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.