water-dynamics → dry-down-curves
After irrigation, water content does not decline linearly.
It follows a dry-down curve shaped by medium properties, root uptake, and evaporation.
Understanding this curve is essential for correct irrigation timing.
A dry-down curve describes how moisture content changes over time after irrigation.
It typically includes: - Rapid drainage phase - Slower depletion phase - Stress threshold zone
Roots experience very different conditions across this curve.
Immediately after irrigation: - Large pores drain rapidly - Oxygen availability improves - Excess water leaves the profile
This phase restores aeration but does not supply sustained water.
After drainage: - Water is removed mainly by roots - Moisture decline slows - Uptake efficiency is highest
This is the optimal operating zone.
As moisture declines further: - Hydraulic conductivity drops - Roots struggle to extract water - Stress increases rapidly
Small changes in moisture cause large stress responses.
Irrigating: - Too early → hypoxia risk - Too late → acute stress
Correct timing: - Maintains roots in the optimal zone - Maximises uptake efficiency - Minimises stress stacking
Different media show very different dry-down behaviour:
Assumptions based on one medium fail in another.
Dry-down rate increases with: - Higher temperature - Higher VPD - Increased airflow
Irrigation timing must adapt dynamically.
Effective strategies include: - Monitoring moisture trends, not snapshots - Avoiding fixed schedules - Adjusting timing with weather and growth - Protecting roots from extremes
Key mistake: - Using the same timing year-round
Roots respond to when, not just how much.