Under-irrigation will compromise yields while over-irrigation can affect root quality, promote excessive fertiliser use, and possibly result in pollution of groundwater and surface water bodies.
- A 16-week summer carrot crop grown on sandy soil near Perth may require 1000mm of irrigation which is 10 000 kilolitres or 10 megalitres of water per hectare.
- Carrots are relatively salt-sensitive, similar to lettuce, onions and strawberries.
- Two key requirements are an efficient irrigation system and simple scheduling approach.
- The irrigation needed depends on the crop growth stage, soil type, crop vigour and weather conditions.
- Use a crop factor, which varies with crop growth stage, to determine the proportion of evaporation the crop should receive. For example, a crop approaching maturity with a crop factor of 1.3 should receive 1.3 times evaporation.
- Evaporation figures can be supplied from direct measurement from a pan evaporimeter or calculated from weather station data.
- Long-term pan evaporation values (see Figure 2, Tables 1 and 3) can be used to guide irrigation. However, using actual daily evaporation is more accurate.
Using actual daily evaporation figures and a crop factor (from Figure 1) you can calculate the daily water requirement and then the system run time to apply this amount as follows:
Water requirement [mm] = (Crop factor) x (daily evaporation)
Run time (minutes) = (Application rate [mins/mm]) x (Water requirement)
Use soil moisture probes to check irrigation adequacy.
Note that: 1000 litres = 1 kilolitre, 1000 kilolitres = 1 megalitre and 1000 megalitres = 1 gigalitre.