GRAVITY-FED IRRIGATION
Gravity-Fed Irrigation Guide: Pressure, Flow & Tips for Better System Design
Gravity-fed irrigation uses elevation to create water pressure. By raising a reservoir above the growing area, gravity creates the pressure needed to move water through the irrigation system without a pump or pressurized water connection. Because gravity systems operate at relatively low pressure, reservoir height, tubing size, water demand, filtration, and system layout all affect performance. This guide explains how those pieces work together and how to design a reliable gravity-fed irrigation system.
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.
Higher-Elevation Gravity Systems May Require Pressure Regulation
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. Height determines pressure; capacity determines how much water you have available. A larger reservoir stores more water and can run longer between refills, but it does not create more pressure unless the water is also higher above the irrigation area.
Design Around the Minimum Pressure Requirement
A gravity system has the most pressure when the reservoir is full and the least pressure when it is nearly empty. Design the system around that lowest-pressure condition. This is why we measure from the top of the soil to the bottom of the reservoir when establishing minimum height. If the bottom of the reservoir is 3 feet 6 inches above the soil, the system still has about 1.5 PSI available when the reservoir is nearly empty; the water above that point when the reservoir is full simply adds additional head pressure. The component with the highest minimum pressure requirement sets the minimum for the system. For example, a Tropf-Blumat sensor can operate at lower pressure, while white BluSoak 2.0 is designed around approximately 1.5 PSI. If a system works when the reservoir is full but slows down or stops as it empties, that usually means the system is operating too close to its minimum pressure requirement, not that the Blumat sensors need to be continually readjusted.
A float valve can automatically refill the reservoir and keep the water level within a smaller range, which keeps the available head height and pressure more consistent and reduces manual refilling. It does not create pressure, so the reservoir still needs enough minimum height for the irrigation system it is supplying.
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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