PET is a standardized value of the plant water need. You must first have an idea on the ratio of the actual need of each crop at its development stage, to PET. As example, a one week old corn crop need less water than the same crop at the flowering stage, due to the very smaller leaf area. At the same stage, a sorghum crop needs less water than a corn crop... That represents the water demand.

Secondly, you must know the water capacity of the soil, i.e. the water content of the soil layer  which is colonized with the crop roots (depending of the crop species and stage, as well as the soil depth and clay and organic matter  composition...) and is easily accessible to the roots. That represents the water tank.

Then you must know the water supply, i.e. rain + irrigation. Take into account that heaviest rain may run-off and any rain on a saturated soil will also run-off and will be loss.

Knowing the water tank capacity, the water demand and the water supply, you are in position to calculate the water balance day after day