Here in Kansas, there is an omnipresent wind. There is rarely a day in spring and summer that is calm. And the rate of evaporation will change dramatically from day to day. This makes establishing a regular watering schedule nearly impossible to achieve. Because of this, I have developed a simple approach to watering: if the trees need watering, then water them, regardless of what time it is. Even if it is true that the afternoon sun will scorch foliage, a few scorched leaves is negligible damage compared to a fully dried out root ball. Since I don't have a watering schedule per-say, I need to check my trees 3 to 4 times a day. This requires a fair bit of attention, but that's one of the main reasons I like bonsai
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Just a few thoughts,
Hoser