Exercise: Design a drip irrigation system for tomatoes
Assumptions:
a. A 16 GPM well with submersible pump is 200 feet from field.
b. The home connected to the water source uses 10 GPM each day.
c. The maximum available irrigation time is 7 days per week for 20 hours per day.
d. Field will have 20 rows of tomatoes 150 feet long, running north-south.
e. Plants are 36” apart in the rows; the rows are spaced 60” apart.
f. The garden slopes down 1% from north to south and 2% down from west to east.
g. Assume a water application rate of 300 gallons per 100 feet of row per week.
h. Use 5/8” dia. drip irrigation tape with 12” emitter outlet spacing, rated at 0.20 GPH flow
@ 8 psi per emitter, or 0.34 GPM flow per 100 ft. @ 8 psi.
Calculations:
a. How many plants will there be per row?
b. How many total plants will there be?
c. What is the total planted acreage?
d. What is the minimum pumping capacity needed & compare to water source
e. How many rows should irrigated at the same time given the available water?
f.
What is the irrigation time per block?
g. What is the mainline pipe size to service each block?
h. Where should the mainline connect to the manifold at each block?
Last revised 2012-12-15 by RAS
Exercise Answers: Design a drip irrigation system for tomatoes
Assumptions:
a. A 16 GPM well with submersible pump is 200 feet from field.
b. The home connected to the water source uses 10 GPM each day.
c. The maximum available irrigation time is 7 days per week for 20 hours per day.
d. Field will have 20 rows of tomatoes 150 feet long, running north-south.
e. Plants are 36” apart in the rows; the rows are spaced 60” apart.
f. The garden slopes down 1% from north to south and 2% down from west to east.
g. Assume a water application rate of 300 gallons per 100 feet of row per week.
h. Use 5/8” dia. drip irrigation tape with 12” emitter outlet spacing, rated at 0.20 GPH flow
@ 8 psi per emitter, or 0.34 GPM flow per 100 ft. @ 8 psi.
Calculations:
a. How many plants will there be per row?
-- 150 ft. ÷ 3 ft. plant spacing = 50 plants per row
b. How many total plants will there be?
-- 50 plants per row x 20 rows = 1,000 plants
c. What is the total planted acreage?
-- 150 ft. long x (20 rows x 5 ft. row spacing) = 15,000 sq. ft. ÷ 43,560 sq.ft./acre
= 0.34 acre
d. What is the minimum pumping capacity needed & compare to water source
-- 20 rows x (300 gal. per 100 ft. of row/week x 1.5 ÷ 7 days/week) = 1,286 gallons per
20-hour day = 64.3 GPH
-- 64.3 GPH ÷ 60 min./hour = 1.1 gallons per minute (GPM) vs. 6 GPM available, so OK
e. How many rows should irrigated at the same time given the available water?
-- Home needs 10 GPM , so field gets 6 GPM maximum
-- Drip tape = 0.34 GPM per 100 ft. of row x 1.5 x 20 rows = 10.2 GPM total demand
-- Maximum water available is 6 GPM, so split into two blocks of 10 rows each for
5.1 GPM per block
f.
What is the irrigation time per block?
-- 0.20 GPH per emitter x 150 emitters = 30 GPH per row = 0.5 GPM per row
-- (300 gal. per 100 ft. row/week x 1.5) ÷ 0.5 GPM per row = 900 minutes/7-day week
= 15 hours/7-day week = 2.14 hours of operation per day, or
= 15 hours / 3 days = 5.0 hours every third day to promote deeper roots
g. What is the mainline pipe size to service each block?
-- 0.5 GPM per row x 10 rows = 5.0 GPM
Use 1” dia. Schedule 40 PVC pipe to keep friction loss below 1 psi per 100 ft. of length,
or use 1.5” dia. Schedule 40 PVC to double water flow capacity to allow for future
expansion of operation
h. Where should the mainline connect to the manifold at each block?
Since west-east slope is <3%, connect in the middle of the manifold.
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Note:
Tomato water application rate of 1.5” of water per acre per week:
3’ wide bed x 100’ long = 300 sq.ft.÷ 43,560 sq.ft./ac. x (27,154 gal./ac.in. x 1.5”) = 280 gallons per 100 ft. row
≈ 300 gallons per 100 ft. row
Last revised 2012-12-15 by RAS