Don,
Great to hear that explanation, especially from one in the desert. I am probably not a very exciting guy, but in my free time I have "noodled" on the "checkbook theory"of irrigation, whereby the supt. determines how much ET there is and replaces accordingly, instead of blindly replacing water each night,
Still, I feel some superintendents do misuse the computer ET values, in that if the daily ET is .25 inches, they apply .25 inches of irrigation....ignoring the fact that the turf does not need to be at full water capacity, and could technically stay alive with only 35% of it. Of course, no superintendent would let turf get periously close to death, but your 70/85% theory sounds about right to me, because if irrigation refills the field capacity of the plant every night, then the rainfall that inevitably comes is wasted as the plant is fully satisfied already!
If irrigation replaces a only a portion of full capacity, gradually diminishing the plant's water content, then rainfall can bring it occaisionally back to full capacity, and the turf can spend most of the summer comfortably at something between full and stress inducing water capacity.
I try to get my irrigation designers to understand this. They typically design for full replacement of ET in the worst month, usually figuring on 6-8" ET per month, and assuming no rain at all. This gives the super maximum flexibility, assists during the heaviest time it will ever be used - when a quick grow in is desired or necessry - and avoids any complaints and/or liability to the designer for poorly irrigated turf, but obviously, in the wrong (either greens chairman or super) hands can lead to overwatering.
I like to see systems designed for about 75-80% of the worst case, certainly no more than 4.5-6" ET per month, or 1.25-1.5 inches per week. Yes, conditions would vary throughout the season, but this should promote some variety and interest. If drought conditions really got bad, the superintendent could (gasp!) request the course closed for a day - or part of a day - to catch up on watering. Construction cost savings and interest alone would pay for an occaisional day of lost revenue.
This system would limit watering somewhat, but usually not detrimentally, conserve water, promote drier conditions, be environmentally sensitive, and oh yes, leave me more money to build the rest of the golf course! In "the good old days" we could reliably estimate the irrigation portion of construction at 20-25% of our budget. Now, when systems are designed to keep the course green in the middle of a drough, 25-33% is more typical.
Another recent trend is to water courses faster and faster, so that maintenance personel can be off the course by the time golfers start! The pump station necessary for watering a course from 9 at night to 3 in the morning (6 hours, rather than the tradition 8-10) to allow pre dawn maintenance is staggering. In the 17 years I have been in Texas, with desire for faster watering, watering of more rough, and more specialized control, the average number of sprinkler heads has gone from 800 to over 1500, pump station has gone from 1500GPM to 2500GPM and more.
My personal opinion is we overshot the mark somewhere in the last few years. The irrigation companies spent decades getting systems reliable, and have have done a great job of developing tools for the superintendent, but the emphasis on watering perfectly comes with some unintended consequences. Also, I find it ironic that the irrigation system has been perfected just as environmental concerns dictate that we use less water.
Jeff