Water Use in Vegetables: Western Head Lettuce
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

• Top Vegetable Crop
• Commercial Irrigation Management
• Water Use by Head Lettuce
• References
  

Top Vegetable Crop

More lettuce is grown in Arizona than any other vegetable crop. Arizona ranks second only to California in winter head lettuce production. Over the past five years, an average of 50,000 acres were planted, making lettuce third in the state for total acreage (behind cotton and hay). Grown mostly in the Yuma area of Arizona, head lettuce yields have average 322 cwt./ac. The highest yield in recent history was 410 cwt./ac in 1995. A high value crop when the price is right, head lettuce production value, at its maximum, yielded more than $353 million. Water stress in head lettuce can cause poor yields (tonnage) or poor quality (firmness, head size, color).

Commercial Irrigation Management

Lettuce is normally irrigated using furrow irrigation. However, it is common practice for growers to use sprinklers for germination. The use of sprinklers helps to reduce the amount of water needed for germination and reduces the potential for salts to enter the seed row and cause emergence problems. Lettuce is moderately sensitive to salt. Water with an ECe of 1.3 or less should be used for irrigation. Lettuce should be kept stress free throughout the growing cycle. The critical period for lettuce is during head development. 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 12 to 18 in. This will give an accurate representation of the moisture level within the plant’s rootzone.

Water Use by Head Lettuce

Two graphs illustrate water use by lettuce throughout the growing season. Figure 1 shows the consumptive use of lettuce as a function of heat units after planting (HUAP). The temperatures used to develop this curve were 38°F for a lower threshold and 70°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 head lettuce is about 2160. Grown mostly during the winter months, this translates to approximately three to four months, depending on the location and time of planting. Peak water use occurs at 2000 HUAP. Total water used for head lettuce for the season is about 9 in.
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 2.7 in. to account for the system’s inefficiency (2 in. /0.75). Figure 2 shows the average daily water use for the Yuma area. This information was developed using crop water use information obtained in studies at the Maricopa Agricultural Center and applying it to weather data gathered over the last seven years at the Yuma Valley AZMET station.

Figure 2. Average daily water use for western head lettuce utilizing a calendar schedule for a September 15th planting date at Maricopa, AZ.

Maximum water use by lettuce is approximately 0.15 in.of water per day. This may vary depending on location and time of planting. This peak occurs after heading, towards the end of the growing cycle, just prior to harvest. The rootzone calculation used for water management in lettuce is a depth of 2 ft., though lettuce roots grow to a greater depth.

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/az1132/
Published August 1999
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