Bill,
Good idea on the evaporation test. I'll do that.
As for the pond going dry between rains, that's not a problem. In fact, I fully expect it. I'm working to restore and improve habitat, not create an aquaculture, at least not in THIS pond. Currently, this arroyo drains of all surface water within 24 hours of rain and is typically bone dry even several inches below grade within two weeks. If I can hold and slowly release water for even a few weeks after a good rain, that will dramatically improve the vegetation near the pond and downstream.
I've seen areas in similar environments that have been dramatically transformed by a series of one and two foot gabion dams (completely permeable, just trap silt and slow flow). In some cases, permanent or semi-permanent surface flow has been restored after decades with no surface flow other than brief flash-floods.
