There is an excellent article in the Green Section Record that explains how to estimate irrigation water requirements.
Based on this approach, where the grass uses a certain amount of water, and then one accounts for the effective precipitation, it seems like one has the answer to the irrigation requirement. If the grass needs the water, but the effective precipitation cannot supply that water, then that difference between the amount needed and the effective precipitation must be the irrigation requirement, right?
This approach works perfectly where it doesn't rain, because the amount the grass uses is the amount required as irrigation.
As I started making these calculations, I realized that it gets a bit trickier where it rains. The reason for that is the effective precipitation. How can one make an accurate assessment of that? And wouldn't it make sense to include something about the ability of the soil to store the precipitation?
Where it rains, one can consider both the depth (and the water holding capacity) of the rootzone, and also consider how much water is in the rootzone at each precipitation event, and thus how much water holding capacity the soil has for each rainfall.
I've found the daily soil water balance to be an easy way to make precise calculations. This approach was also used by Gelernter et al. in their analysis of water use on golf courses in the United States.
Here are some calculations for Khon Kaen, showing the different results obtained by calculation method, year, and changes in rootzone depths or irrigation rules.
For more about these calculations, see: