Wise watering choices

There have been numerous studies that show fall as the season where the most over-irrigation occurs. Typically, summer or peak season irrigation schedules are not adjusted downward as the days become shorter, and the ambient temperatures drop. As the ratio of water applied increases as compared to crop need (a summer schedule irrigating in the fall), beside the obvious waste of water is the fact that when water moves, it transports nitrogen along with other pollutants or contaminants that may be present in the landscape. Cultural practices common in landscape management include frequent applications of nitrogen, and nitrogen-leaching losses are closely tied to the amount of applied water, which leaches below the root zone.

Water waste due to the difficulty in controlling the movement of the water as it floods the soil is the tip of the iceberg. When water becomes gravitational, it is either moving off the site as runoff or into groundwater via percolation. Spray irrigation hardware, with its inherent inefficiencies, adds to the problem.

It is helpful to understand the limitations of irrigation distribution hardware. For many years we have used fixed sprays, impact heads, rotors and multi-stream nozzles – spray irrigation hardware of various configurations. They all share key characteristics and constraints. They irrigate in circles or part circles – landscapes that have straight edges and often – rectilinear areas. So inevitably there is overspray onto non-landscape or a majority of a given area is over-irrigated, to get the least "overlapped" area an adequate minimum of water.

When spraying water on the surface, above the roots, we are seeking to flood the surface to the point where water will infiltrate, and move down with the force of gravity through the pore spaces in the soil. This downward percolation is gravitational water – water in such concentration that it can't be held by the soil and moves downward with the force of gravity. If thought about this way, it points out the high potential for waste inherent in spray irrigation. Although plants will certainly take up water while gravitational water is moving, the practical reality is most of the water is wasted to runoff and deep percolation.

Through scheduling, we are attempting to time the "right amount" of water to infiltrate down through the root zone and stop at the lower perimeter of the root zone.

What we are trying to irrigate is a root zone. If we thought of the whole block of soil around the roots as a reservoir, than we could think of irrigation as an act of sustaining the reservoir in ways that water is available to the plant. The trick is to fill the reservoir (root zone) in a way that encourages and produces more and deeper roots. To create a vibrant and prolific root zone, an ideal ratio of air pores, moisture and nutrients has to be sustained.

To do that takes a basic understanding of four different issues, those being plant water requirement, weather, irrigation hardware, and last and perhaps foremost – soil, and how water is held by and moves through soil.

To state the obvious, plants have different water requirements. Any given plant has a water requirement that changes through the year in direct correlation to the length of the day. So plant water requirement is a moving target, moving in a bell shaped curve on an axis that has January and its short days on the left, peak season (typically July) in the middle, and December on the right. There are several drivers to plant water requirement – but keeping it simple – they are plant genetics, length of day, the strength of sunlight or heat and wind.

What this means for scheduling is if you are using a schedule that keeps your landscape thriving in July, any other time of the year – and particularly in fall as the days are shorter, you are wasting water. In addition to wasting water, over irrigation creates gravitational water, which has a high potential for transporting dissolved substances such as nitrates or pesticides by surface runoff or percolation through the soil into ground water.

For our industry to grow, it may need to embrace irrigation without flooding or creating excess gravitational water. We need to focus on supplying the right amount of water in the right way to supplement what nature may provide through rain or groundwater. Fortunately, there is a new class of technology available that is a significant improvement over existing sub-surface drip irrigation and has gained acceptance around the world.

The new technology incorporates textiles along with pre-embedded emitter tubing to create highly uniform wetting of the soil. The tubing supplies a low volume of water; the textile distributes it in a highly uniform manner. Unlike spray irrigation, which floods localized areas to create gravitational water, sub-surface textile irrigation leverages the principle of capillarity to move water upward though the root zone. The soil simply does not "take up" more than it can hold – therefore no flooding or gravitational water loss.

Since there is no flooding, an ideal soil, air pore and water relationship exists – which promotes vigorous root development. These unfailing principles at work in nature, combined with reliable distribution hardware, provide the opportunity for extremely high irrigation efficiencies.

Regardless of the irrigation hardware used, how much water a given soil can hold, and how long to irrigate before water becomes gravitational water is "knowable." The California Landscape Contractors Association developed the Water Management Certification Program. which teaches science-based concepts in a practical manner. With training like this, and new high efficiency sub-surface textile irrigation hardware, the green industry will continue to evolve and provide the sustainable solutions communities seek.

The author is national accounts director at Irrigation Water Technologies America and is a Lawn & Landscape columnist. He can be reached at jossa@giemedia.com.