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Irrigation Formulas
 

Pressure Related
1 psi = 2.307 Head feet
1 head foot = 0.4335 psi

Water Related
1 acre foot of water = 325,851 gallons
1 acre inch of water = 27,154 gallons

Area Related
1 acre = 43,560 square feet
Acres in circle = (Radius x radius x 3.1415) / 43560
Radius needed for given acres = square root of (acres x 43,560 / 3.14156)
Circumference = Radius x 2 x 3.1415 (or diameter x 3.1415)

Center Pivot Specific
Feet per minute FPM (last tower at 100%) = (Distance traveled x 60) / time (seconds)
Hours per revolution = Circumference / (FPM x 60)

Precipitation Related
Gallons per Minute per Acre (GPMA) = GPM / acres irrigated
Inches per day = GPMA x 0.053
Inches per revolution = (Inches/day x Hrs/Rev) / 24