Both of these practices result in part or all of the field being in water stress. Drought stress can begin in as little as three days after a 1-inch rain or irrigation in such crops as tomatoes in soils like those in the Piedmont of North Carolina. Thus, frequent irrigation is necessary for maximum yield. Soil moisture requirements differ with the crop and stage of crop development. Soil moisture availability varies with the amount of water in the soil and the type of soil.
Soil type is very important in planning for and using an irrigation system.
Various vegetable crops are listed in Table 1 as to the critical stage and irrigation needs. Up to 1.5 inches of water is needed each week during hot periods to keep vegetable crops that have a plant spread 12 inches or more. This need decreases to .75 inches per week during cooler seasons. Droplet size and irrigation rate are also very important in vegetable crops. Large droplets resulting from low pressure at the sprinkler head can cause damage to young vegetable plants and contribute to crusting when soil dries.