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
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
11/1/1998 2/9/1999 5/20/1999 8/28/1999 12/6/1999 3/15/2000
Date
Eto
in
ch
/da
y
Lettuce crop coefficient
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0 20 40 60 80 100 120 140
Days since planting
crop
coe
ffici
ent k
c
Calculation of Evapotranspiration
• Potential Et= 0.3 inches / day
• Et= K*PET
• K= 0.8
• Et= 0.8* 0.3 = .24 inch/day
DailySpring lettuce Evapotranspiration
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0 20 40 60 80 100 120 140
Days since planting
Eva
po
tra
nsp
ira
tio
n (
inch
es/
da
y)
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
• Inputs
• Climate data, paste into spread sheet
• Soil water holding capacity inch/ft
• Irrigation amount applied at each irrigation.
• Output – Date to irrigate on
Spring Lettuce Irrigation
0.00
5.00
10.00
15.00
20.00
25.00
0 20 40 60 80 100 120 140
Days since planting
Eva
po
tra
nsp
ira
tio
n
or
irri
ga
tio
n
(in
che
s)
Irrigations Evapotranspiration
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.