Automated Water Management


Summer, 2007
By: Steven E. Moore

Pipeline: Publication of the Florida Irrigation Society

Water agencies across the country are encouraging more efficient use of water in the landscape. A responsible homeowner responded to the message to conserve water founded by local water agencies. He felt he should do his part to reduce water waste, but he was passionate about his grounds and feared cutting back water use would sacrifice his beautiful landscape. Despite his fears, the homeowner began using the local water agency's, "Guide to Landscape Watering." Every week, he faithfully adjusted the sprinkler timer. Compared to neighboring properties, he used 20% less water. His landscape remained healthy and beautiful.

When "smart" controllers became available, the homeowner wanted to be part of a study to see if it could automate his weekly adjustments. He was fascinated with the smart controller's ability to use weather information to automatically adjust his watering schedule. Because he had already cut back on his water use, He didn't expect to save any water but was glad to see technology take over his weekly routine. He was delighted when he learned he was now using 47% less water compared to neighboring properties. At the conclusion of the two-year smart control study, he saved an additional 160,000 gallons of water.

How could a "smart" sprinkler controller reduce so much water waste without impacting the beauty and health of his landscape? There are three factors a quality smart controller uses to automate landscape water management:

Weather Conditions
The Landscape
Capabilities of the Sprinkler System

Weather Conditions
Weather conditions are the driving force that influence when the landscape needs to be watered. Evaporation and rainfall constantly change the amount of moisture in the soil. When rainfall is not enough, irrigation systems deliver the water landscapes need to remain healthy.
The term ET, short for evapotranspiration, describes water lost as a result of evaporation from the soil and transpiration from the plant. Sunshine and temperature cause water to evaporate from the plants and soil. Windy conditions accelerate evaporation. High humidity decreases the evaporative rate while low humidity causes the landscape to dry out faster. As the weather changes so does the need for water in the landscape.

Golf courses and large landscape water managers have been using weather stations for decades to automate irrigation systems. Weather stations specifically designed to calculate ET include reliable precision sensors to measure solar radiation, temperature, wind, humidity and rainfall. To calculate ET these precision instruments must be properly sited in the community. A weather station requires routine maintenance to assure accurate hourly measurements.

It is not practical for every property to have its' own weather station. Wireless technology provides a means to share data from well maintained precision weather stations in a community. An unlimited number of irrigation systems can receive a wireless broadcast of current weather information. A smart controller calculates ET from this wireless weather signal to provide accurate management of landscape irrigation systems.

In addition to the weather parameters used to calculate ET, rainfall must also be measured as it replaces evaporated water. In some parts of the country, rain plays the primary roll in replenishing lost soil moisture, while in arid climates rain supplements irrigation. True smart controllers measure rainfall and ET to control watering schedules.

The Landscape
Plants in the landscape depend on the soil for water, oxygen, nutrients and stability. Deep healthy roots mean beautiful healthy plants. Roots draw water from the soil reservoir.
Watering replenishes soil moisture. Root depth and soil composition limit the capacity of the soil reservoir. To promote a deep healthy root system, soil moisture must be depleted to draw air into the root zone. Best Management Practices, published by the Irrigation Association, suggest that moisture levels should be depleted by 50% before watering (IA BMP, 2005). The cycle of deep, less frequent watering promotes a deep healthy root system. If the plant root zone is kept at or near a saturated condition, the roots remain shallow because they are deprived of essential oxygen. Frequent shallow watering evaporates faster and does not promote deep healthy roots.

Each landscape is unique. Evaporative rates vary with plant type, exposure and planting density. Landscape adjustment factors are used to fine tune ET-based schedules to each of these conditions. Smart controllers allow water managers to program site-specific settings for precise automated control.

Capabilities of the Sprinkler System
Sprinkler systems deliver water to the landscape. Optimum irrigation applies enough water to refill the soil reservoir. Sprinkler valves must run for the correct amount of time to deliver needed water. To determine station run-time, sprinkler application rates and the optimum irrigation amount must be considered. For example, spray heads apply water at about 1.5" to 2" of water per hour. Sprinklers are not 100% efficient at evenly distributing water. To compensate for inefficiencies, station run-time is increased in relation to water distribution uniformity. For example, given an optimum irrigation amount of 0.50" the runtime for a spray head zone that applies water at 1.75" per hour with 75% efficiency should be about 20 minutes.

Watering schedules consider three things; station run-time, cycle start-time, and watering days. Station run-time is tied to optimum irrigation amounts and sprinkler performance. Cycle start times need to be set so watering is completed within the allotted time. More than one watering cycle will help minimize run-off and help water soak deep into the roots. Advanced smart controllers manage watering days in response to changing weather conditions. Local watering day restrictions and facility use may limit available watering days. If available watering days are limited, a truly smart controller will look-ahead to check if watering is turned off on the coming days and water if necessary.

Automated Water Management
When to water and how much is the challenge water managers face. The purpose of irrigation is to replenish depleted soil moisture. To keep roots healthy soil moisture should be depleted to an allowable level. The optimum irrigation amount is a deep watering to refill the soil reservoir. Station run-times are based on the amount of time needed to apply the optimum irrigation amount.

Watering is not needed until soil moisture is depleted to the allowable level. The rate of depletion is based on ET. For years water managers have used the "Checkbook Method" of irrigation scheduling which compares ET to a "withdrawal" of moisture from the soil moisture balance, while rainfall and irrigation are considered "deposits" (Smith, 1997). Once the "balance" reaches the allowed depletion level, "0", the irrigation system must make a "deposit" to replenish soil moisture.

The following chart demonstrates the application of the Checkbook Method. ET depletes soil moisture so it is deducted from the soil moisture balance. Rainfall and irrigation replenish soil moisture, adding to the moisture balance.

To assure that oxygen is drawn into the soil, moisture levels are depleted to an allowed level. Soil type and rooting depth are used to determine the allowed depletion and irrigation amount. In this example it was determined 0.50" was the optimum irrigation amount. The result is four deep watering cycles in the month.

The question of when and how much to water is answered. How much do we water? Enough to refill the soil reservoir. When do we water? When the soil moisture balance has been depleted to the allowed level, drawing essential oxygen into the roots. A smart controller makes this type of management a reality, assuring optimum plant health without water waste.

Automated water landscape management is not only benefiting homeowners, but landscapes all over the country are using less water without sacrificing the health of the landscape. The landscape irrigation industry is seeing the beginnings of a new era of automation.

Automated "smart" control systems require trained professionals who understand the dynamics of changing weather, sprinkler system performance, and plant needs. Automated landscape water management makes sprinkler control systems efficient by reacting to changing weather conditions and reducing water waste.