GRAVITY-FED IRRIGATION
Gravity-Fed Irrigation: Pressure, Flow & System Design
Gravity-fed irrigation uses the weight of water to create pressure. Instead of relying on a pump or pressurized water connection, the reservoir is raised above the growing area and gravity moves the water through the irrigation system. It is a simple way to supply water, but gravity systems have much less pressure to work with than a hose bib or pump. Reservoir height, tubing size, total water demand, filtration, and system layout all become more important when you only have a few PSI available. This guide covers the basic principles we use when designing and troubleshooting gravity-fed irrigation systems, from small container setups to larger systems supplying multiple beds or growing areas.
1 · START WITH HEIGHT
How Gravity Creates Water Pressure
In a gravity-fed system, the vertical distance between the water level and the irrigation area creates pressure. Every vertical foot of water column produces about 0.43 PSI. For most Blumat gravity systems, we recommend at least 3 ft 6 in from the top of the soil to the bottom of the reservoir. That provides about 1.5 PSI even when the reservoir is nearly empty.
Vertical head | Approx. pressure |
|---|---|
1 ft | 0.43 PSI |
2 ft 4 in | 1 PSI |
3 ft 6 in | 1.5 PSI |
~35 ft | ~15 PSI |
Measure from the bottom of the reservoir
For system setup, measure the vertical distance from the top of the soil to the bottom of the reservoir. Using the bottom of the reservoir accounts for the lowest water level the system will normally see and helps make sure adequate pressure remains as the reservoir empties.
A low-pressure gauge is also useful, especially for setup and troubleshooting. If you do not have a gauge, you can estimate pressure from the vertical height of the water column.
Very tall gravity systems may require a pressure reducer
At about 35 feet of vertical head, gravity alone produces roughly 15 PSI. Blumat systems operating above approximately 15 PSI should use an appropriate pressure reducer to keep pressure within the system's operating range.
2 · DESIGN FOR THE LOWEST WATER LEVEL
Reservoir Height, Water Level, and Capacity
Reservoir size and reservoir height do different jobs. A larger reservoir stores more water and can run longer between refills. The vertical height of the water creates the pressure that moves water through the irrigation system.
Do I need to adjust my Blumats as the reservoir empties?
Normally, no. Pressure decreases as the water level drops, but that does not mean the Blumat sensors need to be readjusted during each refill cycle. Problems occur when the water level drops far enough that the system no longer has the minimum pressure required by the irrigation components. This is why we size the system using the bottom of the reservoir rather than the full water level.
A float valve can be used to refill the reservoir automatically and keep the water level within a much smaller range. This keeps the available head height and pressure more consistent while also reducing manual refilling. A float valve does not create additional pressure, so the reservoir still needs to be installed high enough for the irrigation system.
3 · UNDERSTAND DEMAND
Pressure, Flow, and Water Demand
Pressure is the force available to move water through the system. Flow is the amount of water moving through it. A pressure reading by itself does not tell you how much water the supply line can deliver when several irrigation points are using water at the same time.
Friction loss occurs while water is moving through tubing and fittings. At very low flow, friction loss can be extremely small. As more water moves through the same tubing, friction loss increases. This is why a long supply line may work well at low demand but lose more pressure when many outlets are flowing at once.
Tubing Size and Supply-Line Design
Small gravity systems can work well with 8 mm supply tubing. As the amount of water moving through a shared supply line increases, larger tubing reduces friction loss and provides more flow capacity. For larger systems, we often use a larger trunk line from the reservoir and reduce to 8 mm closer to the beds, pots, or irrigation zones.
Approximate Potential Flow at 1.5 PSI
Tubing size | Approx. potential flow |
|---|---|
3 mm | ~0.04 GPM / ~2 GPH |
8 mm | ~0.5 GPM / ~30 GPH |
1/2 inch | ~2.5 GPM / ~150 GPH |
*Approximate values for demonstration only. Actual flow varies substantially with tubing length, exact inside diameter, fittings, elevation changes, and other restrictions. These figures should not be used as system maximums or precise sizing specifications.
The table shows the difference in flow capacity between tubing sizes at the same low pressure. Larger tubing becomes more useful as more irrigation points share the same supply line. The exact amount of flow available still depends on tubing length, fittings, elevation changes, and other restrictions in the system.
4 · IMPROVE THE LAYOUT
Why Looping a Gravity System Helps
A dead-ended supply line gives water one route from the reservoir to the end of the line. A loop connects the supply line back into itself, giving water more than one route through the system. In larger low-pressure systems, this can improve flow to the far parts of the system and reduce pressure differences across the irrigation area.
