301.1 Capillary Effect

When graywater is applied every day, the top 4 - 6” of soil reaches constant and wide area moisture.
For landscape beds this means we water the soil, instead of the plant. Dripperlines are installed to water the soil, not the plants directly.

Not many people think about how they apply water to the garden. We have been conditioned to apply water deeply only when the soil or a plant is thirsty / dry.

The reality is that watering everyday, with a little water, provides a much better outcome (both in saving water and providing better growth) than watering heavily twice or less per week.

Capillary effects are the key. Before reviewing different methods of using graywater, we will spend a few pages on what Capillary Irrigation is, and why it is the most important concept of all.

Capillary Action

Capillary action occurs when the forces binding a liquid together (cohesion and surface tension) and the forces attracting that bound liquid to another surface (adhesion) are greater than the force of gravity.

A simple way of observing this is to take a teaspoon of water and gently pour it in a pool on a countertop. You'll notice that the water stays together in the pool, rather than flattening out across the countertop. This happens because of cohesion and surface tension.

Cohesion is the attractive force that pulls similar substances together. In this case, the individual water droplets are being pulled together. The force of the pull is strongest at the edge of the pool. 
The water droplets at the edge have fewer neighboring water droplets, so they cling more tightly to those around them; this is known as surface tension. Now gently dip the corner of a paper towel in the pool of water. The water is attracted to the paper and "climbs" up the paper towel - this is capillary action.

capillary effect

Water behaves almost exactly the same way in soil.

  • If the soil is dry, the surface tension of the edge of the water pool prevents the water spreading outwards quickly.
  • The effect of gravity pulls the water down, into the root zone, and if there is more water than the roots can take straight away, it keeps going down, into the subsoil and much of it wasted.

To summarize, for soil, water moves slowly through dry soil, and relatively quickly through wet / moist soil.

Because graywater can be irrigated in the garden every day, the soil builds up moisture levels a significant distance from the immediate irrigation point. Water spreads quickly away from the irrigation point, avoiding issues with surface runoff.

Note that if the garden soil is very dry, surface runoff may occur. This is common in drought areas, where the soil is so dry it has become hydrophobic (i.e. repels water).

Soaps and surfactants in the graywater help to breakdown the water resistance, and surface runoff in most cases ceases within a day.

capillary effect three dripperlines

Wet / dry irrigation requires more concentrated dripperlines to enable water to soak into the soil without runoff.
In this example three dripperlines are needed in the garden bed.

capillary effect one dripperline

Moist soil requires only one irrigation dripperline for the same size garden bed.
When planning and installing a graywater irrigation system, remember to "water the garden bed, not the plant".

After a few weeks of graywater irrigation on a daily or every second day basis, the top 3" to 6" of soil becomes uniformly moist. Think of this zone as being similar to a water blanket. The water moves freely across the blanket, rather than going down into the subsoil.

Because less dripperlines are required, planning and installation is much easier.

Dripperlines do not need to be near the plant, and can be as far as 3' away (in loam / clay soil).

capillary effect layout
Capillary effect layout 2

The effects of capillary irrigation can be far reaching. The front lawn of my previous house in Australia had under turf graywater irrigation (using Netafim Bioline dripperlines). This lawn rose up 5 feet from the front of the house to the sidewalk, and then had a green strip between the sidewalk and the curb. This green strip is another 6" higher than the sidewalk.

Even without any irrigation or rain over the hot summer months, my green strip stayed green, while neighboring strips went brown.

Graywater is being sucked under the sidewalk, uphill into the green strip, all because of capillary action!