Irrigation criteria for drip-irrigated onions

Long-day onion (Allium cepa L. 'Vision') was subjected to five soil water p otential (SWP) treatments (-10, -20, -30, -50, and -70 kPa) using subsurfac e drip irrigation in 1997 and 1998. Onions were grown on 1.1-m beds with tw o double rows spaced 0.56 m apart and a drip tape buried 13 cm deep in the bed center. Soil water, potential was maintained at the five levels by auto mated, high-frequency irrigations based on SWP measurements at 0.2-m depth. Onions were evaluated for yield and grade after 70 days of storage. In 199 7, total and colossal (bulb diameter greater than or equal to 102 mm) yield increased with increasing SWP, but marketable yield was highest at a calcu lated -21 kPa because of greater decomposition in storage in wetter treatme nts. In 1998 total, marketable, and colossal-grade onion yield increased wi th increasing SWP. Onion profits were highest with a calculated SWP of -17 kPa in 1997, and at the wettest level tested in 1998, Storage decomposition was not affected by SWP in 1998, Maintenance of SWP at -10 and -20 kPa req uired, respectively, 912 and 691 mm of water in 1997 and 935 and 589 mm of water in 1998, Onion crop evapotranspiration from emergence to the last irr igation totaled 681 mm in 1997 and 716 mm in 1998.