Drainage below the root zone - generally ignored as difficult to measure and considered minimal during irrigation conditions (unless you are over watering)
Everything is calculated in mm.
You need to irrigate if your PAW is below a value (mm) that you have nominated as your trigger point. This amount may also be referred to as the Soil Moisture Deficit, SMD. The trigger level is often chosen to be about half way between the point at which you can visibly see plants suffering (Wilting Point) and Field Capacity.
Good irrigation management keeps soil moisture between Field Capacity ,without over watering and the trigger (or refill) point, without causing plant stress.
It is easiest to start a water budget when your soil is at Field Capacity. The amount of water (mm) in the soil in these conditions will depend on your soil type and the rooting depth of your crop. List of typical Available Water Capacity values.
Potential Sources of error in a simple water budget:
Irrigation loss - for overhead irrigation in particular, a certain amount of water will be lost by evaporation before it reaches the ground. This loss should be accounted for (and minimised of course)
Surface loss - in parched soils or in highly evaporative conditions (hot, dry and windy) some of the irrigation or rainfall will evaporate before it can infiltrate the soil. In severely parched soils, runoff can also occur before infiltration.
Evapotranspiration estimates - estimates are based on standardised conditions, and non-parched soils. In reality, the rate of evapotranspiration will decrease as the soil becomes parched and plants transpire less