It doesn't . It's almost voodoo, there just isn't a way to represent such a heterogenous system in iro.
There is no common root depth, there is no consistent uniform distribution of water. Configuring this is voodoo, not to mention tweaks to the watering are almost always hardware based.
What worked for me keepin the estimates inline is 1 gallon per foot. For,every 2 gallon emitter, make your area 2 foot.