Agriculture and Plantation Engineering: Irrigation, Soil–Water, Machinery and Post-Harvest Plant
Agriculture and Plantation Engineering in IESL Part III examines whether you can size field irrigation and drainage systems, work with soil–water relationships, select agricultural machinery, and design post-harvest and processing facilities for paddy, tea, rubber and coconut.
Core rules and concepts
- Total available water TAW = (θ_field capacity − θ_wilting point) × root-zone depth; readily available water RAW = manage-allowed depletion (MAD) × TAW, with MAD typically 0.4–0.6 for high-value crops.
- Crop water use ETc = ET0 × Kc, where Kc changes across the initial, development, mid-season and late-season stages. Design on the peak-month ETc, not the seasonal average.
- Convert net to gross depth by dividing by application efficiency (field) and conveyance efficiency (channel) in series.
- Drainage design uses a drainage coefficient (mm/day removed) linked to rainfall exceedance and soil hydraulic conductivity; subsurface spacing follows Hooghoudt-type relationships.
- Machinery sizing uses draft force × speed for power, with allowance for wheel slip, rolling resistance and implement width.
- Post-harvest calculations distinguish moisture content on wet basis (w.b.) and dry basis (d.b.); drying, tea withering and rubber processing are all mass-balance problems.
Worked example
A 10 ha block of a crop has ET0 = 5 mm/day and Kc = 1.1 at mid-season, so ETc = 5.5 mm/day. Effective rainfall is 1.5 mm/day, giving a net requirement of 4.0 mm/day. With field application efficiency 0.70 the gross depth is 4.0/0.70 = 5.71 mm/day. Volume = 5.71 mm × 10 × 10⁴ m² = 571 m³/day, ignoring conveyance losses. If conveyance efficiency is 0.85, the diversion requirement rises to 571/0.85 ≈ 672 m³/day, which is what sizes the pump and channel.
Traps that cost marks
Mixing mm depth with m³ volume; using seasonal average ET for a peak design; forgetting that efficiencies multiply; applying a single Kc all season; switching wet-basis and dry-basis moisture without conversion; and quoting drainage coefficients without stating the return period.