2019 ASHS Annual Conference

Agricultural production in the Lower Umatilla Basin (LUB) in Oregon is under pressure as a potential contributor to nitrate pollution in the groundwater system. Onions are a primary crop produced in the LUB and are commonly irrigated with center pivot systems. Growers often irrigate the crop intensively to achieve optimal yield, which may result in undesirable water losses due to infiltration because of the native coarse-textured soils with a low water holding capacity. Irrigation management and nitrate leaching are closely linked and should be considered together when developing recommendations. Research from other regions with heavier soil types has demonstrated that drip irrigation systems can produce higher onion yields with reduced water and nutrient losses. However, little information is available on drip irrigation systems in LUB. The objectives of this study were to 1) evaluate drip irrigation flow rate and irrigation onset based on soil moisture in coarse textured soil; and 2) model water distribution throughout the soil profile to determine optimal irrigation parameters. Four treatments were evaluated in a random complete block design with five replications. Treatments consisted of two sub-surface drip irrigation flow rates of 0.26 and 0.49 L h^-1. At each flow rate, two values (10 & 20 kPa) of soil tension were used to initiate irrigation onset. Data indicated that onions uniformly established in all treatments. As the crop water demand increased, treatments that irrigated at 10 kPa had significantly higher vigor and plant height than those irrigating at 20 kPa. Onion yield and counts were significantly reduced by the 20 kPa irrigation onset threshold. Optimal yield and counts were found at 10 kPa onset, regardless of flow rate. Modeling of moisture indicated that with the 0.26 L h^-1 flow dripline, irrigation water moved downwards during the initial 4 h after onset, then laterally through the soil profile. A deeper saturation occurred (15-40 cm), while the soil surface (0-15 cm) remained relatively dry. With the 0.49 L h^-1 flow dripline, moisture moved downward within 1 h, then pushed laterally to saturate the uppermost portion of the profile, for the most part, irrigation water did not move downwards past 25 cm, which is optimal for onion root uptake and could minimize excess water and nitrate losses. Experimental procedures are currently evaluating nitrogen leaching correlated to irrigation flow rate and onset.