The hard part is controlling the ammount of water that falls from the sky.
in regards to controlling rainfall, the average mature
tree evaporates 36 to 45 gallons of water daily, creating
mist which becomes clouds and then rain.
One of the chief factors in the making of the Sahara
Desert (it wasn't created that way) was Roman tree felling
for their ships, as the cedars of Lebanon were felled
for Solomon's temple (perhaps it would have lasted had
it been made of stone)