Irrigation and Drainage
What the exam tests
Irrigation and Drainage tests hydraulics, zoning, scheduling, backflow, and stormwater management. You need to know how pressure, flow, and friction loss affect head performance, why hydrozones matter, and how to keep water on site without erosion or off-site discharge.
Core rules
- Design by hydrozone: turf, beds, and drip cannot share a valve if their water demand or precipitation rates differ.
- Use matched precipitation within a zone. Spray heads, rotors, and drip have different application rates; mixing them causes wet and dry spots.
- Check available pressure and flow at the point of connection. Account for friction loss, elevation, and valve/backflow losses.
- Install backflow protection per plumbing code and local water authority; never rely on a vacuum breaker alone for a permanent system.
- Schedule from evapotranspiration (ET), not a fixed timer. Use rain sensors, soil-moisture sensors, and cycle-and-soak on slopes.
- Drainage: grade to swales, French drains, dry wells, or retention areas; protect the outlet from clogging and erosion.
Worked example
A 1,000 sq ft lawn needs 1.0 in of water per week. One inch over 1 sq ft is about 0.623 gal, so the lawn needs about 623 gal/week. If the system applies water at 80% efficiency, the gross requirement is 623 ÷ 0.80 ≈ 779 gal/week. With a 10 gpm zone, run time is 779 ÷ 10 ≈ 78 minutes per week, split into two or three cycles to reduce runoff. Reasoning: calculate net ET demand, adjust for efficiency, then convert to valve run time. Do not schedule by '20 minutes per station' habit.
Traps
- Mixing drip and spray on one valve.
- Ignoring pressure loss and then blaming the heads.
- No backflow device or no permit for the connection.
- Overspray onto pavement, walls, or salt-sensitive plants.
- Setting a controller once and never adjusting for season.
- Sending drainage to a neighbor or storm drain without authorization.