FAO-56 splits crop water use into two questions that can be measured separately, and that separation is the whole method. What does the atmosphere demand? — answered by $ET_0$, the reference evapotranspiration, the water a well-watered clipped grass surface would lose under today's sun, wind and humidity, in mm/day. What does this particular plant do about it? — answered by $K_c$, the crop coefficient, a bare number from a table for that crop at that growth stage.

$ET_c = K_c \times ET_0$ — read aloud E-T-c equals K-c times E-T-nought. $ET_c$ is this crop's evapotranspiration in mm/day, and it is called that because it is two losses in one: evaporation off the soil surface plus transpiration through the leaves. Nobody separates them in the field, and the equation does not ask you to.

Know the range or you will not catch your own mistakes. $K_c$ runs about 0.2 to 0.4 at emergence, where nearly all the loss is bare-soil evaporation, up to about 1.0 to 1.2 at full mid-season canopy. Values past 1.3 belong to tall crops in windy dry climates and to rice. A $K_c$ of 5 is not a thirsty crop; it is an arithmetic error.

Then the division the entire trade runs on. $I = \dfrac{RAW}{ET_c}$ — I equals R-A-W over E-T-c — where $I$ is the irrigation interval in days, $RAW$ the readily available water in mm from the last lesson, and $ET_c$ the daily use in mm/day. Millimetres over millimetres-per-day leaves days, which is the units check that makes this one hard to get backwards.

One operational nugget, and it costs money to learn the hard way: an interval always rounds DOWN. 13.5 days means thirteen. The half-day is real water, and spending it takes the crop past the depletion the schedule was built on. Rounding up looks like efficiency and reads, in the tuber or the grain, as stress.