Nicely colored bee on Mentzelia involucrata. I presume it's a female, from the very "furry" scopae on hind legs. I think the round amber-colored disk at the base of the tibia is called the "basitibial plate". Two more images of what may be the same individual (or at least species) can be found here.

I tried to go through the key in Stephen, Bohart, & Torchio(1) (even though the Sonoran Desert is arguably a far cry from the "Northwestern America" scope of that reference :-). I wasn't very confident of my result: Nomia. Hoping Dr. John or others may recognize this interesting looking bee.

(I actually shot a whole slew of what I think were well framed & focused images of this species from lots of different angles, but my Canon 40D+flash were on the blink and none of those shots came out. Ughh!!)