and dry-down dynamics in response to irrigation method. Increasing soil dry-down rates over years indicated that roots proliferated within the drip-irrigated soil volume. Converting to drip reduced the growth and survival of floor vegetation. Vine vigor, leaf gas exchange, and crop yield were reduced but crop yield recovered in the second year and vigor recovered by the fourth year. Stomatal conductance and leaf gas exchange remained lower under drip irrigation. Transpirational water-use efficiency was higher under drip than sprinkler irrigation in the first three years. Input water use efficiency averaged 2.5 times higher under drip irrigation over the four years. Fruit maturation was advanced by drip compared with sprinkler irrigation each year and was associated with increased cluster exposure and higher ambient temperatures. Cluster thinning also advanced fruit maturation but reduced crop yield substantially each year and had only minor interactions with irrigation method.