Natural drainage ensures that water is distributed in several ways:

• Runs off surface
• Taken up by plant roots
• Absorbed into pores in the soil particles
• Evaporates from the soil surface
• Drains down through the soil

If natural drainage is not sufficient for the rainfall falling on the field, then control methods are necessary.

Improving the soil structure

This can be improved by the addition of farmyard manure (FYM), organic matter, sand, grit and even lime will open up a heavy clay soil. It is therefore necessary to have an understanding of the soil type of the site in question.

Ditches

Ditches are the cheapest method of putting in artificial drainage. If the land slopes, dig a cut-off ditch across the top of the plot to intercept water from higher ground. Connect this with another ditch at the bottom of the slope.

It is important to maintain ditches and their outfalls every few years.

Drainage pipes

Check to see if systems exist - farm records & aerial photography will show any previous drainage. The problem may be isolated if the original system was well installed and in good condition, so try restoring:

• Use divining rods to mark out.
• Check ditches and outfalls.
• Look for patches of wetness or rust coloured staining on the surface.
• Expose pipe and rod from outlet up.

If a new system is needed:

• Survey area
• Draw up plans
• Obtain quotes
• Clean ditches

Ensure that all drainage installation is carried out when the vineyard soil is dry. Clay pipes are no longer used. Perforated plastic is now commonplace, with diameters of 60 - 80 mm for laterals and 100, 125, 150, 250 mm for mains.

Distance between drains depends on soil type. Positioning of drains depends upon the slope of the field and occurrence of springs and wet patches. If the slope is greater than 2% (1:50) the laterals should run across the slope.The minimum fall on laterals should be 1:250. The minimum fall on mains should be 1:400.
The pipe must be surrounded by gravel (v. expensive). The depth of fill is more important than width so narrow trenches should be dug. For large operations use a machine with laser depth control.

Mole drainage

Cheap method usually used on fields with clay subsoil (no stones). Mole ploughs have a torpedo or bullet shaped "mole" attached to a steel coulter and forms a cylindrical channel in the subsoil. They can be mounted on the three-point linkage, on a wheeled frame or as a simple skid.

The best conditions are when the subsoil is damp enough to be plastic and form a good channel but sufficiently dry to form cracks. The field surface should be dry for a good grip, and reasonably even. Dry weather after ploughing allows the surface of the bore to harden and so the effect lasts longer.

The channels should not be less than 75 mm in diameter at least 300 mm below the soil surface and spaced not more than 4 m apart. Some machines will fill the bore with gravel.

Subsoiling

Subsoiling mechanically bursts the soil and artificially creates the passages which enable the free movement of water and air and allow roots systems to fully develop. To obtain the maximum shattering effect subsoiling should be carried out when the subsoil is dry. Subsoiling in wet conditions is of little benefit and may cause further compaction. It should be carried out at right angles to any drains and should clear any drains by 75 mm to avoid damage.