Irrigation uniformity
In Australia carrots are usually irrigated from overhead sprinklers, either permanent or semi-permanent impact sprinklers or centre-pivot irrigators.
An efficient irrigation system is vital before any attempt can be made to schedule irrigation efficiently. The system must be capable of applying water uniformly. A measure of system uniformity, called the coefficient of uniformity (CU) can be calculated from a series of catch cans placed under the sprinklers:
CU = [1- (average catch can variation from mean)] x 100
A CU of greater than 85% is considered acceptable.
Irrigation frequency and timing
On loamy soils in cool climates, crops may only require irrigation once or twice per week, while on sandy soils in hot climates carrots may need irrigating twice per day.
To maximise the use of off-peak power in summer, pre-8am waterings can be applied on sandy soils. These waterings should apply no more than 8mm to a crop that has been watered within the previous 24 hours and will usually be limited to less than 6mm to avoid drainage beneath the root zone.
When more that 8mm/day of irrigation is required, this will be applied in two waterings separated by more than four hours. Soil moisture can be monitored with sand (low-tension) tensiometers or some other soil moisture monitoring device suitable for sand.
Application rate
The application rate in milimetres per hour of the system at a given operating pressure must be known. For example, if a system has a output of 8mm/h, it takes a run time of 60/8 = 7.5 minutes to apply 1mm of water.
Irrigation should be scheduled based on evaporation and guided by soil moisture monitoring devices.
Irrigation for crop establishment
During land preparation before planting, water application will be kept to a minimum to minimise leaching losses of residual or mineralised soil nitrogen.
During the early stages of crop establishment, the soil surface needs to be kept moist to ensure even germination and to prevent heat-induced girdling of young seedlings. This is achieved by multiple light waterings on hot days.
During these early stages, total daily water application should not exceed 0.9 of Class A pan evaporation (Epan) to minimise nutrient leaching.