I am building a house near Idyllwild California elevation 4,600ft. USDA z8b (4 seasons, some but not much snow and rains only in winter) with very sandy soil.

OK the dream is to have a gold fish pond that also act as a very small reservoir to collect winter rainfall. I am thinking of a 5ft avg deep pond with footprint of 15ft x 15ft during dry season. The same pond swell to 25ft x 25ft and 6.5ft deep or so during rain/snow season. I would like to pump water from the pond for irrigation during the dry seasons, then refill the pond with fresh tap water and maintain the 15ftx15ft at 5ft deep.

Question:
Being very sandy, and the top shores will be exposed to sunlight for months. what would be best way to build this pond? No bottom drain right? Being a mud pond, how to keep the pond relatively clear?

Any advice and suggestions are welcome and appreciated

With soils like that, and knowing a little about your climate, you will definitely want a rubber-lined pond. You can get standard off-the-roll sizes 40' wide, so a 40x40 sheet will easily hit a 25x25 pond, if not more. At least I think you still can get 40' wide rolls.

That size hole you can dig by hand, or rent an excavator to do it.

Starting right off with rubber will prevent any worry about leaking or importing clay to the site. Yes, the stuff is expensive, but so it trying to fix leaks, and with your soil, why take chances?

Oh, 25x25 shrinks really quick once you get into the hobby. My 10x15 looks like a driveway puddle to me now, and planning on increasing it to 20x15 this summer along with a small deck. One good pond lily can easily take up a 10x10 area.

Do I need to worry about liner being exposed to sunlight for months every year when the water level is low during dry months?

Perhaps I can put a foot or so of dirt/sand on top of the liner on the part that will be exposed to sunlight when the water level is low?

I'd do the cover with dirt thing. Less chance of poking a hole in it that way too.