I have an outside underground line, #12 type UF, with an outlet in the middle. I'd like to tap into that and run a line about 15 feet to support a pond pump (100watt, 1 amp). I'm out of #12 UF, but have a lot of #14 UF. Is that OK (going on the idea that you can tap off the main line in plumbing with a smaller diameter pipe.)

I would trade for Number 12, or get a short piece of 12 so you can keep the 20 amp Breaker, with #14, you would need to place on a 15 amp breaker. If someone later sees #12 in the panel, they may think it could go on a 20 amp breaker?

Good point, Stratmando. I had forgotten about the breaker. You can't protect a #14 line with a 20 amp breaker. I'll just buy 15 feet of #12 UF.

Although you have 12 awg now are you certain about the existing breaker, is it 20 amps for sure? Also ,since this is outside and will support a pond pump your breaker or outlet should be a GFCI

Breaker is 20 amp and includes GFCI.

Yep, then you need to stick with 12 awg. Some folks would feel its OK, not me, to drop to 14 awg since your 20 amp plugs will not fit into a 15 amp receptacle but you never know what future folks will think or try.