I think we have a current thread on this very topic. I have Earthboxes which (I think) is what Raybo expanded on with his Earthtainers. EB is designed as a closed system, not so much to prevent evaporation as to prevent top water from rain entering the system. The system relies on the growing medium to wick up water from below for optimum dispersal of the ferts. Top watering, or rain causes the ferts to break down much more quickly, as they would in a regular top watered container. I live in SoCal also, and we stop getting rain long about April. I left an EB uncovered last year, and just placed some bark fines on the top to help prevent water loss from evaporation.