LICO Factsheet No. 1 - How a Drain Works

No. 1: HOW A DRAIN WORKS

Soil
A soil has two principal components - the solid material, such as a grain of sand, silt or clay; and, spaces between these soil minerals. The solid materials of a soil occur in a range of sizes. Coarse particles that you can see are called sand. You cannot see with your naked eye single particles of silt or clay - they are too small.

The soil spaces can be filled with water, in which case the soil is said to be waterlogged. The spaces may also contain so little water that plants wither and die. The purpose of soil water management is to establish an equilibrium of soil moisture for optimum plant growth.

Water in Soil
Rainfall striking the soil surface has two main choices. If rain arrives faster than it can enter into the soil then it will run off the surface and may create surface erosion, dirty water in streams and loss of fertilizer and herbicides.

Some rain enters the soil surface. This new water replenishes the soil moisture used by the plants. If it rains for a long time these air spaces may become full of water and additional rain will percolate deeper into the soil profile.

A zone of denser soil often occurs at some depth. Water percolating downward cannot pass through the denser soil as rapidly and a water table builds up on it. The water table might rise to the surface in some cases and create waterlogged situations.

In other cases the slope of the land permits water table to spill sideways and the water table will seldom reach the surface. The speed with which water moves down through a soil depends on the soil structure, the soil particle sizes present and the density, or compactness, of the soil.

Plants
Soil holds the plants erect. The soil is also a reservoir for the fertility used by the plants for growth, as well as a source for the biological activity. It holds the soil moisture that is used to dissolve the fertilizer.

Plant roots spread throughout the soil mass collecting soil moisture and fertility for the above ground plant growth. A plant depends on oxygen being present in the soil air spaces for growth. If the soil spaces are full of water, there is no oxygen and the plant, like a human, will drown of oxygen starvation. Water management is the maintenance of the correct air-water ratio in the soil spaces.

Soil Drainage
Water moves through the soil under the force of gravity. That is, water runs down hill. When a tile drain system is installed in a field all that has been done is to place porous pipe at a depth of 0.75 to 1 metre.

The drain pipe performs three things: it has sufficient structural strength that the soil around it will not collapse into the hole, it has slots or holes in the pipe wall or a crack between adjacent drain tile to let the surplus soil water drip into the pipe (water never enters through the dense pipe wall), it provides the outlet for the excess soil water and it conducts the water from the field to a satisfactory outfall.

The Figure illustrates how a drain works. The soil surface is marked as well as a dense layer of soil upon which a water table has built. The initial water table is noted by a carat (v). In undrained conditions the water table is nearly flat, but tends to follow the soil surface.

When a tile drain is installed an outlet is created for the surplus soil water which flows to it by gravity. (The drain does not "suck" water.) Because the distance from the water table to the drain is less, water closer to the drain drains away first and a curved water table develops. As time goes on the water table drops and the curvature changes, eventually becoming nearly flat at the level of the drain.

The tile drain cannot lower the water table any lower than the bottom of the tile. The tile will only run water when the water table has risen to a height above the tile. A tile drain will have no effect on the recharge of groundwater to wells. Since water flows to the tile by gravity there is no possibility of over draining a mineral soil, other than possibly a coarse sand.