By E.H. Simonne, M.D. Dukes and D.Z. Haman
Proper water management planning must consider all uses of water from the source of irrigation water to plant water use. Therefore, it is important to differentiate between crop water requirements and irrigation or production system water requirements.
Crop water requirements refer to the actual water needs for evapotranspiration (ET) and plant growth, and primarily depend on crop development and climatic factors, which are closely related to climatic demands. Irrigation requirements are primarily determined by crop water requirements, but also depend on the irrigation system, management practices and the soil characteristics in the irrigated area.
The water used for field preparation depends on specific field cultural practices, initial soil moisture conditions, the depth to the natural water table and the type of irrigation system.
Drip-irrigated fields on sandy soils often require an additional system for field preparation because the drip tubes are not installed until after the beds have been formed. Thus, many drip-irrigated vegetable fields also may require a sprinkler or subirrigation system for field preparation.
Subirrigated fields will use the same system for field preparation as well as for crop establishment and plant growth needs. Subirrigation water management requirements depend on the soil characteristics within the irrigated field and surrounding areas.
Sufficient water must be provided to raise the water table level as high as 18 to 24 inches below the soil surface. Water is required to fill the pores of the soil and also satisfy evaporation and subsurface runoff requirements.
As a rough guide, 2 to 2.5 inches of water are required for each foot of water table rise. For example, a field with a pre-irrigation water table 30 inches deep may need about 2 inches of water to raise the water table to 18 inches.
Vegetables that are set as transplants, rather than direct seeded, require irrigation for crop establishment in excess of crop ET. Establishment irrigation practices vary among crops and irrigation systems.
Crop growth & development
Irrigation requirements necessary to meet the ET needs of a crop depend on the type of crop, field soil characteristics, irrigation system type and capacity and stage of crop development.
Soils vary in texture and hydraulic characteristics such as available water-holding capacity and capillary movement. Because sands generally have very low AWHC values (3 percent to 6 percent is common), a 1 percent change in AWHC affects irrigation practices.