What Is Gravity Irrigation?
Gravity irrigation uses the natural pull of gravity to move water from a higher source to a lower growing area. In a tank-fed system, an elevated reservoir creates pressure that moves water through tubing to plants without a pump. On fields, water can also flow across the soil surface through basins, borders, or furrows. The method and the available elevation determine how the water reaches the roots.
The main gravity irrigation methods
Method | How water reaches plants |
|---|---|
Porous tape or soaker lines | Water seeps along a line into a bed or row; components must work at the available pressure. |
Drip emitters | Tubing delivers water to individual plants through outlets designed for the system's pressure. |
Basin irrigation | Water spreads across a level area enclosed by earthen banks. |
Border irrigation | Water flows as a sheet down a long, gently sloping strip. |
Furrow irrigation | Water runs in channels between crop rows and soaks into the adjacent soil. |
How does gravity create water pressure?
In a tank-fed system, the water surface sits above the irrigation outlet. That vertical difference creates pressure: about 0.43 PSI for each foot of water height. A wider tank holds more water, but it does not add pressure at the same water height.
Water height above irrigation | Approximate static pressure |
|---|---|
2 ft | 0.87 PSI |
3.5 ft | 1.5 PSI |
5 ft | 2.2 PSI |
10 ft | 4.3 PSI |
20 ft | 8.7 PSI |
35 ft | 15.2 PSI |
The illustration shows two reservoirs with the same five-foot drop from water surface to outlet. Both create about 2.2 PSI at those outlets while the water is still. Pressure at the plants can differ when the outlet, tubing, and growing area are at different heights.
As a reservoir empties, its water surface falls and the available pressure decreases. An irrigation method must be compatible with the pressure available throughout use. The system design guide explains how to check height, flow, tubing, and the lowest water level.
How water moves through soil
Slow delivery can give soil time to absorb water where it lands, especially when the application rate stays below the soil's infiltration rate. Water applied faster than the soil can take it in may run across the surface or collect in low spots.
The wetting pattern depends on soil texture and structure, slope, mulch, how much water is applied, and how often it runs. Slow watering does not automatically mean deeper watering; check the root zone to see where the moisture actually goes.
Plants need both moisture and air in the root zone. Soil that remains saturated can limit oxygen, while soil that becomes too dry can make it harder for roots to take up water.
A timer releases water on a schedule. A moisture-responsive control, such as a Tropf Blumat sensor, responds to conditions in the soil. Either can be paired with a suitable gravity-fed distribution system; the control and the water-delivery method do different jobs.
Porous tape and soaker lines
Porous tape releases water along its length, making it useful for beds and other areas where plants grow close together. Gravity BluSoak is one example designed to operate with a low-pressure gravity supply.
A porous line wets a continuous area instead of delivering water at individual emitter points. Its reach and uniformity depend on the available pressure, the length and layout of the line, and the soil beneath it.
Porous tape may be run with a compatible control, including a soil-moisture-responsive Tropf Blumat sensor. The sensor determines when water flows; the tape distributes it. Specific pressure, flow, and installation details belong in the system design guide.
Drip irrigation and individual emitters
Drip systems carry water through tubing and release it near individual plants. An emitter or outlet must work at the pressure the gravity supply can provide; a standard drip component designed for pressurized water may not operate well from a small elevated barrel.
A gravity-compatible emitter layout can direct water to pots, trees, or spaced plants. Unlike porous tape, it delivers water at selected points, so outlet choice and placement shape the wetting pattern.
Some pressure-compensating emitters require more pressure to open and regulate than a small gravity system can provide. Other emitters may flow unevenly if the supply and layout are poorly matched. Check the component's operating range before designing the system.
A timer or soil-moisture-responsive control can determine when water is released. The delivery method determines where it goes.
Ollas: a related passive watering method
“Ollas” are unglazed clay pots buried in soil with an opening above the surface for refilling. Water seeps through their porous walls into the surrounding root zone. They use gravity and the movement of moisture through the clay, but they are not an elevated-reservoir tubing system.
An olla serves the soil close to the pot, so the area it waters depends on the pot, soil, and plant demand. It must be refilled and checked in the same way that any other water source needs attention.
Surface irrigation: basin, border, and furrow
Basin irrigation in a wheat field. Photo: Jeff Vanuga, USDA Natural Resources Conservation Service.
Basin irrigation
Basin irrigation uses a level area enclosed by earthen banks, or dikes. Water enters the basin and spreads across it without a defined direction. You can use these for small areas around trees or much larger planted areas. It works best for crops that can tolerate ponding. The land needs to be as level as possible and the dikes must hold water without breaking. Spill structures help accommodate heavy rain or irrigation that runs longer than intended.
Border irrigation
Border irrigation divides a gently sloping field into long rectangular strips. Water advances as a sheet down each strip, then drains from the open lower end. Unlike a basin, a border is not enclosed by a dike at the lower end. The inflow must be large enough for water to advance while remaining low enough to avoid erosion and loss of topsoil. It can serve many field crops but is generally not appropriate for crops requiring ponded water, such as rice.
Furrow irrigation
Furrow irrigation funnels water into small channels between crop rows. Water infiltrates through the bottom and sides of each furrow, wetting the root zone without flooding the entire field surface. Farmers can supply furrows with gated pipe, siphon tubes, or other methods.
Furrows allow more control over where water flows and suit row crops and vegetables, including crops that benefit from light, frequent applications. They require regular attention and can be labor-intensive. Water that advances too slowly may infiltrate unevenly; flow that is too fast can cause erosion and runoff. Surge flow and flow cutback can improve uniformity and reduce deep percolation, runoff, and nutrient loss.
Furrow irrigation in lettuce. Photo: Jeff Vanuga, USDA Natural Resources Conservation Service.
Common questions about gravity irrigation
Gravity irrigation can be as simple as water flowing across a prepared field or as controlled as an elevated reservoir feeding drip lines. The questions below cover the basic distinctions; the design guide handles the calculations and setup decisions.
Learn more about designing a gravity-fed system
These methods all use gravity, but they do not require the same water source, pressure, controls, or maintenance. Porous tape and emitters carry water through tubing; basin, border, and furrow systems guide water across the land. Ollas are a related, manually refilled way to water a small area.
If you are planning an elevated-reservoir system for containers or beds, the separate guide goes into reservoir height, pressure, tubing, flow, layout, and testing.
Read the gravity-fed irrigation design guideSources and further reading
For a fuller explanation of field methods and small drip systems, these university extension resources are useful starting points.
Oklahoma State University: Surface Irrigation SystemsUniversity of Washington: Gravity Fed IrrigationWhat Growers Are Saying
