Sprinkler Irrigation System Capacity Calculator
Where the field has several soils, enter the lowest Wa: the design uses the smallest allowable depletion. One crop per design area.
Crop water use, root zone, soil water holding capacity and irrigation efficiency are turned into the gross application depth, the irrigation interval and the required sprinkler system capacity in gpm for one crop. Source: USDA NRCS, National Engineering Handbook Part 623 Irrigation, Chapter 11 Sprinkler Irrigation (Amendment 80, August 2016), equations 11-9, 11-12, 11-13 and 11-14; 453 for English units as printed. For preliminary estimation only, not a substitute for review by a qualified/licensed engineer.
Sizing a periodic-move sprinkler system from the water balance
A periodic-move sprinkler system has to put back what the crop used before the root zone drops past the point the manager allows. That ties four things together: how fast the crop uses water, how deep the roots reach, how much water the soil holds per foot, and how much of the applied water actually reaches the root zone. Get one wrong and the system is either too small to keep up in peak weeks or oversized for the life of the pipe.
This page is written for irrigation designers, farm managers and extension agents. It takes eight design inputs and chains them into allowable depletion, gross application depth, the maximum irrigation interval and the days of operation, then into the required capacity in gpm. Crop water use, root zone, soil water holding capacity and irrigation efficiency are turned into the gross application depth, the irrigation interval and the required sprinkler system capacity for one crop.
The interval is truncated down to a whole day, as the source does for a periodic-move system, so 9.23 days becomes 9. Days off are taken out of that whole number before the capacity is worked out, which is why a short interval combined with days off can leave no days to operate at all. In that case the page reports the problem instead of a capacity.
How to use it
- Enter the irrigated area in acres and the design moisture use rate in inches per day for the crop.
- Enter peak rooting depth in feet, management allowed depletion in percent, and soil water holding capacity in inches per foot — the lowest value if the field has several soils.
- Enter irrigation efficiency in percent, plus the days off you want between irrigations and the hours per day the system can operate.
- Read allowable depletion and gross application depth in inches, then the maximum interval f′ and the days of operation f.
- Read the required capacity in gpm; if f comes out at 0, the tool tells you days off is the cause and shows no capacity.
FAQ
What soil water holding capacity (Wa) do I enter when the field has several soils?
Enter the lowest Wa in inches per foot, because the source designs for the smallest allowable depletion; allowable depletion is then Wa × Z × MAD/100 in inches.
Why is the capacity 362.4 gpm when the NRCS sample calculation prints 362 gpm?
The source rounds its printed answer, while this page uses the unrounded gross depth dg and shows gpm to one decimal; the sample inputs A 40, Uc 0.26, Z 3, MAD 50, Wa 1.6, Epa 75, days off 1, T 20 give AD 2.4 in, dg 3.2 in, f′ 9.23, f 8 and Q 362.4 gpm.
Why does the calculator truncate the irrigation interval instead of rounding it?
f′ = dn/Uc is truncated down to a whole day as the source does for a periodic-move system, so 9.23 days becomes 9; days off is then subtracted from the truncated value to give f, and if f is 0 or less the tool reports an error naming days off and shows no capacity.
What is the 453 in the sprinkler capacity equation?
Capacity is Q = 453 × A × dg / (f × T) with A in acres, dg in inches, f in days and T in hours per day to give US gallons per minute — 453 for English units as printed in USDA NRCS National Engineering Handbook Part 623, Chapter 11 Sprinkler Irrigation.