Agricultural Engineering
Agricultural Engineering in the PEC Engineering Practice Examination tests applied farm power and machinery, irrigation and drainage, soil and water conservation, and post-harvest handling. Questions are field-oriented: they expect correct unit handling, standard Pakistani irrigation and machinery practice, and defensible design assumptions rather than memorised definitions.
Farm power and machinery
- Theoretical field capacity (ha/h) = working width (m) × speed (km/h) / 10; effective capacity = theoretical × field efficiency.
- Draft rises with specific soil resistance, depth and speed; wheel slip rises with excessive draft and falls with correct ballast and tyre pressure.
- Calibrate seed drills and fertiliser spreaders by catching output over a measured distance, then convert to kg/ha using actual swath and travel.
Irrigation and drainage
- Duty (acres/cusec) and delta (depth) are linked by Q = (delta × area) / base period, in consistent units.
- 1 cusec = 0.0283 m³/s; 1 acre-foot = 1233.5 m³; 1 ha-mm = 10 m³.
- Distinguish crop water requirement, irrigation requirement after effective rainfall, and diversion requirement after conveyance and application losses.
- Drainage design uses a drainage coefficient (mm/day) with hydraulic conductivity and drain spacing.
Soil and water conservation
Contour bunds, terraces and grassed waterways control slope length and runoff velocity. Check peak runoff, channel capacity and safe velocity, and size structures for a design storm rather than average rainfall.
Post-harvest
Wet-basis moisture = water / (water + dry matter); dry-basis moisture = water / dry matter. Drying and storage design use equilibrium moisture content and airflow rate.
Worked example
A 40 ha field has a crop delta of 250 mm over a 100-day base period and the canal runs 12 h/day. Volume = 0.250 m × 400,000 m² = 100,000 m³. Operating seconds = 100 × 12 × 3600 = 4,320,000 s. Field demand = 100,000 / 4,320,000 = 0.0231 m³/s (0.82 cusec). At 70% application efficiency, diversion = 0.0231 / 0.70 = 0.033 m³/s (1.17 cusec).
Traps that cost marks
Mixing wet- and dry-basis moisture; using theoretical rather than effective field capacity; ignoring application or conveyance losses; treating delta as a flow rate; and converting acres to hectares or cusecs to m³/s inconsistently.