Here ya go:
12 gpm (gallons per minute) per irrigation zone
X 20 minutes per zone
= 240 gallons per cycle
X 4 zones
= 960 gallons per day
X 4 days per week
= 4800 gallons per week
X 700 lawns around lake
= 3.36 million gallons per week
= approximately 10.3 acre/feet per week
= 41.2 acre/feet per month
I'd say there is some impact from watering lawns around a good sized lake.
SE Kansas grass grower
I believe your math is sound, but....
Area of Eagle Mountain lake is 13.58 miles.
13.58 miles x 640 acres/mile**2 (640 acres per square mile)=8691.2 acres
So, using the estimate of 700 homes around the lake (may be waaay off, I got no idea) you get
41.2acre-ft/8691.2 acres = almost 1/17 of an inch used by lawn waterers daily. Evaporation can approach an inch a day.
BUT.....add ALL the other lawns being watered from that lake in municipalities (and from other lakes) and the golf courses, and you get about half the consumptive use of water, for lawns and golf courses. That folks watering directly from the lake are no worse (or better) than those watering their lawns with city water, environmentally and water consumption speaking. They do likely water their lawns for a lot less money....
IF the goal is to SAVE water, THEN it would make a lot of sense to make some rules against intensive consumption of water for landscapes in dry places.
Carpetgrass lawns West of I45 make no sense. Golf courses (some are) should be watered with reclaimed waste water.
