Irrigation Interval

This is the whole of irrigation scheduling in a single division: the depth of water the crop is allowed to spend, divided by the depth it spends each day, is the number of days until it must be refilled. Eighty-one millimetres of readily available water against a crop using 6 mm a day gives $81/6 = 13.5$ days. Everything else on this page exists to produce those two numbers honestly.

The arithmetic is trivial and the interval it produces is not a constant. Crop use climbs through the season as the canopy closes and falls away again at maturity, so the interval that is right in June is wrong in August. Rooting depth also grows, which enlarges the reserve, and the two effects pull in opposite directions — a deepening root zone lengthens the interval while a rising crop coefficient shortens it. The practical consequence is that a schedule computed once in spring and followed all summer will be too slow in the hot weeks and too fast at the shoulders.

The interval is a soil-and-crop answer, and a system may not be able to honour it. A set-move sprinkler system that takes eight days to get round the whole field cannot run a five-day interval however dry the soil gets, and where the calculated interval is shorter than the time to complete a round, the schedule is being set by the equipment rather than by the crop. That mismatch is one of the standard arguments for the calculation above it on this page: system capacity exists precisely to make sure the machine can keep up with the arithmetic.

Two refinements are worth knowing. Rainfall interrupts the countdown — an irrigation triggered on a fixed calendar rather than on a running water balance is what causes fields to be watered the day before a storm — so the useful version of this calculation is a daily ledger that adds effective rain and subtracts crop use, with the interval as its forecast rather than its rule. And the interval is the time between the starts of two irrigations, which on a system with a long set time is not the same as the gap between them; on a field that takes three days to cover, a 13.5-day interval means starting again 13.5 days after the previous start, not 13.5 days after the previous finish.