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Understanding Sustainable Irrigation
By David Mandel, M.L.A., A.S.L.A.

According to the Irrigation Association, the amount of water to apply to maintain a healthy and functional landscape can be quantified. It is a scientific calculation based on the type of hydrozones, evapotranspiration data adjusted with appropriate plant factors, site rainfall, and additional considerations such as extra water needed to leach any salt accumulation due to poor water quality.

Weather-based irrigation controllers are designed to save water by setting irrigation routines based on plant need rather than a time schedule. The systems take into account the types of turf, soil and sprinklers to optimize water efficiency. They can also rely on soil soak cycles, that is, irrigation of a zone long enough to allow the soil to absorb the moisture before automatically moving to another zone.

The emphasis is on balance: we cannot put protection of the environment over our need to survive; we also cannot put current profits over future generations' needs if our children are to survive.

Because water is an absolute necessity for human life, irrigation professionals have a critical role in assuring the well being of this generation and of those to come.

Within the general concept of earth's hydrologic systems, one major model is becoming widely accepted in science, planning, and design as the critical unit: the watershed. Cost and maintenance of land uses are dependent on the opportunities and constraints of each watershed.

You will probably plan and design functionally better and sustainable irrigation systems if you seek an understanding of the watershed in which your project is to occur, as well as the watershed that will supply your project. We need, then, to consider four irrigation-related watershed categories, which we can call source, passage, consumption, and deposition watersheds. All four types share certain characteristic responses to irrigation, and each has some unique responses.

The first and most obvious category is the source watershed. Source watersheds are typically rural and relatively pristine. It is because minimal human development and use has occurred in the source watershed that we rely on it as a clean and bountiful water source. Yet human land uses downstream can have major impacts on the source watershed without our once entering it.

The second watershed category is one seldom considered. The passage watershed is typically impacted by irrigation canals or pipelines. While the pipelines may be underground, the construction process and the aboveground control and pumping equipment can be disruptive. Canals and aboveground pipelines create barriers to species migration, while above-ground equipment can disrupt breeding, feeding, or rest areas. Because migrating populations are frequently major pollination agents, everything from crops to shade tree cover can be affected by barriers.

In Seattle, which receives an average of 39 inches of precipitation annually, summer consumption of water is 30 percent more than in the winters. Research has shown that this increase is almost entirely attributable to residential irrigation.

"De-watering," using more water from sources for irrigation and other needs than is returned to the sources by man and nature, has been directly linked to large-scale damage including, according to the Florida Department of Environmental Protection, pollution in the Florida Everglades.

The consumption watershed typically has the greatest number of human land uses and is likely to suffer the greatest disruptions to natural systems. Yet the consumption watershed also gives us an opportunity to balance the positive results of irrigation against disruptive effects. While agricultural irrigators may be the prime users of waters from and in some watersheds and aquifers, non-agricultural irrigation can be a significant - if not the exclusive - usage in others. Corporate campuses, parks, residences, institutions, and commercial properties use significant amounts of irrigation water.

The final category, the deposition watershed, was historically ignored until ignorance became too costly. When we pump fertilizer and pesticides into water borrowed for irrigation from the Colorado, Sacramento, Columbia, Mississippi, Savannah, Cimarron, and Red Rivers, the salt-laden run-off returns to those channels and ultimately affects algal blooms and commercial fisheries in the coastal oceans.

Recharging the Sources - Effective Cases
Texas Tech University researchers found that placing filters in shallow trenches in playa lake bottoms and draining overflow runoff from the lakes to nearby wells may increase the amount of recharge available to the Ogallala Aquifer (editor's note - cited as one of the world's largest aquifers, the Ogallala's expanse is approximately 174,000 square miles in portions of South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas).

A study by the U.S. Geological Survey (USGS, 1998) reports that water levels in the Ogallala Aquifer actually rose in some areas. The rise is attributed to above average rainfall, a decrease in the total amount of irrigated acreage, and a decline in the amount of water pumped for irrigation.

Drainage swale patterns and recharge wells have been successfully used for city-wide urban runoff in Davis, Calif. Detention ponds for water-table recharge are mandated and in widespread use in several states, notably Washington and California.

Live and Learn
It would be easiest to just quote the old Hawaiian proverb, "E malama I ka wai": cherish the water. However, we will only achieve sustainability of our irrigation waters through seeking greater knowledge of watershed needs.

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August 25, 2019, 5:39 am PDT

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