After The Rain

Irrigation provides supplemental precipitation – rain – to landscapes. But the water must reach its intended destination to be effective. What happens after the “rain” is critical.

It’s easy and convenient to think that precipitation in an arid environment is like precipitation anywhere else: rain falls to the ground and quickly sinks into the soil to nourish plant roots. At least that’s how it’s supposed to work… and so it does in most areas of the country. For those of us who’ve come to Arizona from other States (and that probably includes most of us), our assumption that gravity causes water to infiltrate and move through the soil is logical, and our experience has verified it.

Prior to coming to Arizona in 1990, I had lived and worked in areas with abundant water: the lowland hammocks and swamps of Florida, tropical rain forests in Central America, and the cypress-laden bayous of Louisiana. Arizona is more than just different; it is strange. Its alien desertscapes are hot and dry, seemingly devoid of water. Except in residential and commercial developments, where complex and expensive irrigation systems have been installed to supplement the meager natural precipitation.

Yet, it does rain here… sometimes, and in spurts. Arizona averages about 8 to 10 inches per year, spread out over two distinct climatic periods, known as monsoon seasons. The Arizona rain runs off into “washes”, low finger-like temporary passageways that mark the land, and then quickly flows away leaving precious little behind. As a result, within hours of the sun’s re-emergence, the desert returns to its normal dry condition. This is quite different from other environments, where rain is absorbed into the soil and held there before moving down into the water table or laterally into nearby waterways.

This is the desert, and as Dorothy famously said to her pup Toto in The Wizard of OZ “I have a feeling we’re not in Kansas anymore”. She was right, everything was different as she would discover.

So, water movement through arid soil is unlike other environments. Since irrigation systems are a mechanical technology deployed to supplement precipitation, are they affected by the arid soil?

Absolutely, and more than we realize.

Irrigation in the Desert

Irrigation involves more than just the electro-mechanical system that transports water to the site through pipes, valves, and emitters. The water must continue its journey through the soil to arrive at the root zone. Therefore, the soil’s capacity to receive, hold, and distribute water is critical. This makes the soil an essential component of a complete irrigation system. Just as the type, configuration, and functionality of the components in the mechanical system determine its effectiveness, the type and condition of the soil are factors that determine its effectiveness.

Given this relationship, the following items warrant careful consideration by professionals responsible for installing or maintaining irrigation systems:

• A complete and properly functional irrigation system includes two (2) phases that must work together: electro-mechanical (pipes, valves, emitters) and natural (soil).

• Pressure moves water through the mechanical system. Moisture potential moves water through non-saturated soil (force to move water from one place to another).

• Moisture potential is created by the attraction between water molecules and soil particles, and climate factors, such as solar radiation, temperature, humidity, and wind.

• Soil type and structure are critical factors that determine the effectiveness of irrigation.

• Soil compaction commonly found at commercial, residential, and other urbanized areas reduces the soil’s capacity to receive, hold, and distribute water.

• Compaction restricts infiltration of water and causes pooling on the surface, which exposes water to high evaporation losses.

• In arid environments water does not move primarily through the soil matrix, but via preferential pathways (cracks, macropores, animal tunnels, and along surfaces).

• Drip irrigation was developed for use on loose agricultural soils with low bulk density and seasonal plants with contained roots. Residential and commercial landscapes typically have compacted high bulk-density soils and perennial plants with expansive roots.

Due to the combination of these factors, water loss from irrigation technologies in urbanized arid environments are much higher than realized. Some estimate such loss to be approximately 70% or more. Our configuration and management of irrigation systems must take this into consideration to improve both their effectiveness and efficiency. The soil-water relationship has never been more important than now, as we endure a decades-long megadrought that is projected to continue into the foreseeable future. “Your

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