With the straight line interior wires you could then make even better use of the rotational grazing area with fewer cattle by putting cross wires across paddocks like your #2 and gradually move it away from the water. This will make much better use of your grass, better distribute the manure and really is not much more work.

For example assuming to start you have approx 50 cow-day/a (average) grass and your initial 15 head that means you will get a bit more than 3 days grazing (50CD/15C) per acre.  For your one week changes, give them about 2 acres per week. At the end of the week, put up a new cross wire 2 acres farther ahead, take up last week's wire and you are done for another week.

Especially at first you can keep an eye on the grass and how long it lasts and adjust the area until you find what CD grass you really have. I like to start grazing at about 10" tall grass and move them when it gets to 3" or 4" but this is on northern grasses and clover.

The only problem with this for 15 head on 40 irrigated acres is that the grass will grow faster than it is consumed and will get beyond prime nutrition condition. You will need more cattle. A nice problem to have. Or clip it or bale it. Good luck.

Jim at Dawn