Donnie and Scott,
Essentially your numbers mean that you are returning 53.22% and 42.86% of the water loss by the plant through evaporation and transpiration (ET) via your irrigation. These numbers being significantly lower than mine can mean a couple things but I would wager on one of these two: (1) you both are growing warm season grasses which don't have as high a water requirement as the ryegrass and bentgrass that I grow or (2) your total ET doesn't have the rainfall subtracted from it. Often, the ETo is the number reported by weather stations, which is the gross ET disregarding any rain. ETa is actual ET, which is water lost by the plant less any rainfall received. For example, my ETo for 2008 was 68.04, less my rainfall of 15.21 inches gives me an ETa of 52.83, which is the correct number to use for the equation. BTW, the ETo of 68.04 essentially means that the grass on my course, through evaportaion from the soil and transpiration through the plant, used the equivalent of 68.04 inches of rain. So you can see with only 15.21 inches of actual rain why we need to water so much.
Moving on to more info about irrigation systems, the conversation could get very far ranging. Almost all golf courses have automated irrigation systems nowadays, which is good at least to not leave watering up to an individual running around in the middle of the night trying to get it just right. However, some systems are much more automated and easily manipulated than others. Many supers still have older systems and no on-sight weather station and water very much by feel, simply choosing from experience how many minutes of water they think the course needs any given night. Admittedly, some supers are very good at this, and even those with brand new fancy systems understand their course and conditions enough such that they could operate in this fashion fairly well too.
However, with the newer compertized system with a central control in the office, you can easily get much more precise. I'll try to simply walk through a number of ways our system is set up here. First, we have an on-site weather station, which measures ET, temperature, rainfall, wind, humidty, and solar radiation. That information is sent to the central computer to use. The first step is I have entered a "crop coefficient", which effectively reduces whatever the weather station spits out as an ET to a lesser number closer to what my particular grass type needs. For us, watering mainly 100% ryegrass, that coefficient is around 0.8, or 80% of actual ET. For warm season grasses like bermuda, a coefficient of 0.6 is usually more reasonable. This same effect can also be achieved by using what some programs call a "global adjust" that simply allows you to reduce the output of the whole system by a flat percentage.
Next, we have individual programs created to run irrigation on every part of the golf course: greens, tees, fairways and roughs, even native areas, landscape, lake fills, etc etc. You can make a program for anything. Ours are broken down into front 9 and back 9, then by hole number, all of which are individuallly adjustable. Some supers go farther than that and make seperate programs for hilly areas, shady areas, typically wet and typically dry areas. If you run your system off ET, as I do, all of these programs have their own percentage which can be manually adjusted to put out a certain percent of the days ET (which has already been partially reduced system-wide from the crop coefficient remember).
So for example, my tees are usually at 100%, as they are sand based, drain well and tend to dry out quickly. My fairways on native soil are usually between 80-110% depending on weather and time of year. However, I can have my front 9 fairways at 100%, but if #3 and #5 are really baking still, I can turn up JUST those fairways to 110 or 120% for a night or two. What's more, if there's just a few spots that are baking, we can go in and adjust the INDIVIDUAL SPRINKLER to 110 or 120% or whatever gives us the right amount of water to keep the area alive, but still not overly wet. It's a daily balancing act of checking the whole course, making lists and adjusting areas as needs require.
So what goes on in the computer is that it takes the ET, reduces it by the crop coefficient, then adjust up or down for program percentage adjustments, then adjusts again for any sprinkler percentage adjustments, then takes the individual sprinklers make, model, spacing, nozzle size, and rotation speed to figure out the equivalent minutes that sprinkler needs to run to put out the desired amount of water.
Does your brain hurt yet?
One last thing, as all I've described above is simply how most irrigation systems are run nowadays by supers, there are always other ways to do it, which are by no means less effective and highly dependant on the site and experience of the super.
I don't water my greens off ET at all. I have bentrgrass USGA spec greens. This means they are sand based with a perched water table. So when I water heavily, or it rains alot, the green drains well but water will perch about 8-14" below the soil surface due to the construction of the green to make a "pool" of water for the plant to pull from. I choose to water my greens 30 minutes 2-3 times a week. If I water every night, the soil surface will be kept wetter, and will be ideal for Poa annua, with shallow roots, to take hold, grow and thrive. To combat this, by watering deeply and less often, the water drains away from the surface, pools farther down in the profile where only the deeper rooting bentgrass can get at it and keeps the playing surface more dried out and firm without overly stressing the bentgrass plant. Of course, this only works if your green soil profile is functioning properly and water actually DOES drain quickly and perch at the sand/gravel interface deep in the profile.
So there you have it......that's how you use ET to water a golf course. And the equation I posted is simply a measure of how much of that ET lost by the plant is being replaced by the super. Obviously, if a super is returning over 100% of ET, I would venture to say they're definetely overwatering.....or their calculations are way off.
But any numbers below 100% can give a good indication and level ground for discussion of water use efficiencies over a wide range of courses, locations and situations.
I must also preface the above conclusion with the fact that use of this equation requires diligent, excellent record keeping by the super to make sure they know exactly how much water is being used to irrigate and not fill lakes or pools or is also being used by the clubhouse, etc etc. Also, the addition of an on-site weather station is going to give much more accurate ET information than data taken from the nearest publicly available weather station, which could be located in a very different microclimate or surroundings.