1. Rainfall must be greater than 10 inches per year (250mm).
2. Wind and heat must not cause excessive evaporation at critical stages of plant growth.
3. Soil should be deep (preferably 10 feet - 3 meters) with no clay, sand, or gravel seams to interfere with capillary movement of water. The minimum feasible soil depth is 18 inches (450mm) but water storage capability and drought resistance increases with increasing soil depth.
To obtain maximum storage of moisture under any rainfall condition, the soil must absorb as much water as possible when it rains and losses by evaporation or transpiration must be kept to a minimum.