Subsurface Drainage & Spacing the Buried Drains
Reclaims waterlogging
Enter soil permeability, the drainage coefficient, the allowable head and the barrier depth to get the drain spacing, the drain length per hectare and a design status.
Size your subsurface drains
Next: install drains 42.4 m apart at the design depth — that needs about 236 m of drain pipe per hectare; check the outlet can pass 5 mm/day.
Hooghoudt steady-state equation: S = √((8·K·De·h + 4·K·h²) / q). De is the equivalent depth to the impermeable barrier; field K should come from auger-hole or pumping tests.
Subsurface drainage — key facts
- Method
- Hooghoudt steady-state
- Drives spacing
- K, coefficient, head, barrier
- High K (sandy)
- Wider spacing
- Low K (clay)
- Closer spacing
- Drain depth
- ≈ 0.9–1.8 m
- Closer spacing
- Drains faster, costs more
- Reclaims
- Waterlogged & saline land
- Privacy
- Runs in your browser; nothing uploaded
How far apart should the drains go?
Waterlogged and saline fields are reclaimed with buried perforated drains that lower the water table and carry salts away. The hard question is how far apart to lay them: too wide and the middle of the field stays wet, too narrow and the pipe, trenching and outlets cost far more than they need to. The Hooghoudt steady-state equation answers it by balancing the water flowing into the drains against the drainage coefficient you want to achieve.
This tool sets the spacing between drains from soil permeability (K), the drainage coefficient in mm/day, the allowable water-table head and the depth to the impermeable barrier, and reports the drain length per hectare and a design status. Closer spacing drains faster but costs more — use it to weigh options before you dig, then pair it with the Soil Infiltration and Soil Salinity tools for a full reclamation plan.
Beat waterlogging
Lower a high water table below the root zone.
Reclaim saline land
Carry leached salts down and out of the field.
Right-size the cost
See drain length per hectare for each spacing.
Design with physics
Hooghoudt balances K, coefficient and head.
Frequently Asked Questions
What is subsurface drainage?+
Subsurface drainage uses buried perforated pipes (tile drains) laid below the root zone to carry away excess water. It lowers a high water table so the root zone stays aerated, and in saline areas it flushes salts down and out of the soil. It is the standard way to reclaim waterlogged and saline fields and keep them productive.
How is drain spacing calculated?+
This tool uses the Hooghoudt steady-state equation, which balances the water flowing into the drains against the drainage coefficient. The spacing depends on soil permeability (K), the drainage coefficient (the mm/day you need to remove), the allowable head of water above the drains midway between them, and the depth to the impermeable barrier below the drains.
What is the drainage coefficient?+
The drainage coefficient is the depth of water, in mm per day, that the system must remove to keep the water table low enough for the crop. Humid regions and sensitive crops need a higher coefficient; arid leaching needs a value matched to the salt load. A larger coefficient drains faster but forces closer, more expensive spacing.
What is soil permeability (K)?+
Hydraulic conductivity K is how fast water moves through the soil, in metres per day. Sands and well-structured loams have a high K and water reaches the drains easily, allowing wide spacing. Clays have a low K, so water moves slowly and the drains must be placed closer together to remove the same amount of water.
Why does the impermeable barrier depth matter?+
The Hooghoudt equation accounts for water flowing both above and below the drain level toward the drains. A deeper impermeable layer leaves a thicker layer of conducting soil beneath the drains, which carries more flow and lets you use wider spacing. A shallow barrier restricts that flow and pulls the spacing in.
What is the allowable water-table head?+
It is how high the water table is allowed to rise midway between two drains, measured above the drain level, during the design drainage. A smaller allowable head keeps the root zone drier but demands closer drains; the crop's tolerance to a high water table sets this value.
Does closer spacing always cost more?+
Yes. Closer spacing means more drain pipe, more trenching and more outlets per hectare, so the cost rises sharply. The design balances faster, more reliable drainage against installation cost — the tool reports the drain length per hectare so you can see the material implication of a given spacing.
How deep should the drains be?+
Drain depth is usually 0.9–1.8 m, deep enough to keep the water table below the root zone midway between drains but above the impermeable barrier. Deeper drains can be spaced wider for the same protection, but trenching cost and outlet levels limit how deep you can practically go.
Does this help with soil salinity?+
Yes. Subsurface drains are the engine of salt reclamation: applied leaching water dissolves salts in the root zone and the drains carry that saline water away. Without working drainage, leaching just raises the water table and salts come back. Check the Soil Salinity (EC) tool to see how much leaching your field needs.
Are the results a final design?+
The Hooghoudt result is an excellent planning estimate, but a final design should use field-measured K, observed barrier depth and local crop and drainage-coefficient standards. Treat the spacing here as a starting point to size the system and compare options, then confirm with a drainage engineer for installation.