Fundamentals of Irrigation and Soil-Water-Plant Relationships
Why soil water matters
This domain tests how water enters, is held by, and leaves the root zone, and how those facts set irrigation depth and interval. Expect definitions of field capacity (FC), permanent wilting point (PWP), available water capacity (AWC), infiltration, evapotranspiration (ET), crop coefficient (Kc), and management allowed depletion (MAD).
Core rules
- AWC is the water held between FC and PWP; only a managed fraction (MAD) should be depleted before irrigation.
- For a rooting depth D, water depth = (θFC − θPWP) × D when θ is volumetric; use consistent units.
- ETc = ET0 × Kc; net irrigation = ETc − effective rainfall; gross irrigation = net / application efficiency.
- Coarse sandy soils have low AWC and need smaller, more frequent doses; clay soils hold more but infiltrate slowly.
- Irrigate before stress, not after visible wilting.
Worked example
A soil has volumetric FC 0.30 and PWP 0.12. Root depth is 300 mm and MAD is 50%. AWC = (0.30 − 0.12) × 300 = 54 mm. Allowable depletion = 54 × 0.50 = 27 mm. If peak ETc is 5 mm/day, the interval is about 27 ÷ 5 = 5.4 days, so irrigate every 5 days with about 27 mm net. At 90% efficiency, gross depth = 27 ÷ 0.90 = 30 mm.
Traps
Do not mix gravimetric and volumetric moisture; do not use full AWC instead of MAD; do not schedule on average ET when peak demand governs; do not ignore effective rainfall or root depth changes with crop stage.