Irrigation and Drainage Systems
Domain 3 tests hydraulic common sense: how much water each zone receives, whether it arrives evenly, and where it goes once it leaves the surface. Expect precipitation-rate and runtime calculations, plus backflow and hardware identification.
Irrigation principles
Group plants into hydrozones by water need, sun and soil so one valve serves similar demand. Precipitation rate (mm/h) for overhead sprinklers is 1000 × flow (m³/h) ÷ area (m²); for a drip grid it is emitter output (L/h) ÷ (spacing × spacing in m). Matching precipitation rates across heads on one valve prevents over- and under-watering. Distribution uniformity below about 70% signals an audit or redesign. Drip typically runs at 1–4 bar with pressure regulation and filtration; sprinklers need correct nozzle pressure, usually 2–3 bar. Every system connected to potable supply needs a tested backflow assembly - an RPZ for high hazard, a PVB or double check for lower hazard, per local code. Schedule from evapotranspiration and a crop coefficient rather than habit: runtime = (ET × Kc × area) ÷ (PR × DU). Cycle-and-soak avoids runoff on clay.
Drainage
Grade to 2% away from buildings, 1% minimum in swales and pipes. French drains use perforated pipe in washed gravel with filter fabric, outletting to daylight or a dry well sized from runoff volume, not rainfall intensity.
Worked example
A 200 m² lawn needs 5 mm/day; the zone's PR is 20 mm/h and DU 0.75. Runtime = 5 ÷ (20 × 0.75) = 0.33 h, about 20 minutes. Split into two 10-minute cycles on clay so water infiltrates instead of running off.
Traps that cost marks
Mixing drip and spray on one valve. Sizing a dry well for intensity rather than volume. No backflow test record. Ignoring winterization and freeze damage.