A gravity water system needs a source higher than every tap it serves, because each metre of height gives about 0.1 bar of pressure, so a source 10 m above the tap delivers about 1.0 bar. Measure the height difference with a water-filled hose or a level before you commit to a source, and if the source is not high enough, lift water into a storage tank on a hillside or tower above the highest tap.
Water source and elevation are the two facts that decide whether a gravity system can work at all: how much water the source gives, and how far above the taps its surface sits. That height difference is called head.
Height Is Pressure
Water pressure at a point is the weight of the water column above it. Every metre of height between the water surface and the tap gives about 0.1 bar, or 1.4 psi. The parent page gives the practical meaning:
| Head | Pressure | What it supports |
|---|---|---|
| 5 m | 0.5 bar | A trickle from a tap |
| 10 m | 1.0 bar | A simple kitchen tap and garden hose |
| 20 m | 2.0 bar | Good working pressure |
| 30 m or more | 3.0 bar or more | Several taps running at once |
How the 0.1 bar comes from the weight of water is worked out in Calculating Head Pressure.
Head And Flow Are Different Things
Head sets the pressure; the flow depends on how much the source yields and how wide the pipe is. A high spring with a trickle of water gives high pressure and little water; a big stream a metre above the house gives lots of water and almost no pressure. A gravity system needs both: enough head and enough yield.
How Much Head You Need
The parent page says the minimum practical head for indoor plumbing is 8-10 m, and that below this the flow is too slow for comfortable use. Pipe friction takes some of it away on the way: Pipe Sizing asks for at least 0.5 bar (5 m) left at the tap. The two fit together if friction uses about 3-5 m of the 8-10 m: 8 − 5 = 3 m and 10 − 5 = 5 m.
What You Need
- A candidate source: a spring (Spring Identification) or a stream (River & Stream Assessment)
- A clear hose of 10-20 m and a way to fill it with water without bubbles, or a level
- A tape or pacing string to measure route length
- A bucket and a watch for a flow test
Method: Survey The Source
Step 1: Measure the height difference. Fill the hose with water, hold one end at the source water level and the other at the tap position. The water level in each end shows the same height. Where the hose is not long enough, step down the slope in stages and add the drops. The parent page also allows an altimeter or a topographic map.
Step 2: Measure the route along the ground from source to settlement; the pipe length is longer than the straight line.
Step 3: Measure the flow. Time a bucket filling (Flow Measurement). Filling 20 L in 60 s is 0.33 L/s, about 28,800 L a day.
Step 4: Decide. If the source is 8-10 m or more above the highest tap, run a pipe from it. If it is not, pipe the water to a storage tank placed high enough (Tank Sizing & Placement).
Step 5: Test the water. A spring or stream still needs protecting and treating before drinking (Source Protection, Water Purification).
Common Mistakes
- Measuring the straight-line distance instead of the pipe route.
- Measuring to the base of the source instead of its water level.
- Choosing by pressure alone and ignoring dry-season yield.
- Choosing a source with livestock, latrines or fields above it.
Safety
Untreated source water can carry cholera, typhoid and parasites, and a high pipeline fails under pressure
Do not drink straight from a spring box or stream intake until it is protected and treated. Pipe above a few bar can burst or blow off a fitting; keep the line below the pipe’s rated pressure with a break pressure tank where needed (Break Pressure Tanks). Take care on steep slopes and near streams when surveying.
What’s Next
- Learn next: Calculating Head Pressure — turn metres into bar and subtract friction
- Combine with: Spring Boxes & Collection Points — capture a spring