- More available flow through multiple supply paths
- Better pressure balance across the irrigation area
- Redundancy if one section becomes partially restricted
- Easier filling and purging than a long dead-ended supply line
Larger gravity systems can also be supplied from two reservoir outlets into the same loop. This increases the number of paths available for water to enter and move through the supply line. Looping can also make filling and purging easier because air and water are not confined to a single dead-ended path.
5 · CHOOSE THE DISTRIBUTION METHOD
Distribution Methods
Distribution and control
The reservoir provides the water and pressure, but the irrigation method determines how that water is delivered to the plants. A gravity supply can be used with Tropf-Blumat sensors and distribution drippers or rings, or with white BluSoak 2.0 for broader coverage in beds and larger containers. Blumat Classic also waters from a reservoir, but it uses siphon action rather than the pressurized supply-line design covered through most of this guide.
Choose a distribution method that is designed to operate within the pressure and flow available from your gravity supply.
Keep the Reservoir and Water Path Clean
Gravity systems don't have much extra pressure available to push through restrictions, so a partially clogged filter or dirty reservoir can have a noticeable effect on performance. Keep the reservoir covered, cool, and out of direct light whenever possible. This helps keep leaves, insects, soil, and other debris out of the water and also reduces algae and biofilm growth. If buildup is developing in the reservoir, it can also develop inside the supply tubing and fittings.
A filter needs to catch debris without unnecessarily restricting the water supply. In a low-pressure gravity system, filter surface area matters. Larger reservoir filters and serviceable Y-filters give water more filtering area to pass through. Small hose-washer-style screens have very little surface area, so a leaf or small amount of debris can cover a large percentage of the screen and noticeably reduce flow. They're also easy to forget about because they're hidden inside a fitting.
Whatever filter you use, make sure it is easy to inspect and clean. If flow gradually decreases over time, check the filter and reservoir outlet before adjusting the irrigation system.
6 · PROTECT THE WATER PATH
Reservoir Maintenance & Filtration
A clean reservoir and water path are especially important in a low-pressure gravity system. Keep the reservoir covered, cool, and out of direct light whenever possible. This helps keep debris out and reduces algae and biofilm growth. Aeration is not automatically beneficial in every irrigation reservoir, especially when the goal is to keep the water path clean. Depending on the water and what is being run through the system, an appropriate water-treatment program such as hypochlorous acid, hydrogen peroxide, or properly diluted chlorine can also be used to help control biofilm. Specific products and concentrations depend on the application.
Filtration is not always necessary
If the water source and reservoir stay clean, a filter may not be needed. Filtration is useful when the water contains sediment, debris, or other material that needs to be kept out of the irrigation system, but an unnecessary filter can also become another restriction. That matters more in a gravity system because there is very little extra pressure available to push through a partially clogged screen. When filtration is needed, use enough filter surface area for the available flow. Larger reservoir filters and serviceable Y-filters provide more area for water to pass through than small hose-washer-style screens, which can become restrictive quickly and are easy to forget about inside a fitting.
Keep the reservoir and water path clean first, then use filtration where the water source or system actually calls for it. If flow gradually decreases, check the reservoir outlet and any filter before adjusting the irrigation system.
6 · TEST BEFORE YOU RELY ON IT
Testing a Gravity System
Before adjusting Blumat sensors or individual irrigation outlets, check the water supply. Open the end of the supply line or a flush point and look at the flow. A gravity system may operate at low pressure, but an unrestricted supply line should still produce a steady, free-flowing stream of water.
If the flow is weak, check the reservoir height and water level first, then work through the outlet, filter, valves, tubing, and fittings. Kinked tubing, a partially clogged filter, trapped air, or an undersized supply line can all limit flow. If the reservoir has run dry, refill it and purge the supply line before making adjustments elsewhere.
Gravity Irrigation Troubleshooting
What you see | Where to look first |
|---|---|
Weak flow everywhere | Reservoir height, water level, filter, main outlet or restriction |
Good flow near the source, weak farther away | Supply capacity, tubing layout, restrictions, or looping |
Works until many outlets open | Combined demand is exceeding what the shared supply path can deliver |
Problems after the reservoir ran dry | Trapped air. Refill and purge the supply. |
Performance gradually declines | Filter, sediment, biofilm, or another developing restriction |
Gravity System Basics
Reservoir height determines the available pressure. Tubing size and the amount of water moving through the system affect how much of that pressure is lost to friction.
A reliable gravity system starts with enough reservoir height, then uses supply tubing and a layout that can carry the required flow. Keep the reservoir and filter clean, and test the supply before adjusting the irrigation components.
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