A barrel on a stand 4-6 feet above the bed will drip water through small holes in a hose or tube with no pump, but the holes must be small and few, because a single 1 mm hole at that height passes about 8-10 litres an hour and a hose pierced with nail holes empties a barrel in an hour or less. Cut the flow with fewer or smaller holes, a valve, and a filter against clogging. Flow falls as the barrel empties, so check the beds at the end of a run.
Salvage drip irrigation is a low-pressure watering system built from reclaimed hose, tubing, pipe and a raised barrel, which delivers water slowly to the root zone of each plant.
The Head Is Small, And Orifice Flow Is Faster Than Drip
The carrier guide (Drought Resilience & Water-Efficient Agriculture) lists salvage materials for drip: garden hose with small nail holes every 12-18 inches, medical IV tubing (precise flow control), perforated PVC buried 4-6 inches, and soaker hose; a gravity-fed barrel (55 gallons) raised 4-6 feet; and says one barrel waters a 100-square-foot bed at drip rate for 2-4 hours. For the general method, see Drip Irrigation and Bottle Drip Irrigation.
Worked Example: How Fast Does A Hole Flow?
Pressure at the hose from a barrel at height h is ρ g h = 1,000 kg/m³ × 9.81 m/s² × h, which is 9.81 kPa per metre of head. The carrier’s 4-6 ft is 1.22-1.83 m, so 12-18 kPa.
Flow through a small sharp-edged hole is Q = Cd × A × √(2 g h), where Cd is the discharge coefficient (about 0.6 for a sharp-edged round hole, a standard textbook value), A the hole area and h the head.
- A 1 mm hole: A = π × (0.0005 m)² = 7.85 × 10⁻⁷ m².
- At h = 1.22 m: √(2 × 9.81 × 1.22) = 4.89 m/s; Q = 0.6 × 7.85 × 10⁻⁷ × 4.89 = 2.30 × 10⁻⁶ m³/s = 8.3 L/h.
- At h = 1.83 m: √(2 × 9.81 × 1.83) = 5.99 m/s; Q = 0.6 × 7.85 × 10⁻⁷ × 5.99 = 2.82 × 10⁻⁶ m³/s = 10.2 L/h.
- So a 1 mm hole passes roughly 8-10 L/h across that range of head.
- A 2 mm hole has four times the area, so about 33-41 L/h.
A 55-gallon drum is 208 L. Twenty 1 mm holes at the low head pass 20 × 8.3 = 166 L/h, so the drum is empty in 208 ÷ 166 = 1.25 hours. The carrier’s 2-4 hours for a 100 ft² bed (about 9.3 m², so 208 ÷ 9.3 = 22 mm of water) needs a total flow of 52-104 L/h, which is about 6-12 holes of 1 mm.
Flow depends on the square root of head. If the water surface falls 0.9 m (a little more than the drum’s height) from a starting head of 1.8 m, the flow at the end is √(0.9 ÷ 1.8) = 0.71 of the start, 29% less. Friction along a long hose lowers it further, so holes far from the barrel flow less than near ones.
What You Need
- A barrel on a firm stand at 1.2-1.8 m.
- Tubing or hose, clean and food-crop-safe (see the safety note).
- A nail, drill or hot wire to make holes of known size.
- A valve, if you can get one.
- A cloth or mesh filter at the barrel outlet.
Method
Step 1: Plan the holes by the arithmetic. Choose the total flow you want and divide by the flow per hole.
Step 2: Make holes no larger than needed and space them at the plants.
Step 3: Lay the line along the row, level or with a gentle fall, and cap the far end (or leave a slow flush).
Step 4: Filter the water to keep silt out of the holes.
Step 5: Run it and measure. Time the barrel to empty and look at the wet patch at the near and far ends.
Step 6: Adjust by closing holes, throttling the valve or lowering the barrel.
Common Mistakes
- Too many or too large holes — the barrel empties in minutes.
- No filter — silt blocks the holes, and flow becomes uneven.
- Assuming equal flow along a long hose.
- Burying perforated pipe where roots enter the holes.
Safety
Do not reuse tubing that carried fuel or chemicals
Hose and tubing that carried fuel, solvents, pesticides or other chemicals can release them into soil and crops. Use only tubing known to have carried water. Support a filled barrel on a firm base: 208 L weighs over 200 kg.
What’s Next
- Learn next: Scheduling: When and How Much — how long to run it
- Combine with: Olla (Clay Pot) Irrigation — a method with no moving parts