Irrigation and Farm Machinery: Water Quantities, Methods and Power Matching
Irrigation and Farm Machinery asks you to quantify water and to match machines to jobs and power sources. Numerical questions on duty and delta appear often, so keep the units of every term visible.
Water terms
- Consumptive use = transpiration plus evaporation from the plant and soil surface.
- Duty (D) is the area irrigated per unit discharge, in ha/cumec; delta is the depth of water applied over the base period.
- Relation: D = 8.64 B / delta, with B in days and delta in metres.
- Irrigation efficiency = conveyance efficiency x application efficiency. Drip gives roughly 85-95% application efficiency, sprinkler 70-80%, and surface methods 40-60%.
Machinery basics
Tractor power is quoted at the engine, at the PTO and at the drawbar; each step loses power to transmission and wheel slip. Implements: mould-board plough inverts and buries; disc harrow breaks clods; cultivator weeds and aerates; seed drill places seed and fertilizer at controlled depth; thresher separates grain from straw. Match implement width to available draught and keep working edges sharp.
Worked example
Base period 120 days, delta 0.6 m. D = 8.64 x 120 / 0.6 = 1,728 ha/cumec. Reasoning: 8.64 converts a discharge-day volume into hectare-metres when depth is in metres, so entering 60 cm instead of 0.6 m inflates the answer a hundredfold. Always convert depth to metres first.
Traps that cost marks
- Using delta in centimetres inside the duty formula.
- Confusing duty (area per unit flow) with discharge (volume per unit time).
- Quoting engine power as useful drawbar power.
- Recommending drip for every crop when cost and topography matter.
- Ignoring conveyance losses when stating field application efficiency.