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.
Irrigation systems are generally rated with respect to application efficiency (Ea), which is the fraction of the water that has been applied by the irrigation system and that is available to the plant for use. Applied water that is not available may have been lost from the crop root zone through evaporation or wind drifts of spray droplets, leaks in the pipe system, surface runoff, sub-surface runoff or deep percolation.
Irrigation requirements are determined by dividing the desired amount of water to provide to the plant by Ea as a decimal fraction. For example, if it is desired to apply 0.5 inches to the crop with a 75 percent efficient system, the system should apply 0.5/0.75=0.67 inches.
Irrigation systems are often used for delivery of chemicals such as fertilizers, soil fumigants or insecticides.
Fertigation should not begin until the system is pressurized. It is recommended to always end a fertigation/chemigation event with a short irrigation with clear water to avoid the accumulation of fertilizer or chemical deposits in the irrigation system and/or rinse crop foliage. The length of the flushing cycle should be 10 minutes longer than the travel time of the fertilizer from the irrigation point to the farthest point on the system.
Irrigation systems require periodic maintenance throughout the growing season. Drip irrigation systems may require high levels of maintenance to prevent clogging and system failure. Typically, cleaning agents are injected weekly, but in some instances more frequent injections are needed.
For some crops, irrigation is used for frost protection during the winter. For strawberries, sprinkler irrigation is primarily used with application rates of about .025 inches per hour during freeze events.
Overhead freeze protection is efficient for air temperatures as low as 26 to 28 F, but seldom below. For vegetable fields with subirrigation systems, the heat properties of groundwater can be used for cold protection.
Irrigation scheduling is used to timely apply the proper amount of water to a crop. The irrigation system, crop needs, soil properties and atmospheric conditions must all be considered.
Poor timing or insufficient water application can result in crop stress and reduced yields from inappropriate amounts of available water and/or nutrients. Excessive water applications may reduce yield and quality, are a waste of water and increase the risk of nutrient leaching.
The recommended method to schedule irrigation for vegetable crops is to use together (1) the crop water requirement method that takes into account plant stage of growth, (2) a measurement of soil water status and (3) guidelines for splitting irrigation.
Other irrigation uses vary according to the type of crop, system characteristics and field location. Some examples include: periodic overhead irrigation for dust control; wetting of dry row middles to settle dust and prevent sand from blowing during windy conditions; and wetting of roadways and drive aisles to provide traction of farm vehicles.
Soil water status and tension
Soil water tension represents the magnitude of the suction (negative pressure) the plant roots have to create to free soil water from the attraction of the soil, and move it into the root cells. The dryer the soil, the higher the suction needed; hence, the higher the soil water tension. Soil water tension is commonly expressed in centibars (cb) or kilopascals (kPa; 1cb = 1kPa; 7kPa = 1psi).
For most vegetable crops, soil water tension in the rooting zone should be maintained between 6 (field capacity) and 15 cb. Because of the low AWHC of Florida soils, most full-grown vegetable crops will need to be irrigated daily. During early growth, irrigation may be needed only two to three times weekly.
Crop water requirement
Crop water use is related to reference evapotranspiration (ETo) by a crop coefficient (Kc), which is the ratio of crop water use to the reference value ETo. Because different methods exist for estimating ETo, it is very important to use Kc coefficients that were derived using the same ETo estimation method as will be used to determine the crop water requirements.
E.H. Simonne is an associate professor at UF/IFAS, (352) 392-1928 ext. 208. E-mail: firstname.lastname@example.org. M.D. Dukes is an assistant professor at UF/IFAS, (352) 392-1864 ext. 107. E-mail: email@example.com. D.Z. Haman is a professor at UF/IFAS, (352) 392-1864 ext. 285. E-mail: firstname.lastname@example.org.