Scheduling irrigations for lettuce
using climate data
Ted Sammis
The Water Budget Method
•
•
•
•
The lelttuce root zone is a reservoir
Water is added by irrigation or rainfall.
Water is removed by evapotranspiration.
Added water can be lost by deep percolation if the
amount exceeds the field capacity of the soil.
Management Allowed Depletion
• Management Allowed Depletion Is the percent of
water that can be removed before soil moisture
stress limits growth.
• (For lettuce MAD is % 30.)
Evapotranspiration
• Evapotranspiration is determined from climate
data to calculate potential Evapotranspiration (water used
by grass) and a scaling coefficient or crop coefficient for
lettuce
• The crop coefficient depends development of the crop or
the land area covered by the crop. The crop coefficient
changes throughout the growing season increasing to a
maximum at maximum leaf cover.
Potential Evapotranspiration
0.4
0.35
Eto inch/day
0.3
0.25
0.2
0.15
0.1
0.05
0
11/1/1998
2/9/1999
5/20/1999
8/28/1999
Date
12/6/1999
3/15/2000
Lettuce crop coefficient
crop coefficient kc
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0
20
40
60
80
Days since planting
100
120
140
Calculation of Evapotranspiration
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•
•
•
Potential Et= 0.3 inches / day
Et= K*PET
K= 0.8
Et= 0.8* 0.3 = .24 inch/day
DailySpring lettuce Evapotranspiration
Evapotranspiration (inches/day)
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
20
40
60
80
Days since planting
100
120
140
Calculated when to irrigate
• Readily available water (RAW) = available water
(AW) * management allowed depletion (MAD)
• Example:
• Aw = 2 inches/ft * 1.5ft root zone
• Mad = 30%
• RAW= 2 inches/ft*1ft*0.3 = 0.9 inches
Irrigation Interval
• Irrigation interval = RAW / Et / day
• Irrigation interval = 0.9 inches / 0.24 inches /day
= 4 days
• Irrigation interval changes with changes in the Et
or soil type.
• Et changes with cover of lettuce and weather.
Source of Potential
Evapotranspiration
• New Mexico Climate Center Homepage
• http://weather.nmsu.edu
Temperature sensor and radiation
shield
The Water Balance Spread Sheet
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Inputs
Climate data, paste into spread sheet
Soil water holding capacity inch/ft
Irrigation amount applied at each irrigation.
Output – Date to irrigate on
Evapotranspiration or irrigation
(inches)
Spring Lettuce Irrigation
25.00
20.00
15.00
10.00
5.00
0.00
0
20
40
60
80
100
120
Days since planting
Irrigations
Evapotranspiration
140
Conclusion
• The water balance-check book method can be
used to schedule irrigations for lettuce.
• The water use of lettuce can be calculated from
climate data, and this is all that is needed to apply
the correct amount of water at the correct time
• The check book method can also determine the
decrease in yield and amount of water stress when
time between irrigation is to long.