cannabis needs a lot of moisture, 3/4- 1" of rain per week average during the growing season. The drier the ground and weather gets and the higher the temps, the more water. General rule 12 days since rain and in the 90's= 2 gallons every 5 days 20 days without rain and temps in the 90's, 3 gallons per plant every 4th day. The more organic the soil, the faster it dries out - soil containing some clay will hold moisture much better. The effects of a lack of moisture on cannabis can be serious. A cannabis plant that lacks adequate moisture for extended periods may never fully mature properly, even if the plant looks normal. Also, each extended dry period the plant goes through adds 5 days to the finish date. 3 weeks of dry hot weather moves the harvest date from sept. 30, to Oct 15.