Description of soil moisture strategies for irrigation systems

When an irrigation event happens based on a calculated soil moisture assessment, a soil moisture strategy is required to be entered. The basic premise is that irrigation can be modelled based on depth per application and frequency (return period), and whether they vary (V) according to soil water contents or are fixed (F). If they vary, then this implies that soil water contents are being monitored, and either the depth per application or return period, or both are modified according to the soil water content.

3 different options can be selected:

• Trigger point; fixed depth applied (FV)
• Depth applied to achieve target; fixed return period (VF)
• Trigger point and depth applied to achieve target (VV)

Trigger point is defined as the soil water content that triggers an irrigation event. It is expressed as ‘% of PAW’, where PAW is the Profile Available Water or ‘mm deficit’. A typical trigger point is between 50% and 60% of PAW. It is normally set above the soil water content that crops or pasture yield start to decrease due to insufficient soil water.

Target is the soil water content, irrigation is applied to achieve. It is expressed as ‘% of PAW’ or ‘mm deficit’. This is sometimes referred to as the refill point. A typical target is 95% of PAW, so that soil water is raised to just below field capacity (FC).

Depth applied is the application depth of water applied in mm/ha

Return period is the number of days between irrigation events

When Override management defaults is selected, the user is required to input the following information.