Fundamentals of Irrigation and Soil-Water-Plant Relationships
What the exam tests
This domain checks whether you can convert soil moisture, root depth and crop water use into a practical irrigation interval, and whether you understand why drip wets only part of the root zone.
Core rules and concepts
- Field capacity (FC) is the water held after free drainage; permanent wilting point (PWP) is the lower limit for most crops. Available water = FC − PWP.
- Total available water (TAW) in mm = (FC% − PWP%) × root depth in mm. Remember 1 cm = 10 mm.
- Management allowed depletion (MAD) is the fraction of TAW that can be used before stress; readily available water RAW = TAW × MAD.
- Crop water use ETc = ET0 × Kc, where ET0 is reference evapotranspiration and Kc is the crop coefficient for the growth stage.
- Irrigation interval = RAW ÷ ETc, with both in mm/day or mm per interval, adjusted for rainfall and application efficiency.
- Under drip, only the wetted bulb is replenished, so scheduling should use the wetted volume and wetted fraction, not the whole field area.
Worked example
A sandy loam has FC 22% and PWP 10% by volume, root depth 600 mm, MAD 50%, and ETc 5 mm/day. TAW = (0.22 − 0.10) × 600 = 72 mm. RAW = 72 × 0.50 = 36 mm. Interval = 36 ÷ 5 = 7.2 days, so irrigate about every 7 days. If drip wets only 40% of the area, the stored water in the wetted zone is much smaller; many drip designs therefore apply smaller, more frequent doses rather than 36 mm over the whole field.
Traps that cost marks
- Mixing cm and mm: 7.2 cm is 72 mm, not 7.2 mm.
- Using ET0 without multiplying by Kc.
- Treating FC and PWP as the same for every soil texture.
- Applying whole-field RAW under drip while only 30–50% of the soil is wetted.
- Ignoring application efficiency, leaching requirement and rainfall.