It is a water tower for the local cattle ranchers.
Anonymous
@ 2009-02-18 19:45:07
The smaller cylinder looks like a 5 million gallon water tank. The 2 little out buildings look like an engine/generator and a pump house. The main 200 ft. diameter structure looks like a first order approximation Fresnel lens antenna - OR a solar cell powered reverse osmosis plant. Just a SWAG - Scientific Wild Ass Guess!
Note the radial supports holding it up and the fact that it appears to be suspended off the ground.
Anonymous
@ 2013-09-24 11:18:40
It's not an awning over anything. Neither is it a tent. The sides are solid. Whatever it is, it does not appear to be something requiring high security levels, since if you are on the base you can drive right up to it. It's about 30 or so feet tall. Given that the base is currently trying to remove about a bajillion tons of tricholoroethylene from both the soil and the entire water table beneath the base, I'd strongly suspect it's a water treatment plant. I know somewhere on the base they are pumping the water up from the water table, cleaning it up, then re-injecting it back into the water table. Maybe this is the spot where that happens?
The bullseye and the two small buildings appear in Google Earth's 1993 satellite photo. The storage tank is shown being built in 2003. Also, the bullseye is casting a very, very small shadow in a few historical photos (compared to the storage tank and buildings), so it is not elevated off the ground by much.
Military - Bases, Info Requested 
Note the radial supports holding it up and the fact that it appears to be suspended off the ground.
http://www.youtube.com/watch?v=kXGkxuL2Xl0
Which looks like this to the southwest of the base:
https://www.google.com/maps?ll=39.088473,-121.424139&spn=0.008452,0.013078&t=h&z=17
That area is also listed as "BAFB WWTP" (waste water treatment plant?) in Google Maps.