Why drip irrigation?
Drip systems deliver water slowly at the root zone through emitters. Little is lost to evaporation, runoff or wind, so a drip line typically reaches 85% to 95% application efficiency, compared with 60% to 75% for sprinklers. The trade-off is that each emitter delivers a small flow, so knowing the run time matters.
The calculation
Total flow = number of emitters × flow per emitter
Water needed = area × depth ÷ efficiency
Run time = water needed ÷ total flow
Example
A 100 ft² bed with 20 emitters of 1 gph, aiming for 1/2 in of water at 90% efficiency:
- total flow: 20 gph;
- water needed: 100 × 0.5 ÷ 12 = 4.17 ft³, about 31 gallons, or 34.6 gallons after efficiency;
- run time: 34.6 ÷ 20 ≈ 1.7 hours, about 104 minutes.
How much water do plants need?
In summer, many vegetables and shrubs need about 1 to 1.5 in of water per week, including rain. In hot, dry, windy weather it can be more; in cool or cloudy weeks, much less. Watering deeply two or three times a week usually beats short daily runs, because it encourages deeper roots.
Emitter flow and soil
| Soil | Suggested emitter flow |
|---|---|
| Sandy | 1 to 2 gph, emitters closer together |
| Loam | 1 gph |
| Clay | 0.5 gph, longer runs |
Water spreads sideways more in clay and goes straight down in sand, so emitter spacing is often 12 in in sand and 18 to 24 in in clay.
Supply limits
A garden hose bib typically supplies 4 to 8 gallons per minute. Divide that by the emitter flow in gallons per minute to find how many emitters one zone can feed: 5 gpm feeds up to 300 emitters of 1 gph. In practice, keep a margin of about 20%, and check the maximum run length of your tubing.
Tips
- Install a backflow preventer, a filter and a pressure regulator (often 25 psi) at the faucet.
- Use pressure-compensating emitters on slopes or long runs.
- Water early in the morning, and check the soil a few inches down before adjusting the schedule.
- Flush the lines at the start of each season.