Water Use in Vegetables: Dry Bulb Onions
Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona

Written by
Edward C. Martin, Ph.D. Extension Irrigation Specialist
Donald C. Slack, Ph.D. Department Head
E. James Pegelow, Ph.D. Senior Research Specialist

• Introduction
• Commercial Irrigation Management
• Water Use by Dry Onions
• References
  

Introduction

Onion production in Arizona has remained stable since 1992. The average acreage planted is about 1,700 acres and the yields average in the low 400 cwt./acre. Market prices have varied widely during the past sevenyears from a high of $16.40 per cwt. in 1993 to a low of $7.72 per cwt. in 1994. Onions are grown in Maricopa, La Paz, Pinal, Yuma and Cochise counties.

Commercial Irrigation Management

Most dry onions in the state are irrigated using furrow irrigation. There are some fields utilizing drip irrigation, but economics make it difficult for most growers to switch. However, it is quite common for growers to use sprinklers for germination. The use of sprinklers reduces the amount of water needed for germination and decreases the potential for salts to accumulate and cause emergence problems. Onions are sensitive to salts; an ECe of 1.2 or less should be used for irrigation.

Onions should be kept stress free throughout the growing cycle. The critical period for onions is during bulb swelling. An irrigation threshold of 40% soil water deficit should be targeted to avoid water stress. If using tensiometers or resistance blocks, –40 to –60 centibars should be used to trigger irrigation. Set the tensiometer or resistance block at a depth of 6 to 12 in. This will give an accurate representation of the moisture within the plant’s rootzone.

Water Use by Dry Onions

Onions are grown in the winter, with planting starting some time in September/October and harvest taking place in March through May. The peak water use occurs toward the end of the growing cycle when the stems begin to swell and the onions mature. At this point, the water is removed, the tops are knocked down and the onions are allowed to “dry down” in the field. Dry onions use approximately 20 in. total water for the season.

Two graphs illustrate water use by dry onions throughout the growing season. Figure 1 shows the consumptive use of dry onions as a function of heat units after planting (HUAP). The temperatures used to develop this curve were 43°F for a lower threshold and 68°F for an upper threshold. The heat units should be calculated using the sinusoidal approach by Snyder, 1985. Information on daily maximum and minimum temperature can be obtained from AZMET. Total heat units required for the dry onions is about 2300.

Figure 2. Average daily water use for dry onions utilizing a calendar schedule for a mid-September planting date at Maricopa, AZ.

Figure 2 shows the average daily water use for dry bulb onions in central Arizona. This information was developed using crop water use information obtained in studies at the Maricopa Agricultural Center and applying it to weather data gathered at the Maricopa Agricultural Center AZMET station.

When determining irrigation water needs, do not forget to incorporate the system efficiency. For example, if the irrigation system has an efficiency of 75% and the crop requirement is 2 in., apply about 2.7 in. to account for the system’s inefficiency
(2 in./0.75).

Maximum water use by dry onions is about 0.2 in. of water per day. This may vary depending on the planting date and location and can be as high as 0.35 in. Peak use occurs late in the season when the stalks begin to swell, just prior to irrigation termination. The rootzone calculation used for water management in onions is a depth of one foot.

References

Snyder, R.L. 1985. Hand calculating degree days. Agric. Forest Meteorol., 35: 353-358.
AZMET, Arizona Meteorological Network on the web at http://ag.arizona.edu/azmet/

For further information contact your local Cooperative Extension office.

The University of Arizona is an Equal Opportunity/Affirmative Action Employer. Any products, services, or organizations that are mentioned, shown, or indirectly implied in this publication do not imply endorsement by the University of Arizona.
Document located http://ag.arizona.edu/pubs/crops/az1131/
Published August 1999
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