Where irrigation water and plant roots are evenly distributed over the whole planting area, water storage and plant water use can be measured in millimetres. Drip irrigation distributes water over a small part of the whole block and roots follow this water distribution. In these cases, it is often easier to use litres to describe both water use and storage in the plant root zone.
This also allows simple calculation of irrigation time as the discharge from drip systems is commonly reported in litres per hour.
Rule to remember
1mm depth of water = 1L applied to 1 square metre.