I think only Arabia has a start bias for desert. Japan has no start bias at all, so your starts are essentially an unweighted dice roll. Possibly you are selecting map conditions or scripts that make deserts more likely to appear. As Browd mentioned, the only real way to avoid a desert start is to simply restart the game repeatedly. Fortunately, the map can now be rerolled without quitting to the main screen and setting the whole game up again.