I am interested in knowing how deep I would have to bury a horizontal closed loop system to reach the average 73 degree ground temperture here in  Phoenix?   I understand approx. 2000 feet of pipe would  be enough for my single level ranch home (1800 sq ft).

Not sure. You could call Al Winters in Southern Utah. I think he's installed some DX systems in your neck of the woods (desert).

His phone is: 435-229-1608

Standard literature I've seen from IGHSPA states that the stable year round earth temp is available only 30+ feet down - hence the appeal of vertical systems. Any horizontal system represents a compromise.

The compromise is reasonable given the often much greater cost of going vertical.

Make sure your "understanding" of the required loop field is based on rigorous design for your site specific situation - climate, load, soil composition, etc.

You need to find your local experts. As engineer states "stable temperatures" are usually accompanied by diminished returns. Heating our homes in MI we only go 5 to 6 feet on horizontal excavation not because it's stable but because it's cheap to install. Greater depth horizontally (even 1 foot) could involve twice the digging due to safety requirements (wider trenches, shoring etc.). Vertical loops are nearly double because of the equipment involved. If you have your local guy show you the operating cost projection differences, you'd be suprised at how small they are.