Duration, frequency determine irrigation efficiency

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Many homeowners irrigate too often and for too short a period to meet lawn and especially landscaping (tree and shrub) needs and often compromise the health and vigor of their landscape’s plant community.

Others tend to leave the water running too long resulting in wasted water and high water bills. Properly adjusting automatic watering systems is an important practice generally overlooked or often incorrectly done.

Irrigation requirements are a function of four things: plant adaptation, soil properties, precipitation and evapotranspiration rate. Any of these factors can become the most critical component of an irrigation strategy’s success or failure. There are plants adapted to dry environments while others are adapted to more moist environments.

Soils can vary tremendously in their effective depth and composition (of clays, silts, sands and organic content). These factors affect the amount of water holding capacity. Coarse (sandy or gravelly) soils do not hold as much water as finer (silt or clay) soils. Water infiltration is quicker, and water may move beyond root zones in the coarse soils. Water holding capacity determines the relative length of time between irrigations. Infiltration rate affects the speed at which we can effectively apply water.

Evapotranspiration is a combination of the disappearance of moisture from the soil through surface evaporation and the consumption of soil moisture by plant transpiration. Evapotranspiration is affected by many factors including temperature, wind, slope, aspect and relative humidity. Evapotranspiration rates will vary over the course of the year.

The amount of moisture available in the root zone (0-12 inches) of most landscape plants, when balanced against the evapotranspiration rate, indicates most lawns only require irrigation once every four to eight days to stay healthy. This prediction should be periodically fine-tuned depending upon observed weather conditions and, particularly, sprinkler zone variations in aspect, slope, shading and distribution efficiencies.

Severe conditions could double evapotranspiration rates. Providing periodic irrigation sufficient to thoroughly wet the top 12 inches of the soil profile guarantees deep percolation sufficient to meet the needs of landscape plants like shrubs or even trees. Allowing depletion of water in the soil profile (not quite to the point of wilting small-stature plants) provides oxygenation to the soil’s micro-flora and fauna and encourages plants to extend root systems deeper into the soil.

Shallow-rooted plants result from irrigating every day. Irrigating less often and applying more water per irrigation results in deeper-rooted plants and healthier turf. Plant roots grow deeper into the soil and the plants become more vigorous if enough water is applied when you do irrigate. Deeper-rooted plants use water and nutrients from a larger volume of soil and are well-prepared to withstand occasional neglect and short-term drought conditions (or system outages).

The system’s application rate is easily determined by placing something as simple as empty soup cans in the sprinkler zone and measuring the time taken to accumulate a measured depth of water.

Automatic irrigation system owners should change controller run times to meet seasonal plant needs. Some controllers allow watering a percentage of peak summer run time settings. With one setting change, systems can easily be reset to 60 percent for spring and fall watering. Practicing seasonal adjustments conserves water and results in healthier plants by matching water application to plant needs. Too much water is often applied to clay soils depriving plant roots of the oxygen needed to function. Excess watering also tends to leach available nitrogen through and beyond the root zone. Applying the right amount of water produces healthier roots.


Editor’s Note: Trade or brand names used in this publication are used only for the purpose of educational information. The information given herein is supplied with the understanding that no discrimination is intended, and no endorsement information of products by the University of Wyoming Extension is implied. Nor does it imply approval of products to the exclusion of others, which may also be suitable. The University of Wyoming is an equal opportunity/affirmative action institution.


Scott Hininger is with the Sheridan County Extension office.


By |August 2nd, 2013|

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