Who needs to know anything about pumps? You do!
Mechanical pumps are the second most common machine in the world, after electric motors.
The following formulas assume 55% pump efficiency (the standard assumption).
Note: Horsepower is Brake horse power for an electric motor. Do not use for fuel-powered pump engines!
Sprinkler Precipitation Rate:
Note: "head spacing" is the distance between sprinkler heads. L = length between heads in the row, W = width between rows. Therefore "head spacing " = L x W. Use the GPM value for a full circle sprinkler. So if your sprinklers are all half circles, you need to double the GPM value so it is equivalent to a full circle value.
Example: Rotors arranged in rows. The rows are 25 feet apart and the sprinklers are 30 feet apart in the row. The heads are arranged in a triangle pattern. The heads use 6 GPM.
(6 x 96.3) / [(25 x 30) x 0.866]
577.8 /  x 0.866
577.8 / 649.5
0.89 inches per hour