I've found that a thin layer of gravel works great att keeping down evaporation. I use granite but I'm sure any gravel would be fine as long as it doesn't affect the pH-level of the substrate.
In areas where you depend upon irrigation for the majority of your bonsai watering, and you have poor water, you will struggle to grow moss. Out of the tap, our water in Southern California has an average pH of 8.2 - 8.5, in addition to being extremely hard. Rainwater has a pH of 5.6 and has no dissolved minerals. The moss you find in the wild only knows rain water.
Well, that explains a lot. I know some bonsai folks here can get moss to grow, but I've struggled in my attempts and always assumed it died in the summer because it got too hot/dry and I didn't water it enough. Didn't even consider the water PH. A new experiment is in order using RO water and semi shade...