Soil-Water-Plant Relationships and Irrigation Scheduling
What the exam tests here
This domain tests whether you can convert soil water storage and crop water use into a real irrigation interval and depth - a calculated schedule, not a guessed one.
Core rules
- Total available water TAW (mm) = (field capacity − permanent wilting point) × root-zone depth, usually expressed as mm per metre of soil.
- Indicative available water capacity: sand 60–80 mm/m, sandy loam 100–130, loam 140–180, clay 160–200.
- Readily available water RAW = MAD × TAW. Management allowed depletion is about 50% for most field crops and 30–40% for sensitive ones such as vegetables and banana.
- ETc = ET0 × Kc. Use peak-month ET0 and peak Kc for design.
- Interval (days) = RAW ÷ ETc; net depth per irrigation (mm) = RAW; gross depth = net ÷ application efficiency.
- Drip wets a bulb rather than the whole profile, so roots concentrate there and fertigation and salinity management must follow that wetting pattern.
- Verify with a tensiometer: refill point is roughly −30 to −50 kPa for most crops.
Worked example
Tomato on loam, root depth 0.4 m, AWC 150 mm/m, peak ET0 5 mm/day, Kc 1.15. TAW = 150 × 0.4 = 60 mm. With MAD 40%, RAW = 24 mm. ETc = 5 × 1.15 = 5.75 mm/day. Interval = 24 ÷ 5.75 = 4.2, so irrigate every 4 days. At 90% drip efficiency, gross depth = 24 ÷ 0.9 = 26.7 mm, which is about 267 m³ per hectare per irrigation.
Traps that cost marks
- Using a seasonal average ET in place of peak ET.
- Forgetting to convert net depth to gross depth using efficiency.
- Applying one RAW figure across all soil types.
- Ignoring deep percolation losses when a sandy soil is scheduled like a clay.
Quick recall
RAW = MAD × TAW; irrigation depth = RAW; interval = RAW ÷ ETc.