Growing Magazine - July, 2014

IRRIGATION & WATER MANAGEMENT

Intelligent Irrigation

Reduce water use with technology and scheduling
By Tamara Scully


Orchard bubbler irrigation in Pakistan.
Photos courtesy of Joel Schneekloth.

On commercial farms, the use of irrigation may be a necessary tool, but it's one with many variables. Using an irrigation system properly is the key to maximizing efficiency. According to the U.S. Geological Survey, over one-third of our nation's available water was withdrawn for irrigation purposes. Conserving this limited resource is crucial, as drought has become a real concern in many regions.

A variety of technology exists, from basic moisture sensors to state-of-the-art computer-programmed wireless irrigation systems, to help growers get maximum yields with minimal water use.

Water loss

"What you really want is the transpiration, but you can never eliminate the evaporation," said Joel Schneekloth, regional water resource specialist for Colorado State University.

Evaporative losses are going to occur when irrigating crops. Minimizing those losses is one simple step growers can take to increase irrigation efficiency and conserve water. Increasing the soil's water infiltration rate is one way to decrease evaporative loss. No-till techniques decrease disturbance of the soil structure, and good soil texture translates to better water infiltration.

"All the soil is, is a vessel to hold the water and the nutrients for your crops," Schneekloth said. "Good soil texture really helps out with your soil management."

Using plastic or organic mulches will help the soil to retain water, decreasing evaporative water loss and the need for irrigation. He said that watering less frequently and watering deeply will increase the efficiency of any irrigation system.

Over and underwatering are equally bad. Too much water leads to nutrients leaching out of the soil, erosion and saturated roots. With too little water, the plant can't transpire. Keeping the soil moisture constant, rather than going to extremes, is critical.

Different crops have varying water requirements. Soils differ, not only across regions or from farm to farm, but within the same fields or even rows. Inevitably, there is always a spot with a low holding capacity. Since that spot requires additional water, the entire field will receive water that isn't needed. Targeted zone irrigation would eliminate that problem. While this type of system requires money, time and labor, it would maximize water efficiency.

"Zone irrigation would be ideal," Schneekloth said.

While such a system is out of reach for most growers, there are other ways to optimize water use. Properly monitoring soil moisture and adjusting irrigation to site-specific soil and crop characteristics is the path to efficient and effective irrigation management.

"The biggest water saver that's out there is actively doing irrigation scheduling," Schneekloth said.

Irrigation scheduling

"To schedule irrigations efficiently, the irrigator needs to consider the type, size and root depth of the plant; the weather; the soil type; and the precipitation rate of the irrigation system," said Dan Smeal, irrigation researcher and professor at New Mexico State University's Agricultural Science Center. "Efficient irrigation scheduling provides plants with sufficient water for [the] desired production or quality while minimizing water losses through runoff or deep drainage below the crop root zone."

Scheduling irrigation to get the most from your plants while conserving water takes a multifaceted approach. Several methods exist.

"In climate-based scheduling, plant water use, or evapotranspiration [ET], is estimated based on a reference ET calculated from weather data, with adjustment factors to account for plant type, size and growth stage," Smeal explained. "In soil moisture-based scheduling, soil sensors are used to monitor the moisture content of the soil, and irrigations are applied when a certain critical soil moisture level is reached."

ET rates vary by location, by crop and with the weather. Factors that impact ET are solar radiation, air temperature, wind speed and humidity. State or regional ET data is typically calculated and available from official weather monitoring stations. Individual on-farm weather monitoring stations are available and vary in cost.

Expressed in inches per day, 1 inch of ET means that growers would need 27,154 gallons of water per acre to replace the amount of water lost to the atmosphere. Compare the ET rates with the predicted rainfall amounts and irrigate only to meet the crop's specific needs.

"Smart irrigation controllers that estimate crop water needs using climate data from weather station networks and internally programmed crop coefficients are available that can turn on and off irrigation valves," Smeal noted.

Soil moisture monitoring is another piece of the irrigation scheduling puzzle. From the low-tech approach - feeling the soil and seeing if it clumps - to hand-held probes, remote monitors, or monitors linked to an irrigation timer or controller, measuring the soil moisture level and adjusting irrigation scheduling accordingly are good management practices.

However, simply knowing the soil moisture level isn't the entire story. Soil has a holding capacity - the point at which it is full of water, but none is running off. Typically called the field capacity, growers often mistakenly use it to determine if they should irrigate, but if a field is irrigated to its capacity, any rainfall will be lost.

Field capacity, minus the wilting point of the soil, provides a measure of plant-available water. This is the critical measurement for irrigation purposes. At the wilting point, a plant cannot retrieve any more moisture - even though it is there - from the soil. The difference between the field capacity and the wilting point is the plant-available water. The ideal soil will have a high holding capacity and a low wilting point, a combination that makes the most water available to the crop for growth. Knowing this measurement allows more accurate irrigation than the soil field capacity and prevents water waste.



Intensive drip irrigation in a vineyard in Pakistan.

Soil moisture measurements can be taken with a variety of tools, from basic to complex. These include a gypsum block sensor that uses electrodes to measure electrical resistance in the soil, which can then be translated to water pressure measurements. A tensiometer measures the water as a function of pressure. A plant's roots use capillary action to move water up the stem. The soil tension indicates how much pressure a root system must have to move the water from the soil.

Dielectric sensors use electronics to measure the dielectric constant, which is highly dependent on the soil's moisture content. Capacitance sensors and time-domain reflectometry probes are both dielectric sensors. Correct calibration for the soil is necessary when using these probes. They can be left in place to continuously log readings.

Today's probes are available in single-level or multilevel configurations. Connecting moisture probes to an irrigation system allows for implementation of automated watering based on actual plant-available water measurements, ET rates and up-to-the-minute weather data. Such systems can be monitored remotely.

"There are varying levels of sophistication," Schneekloth said. Some systems must be actively monitored, while others can "download information off of the Internet after they are installed in your fields."

Effective irrigation systems

"Efficient irrigation design and efficient irrigation scheduling are the two most important keys to wise irrigation management," Smeal said. "There is no substitution for efficient irrigation system design and maintenance."

The same irrigation system can produce different results depending on how it's managed. The various types of irrigation systems - drip, overhead, furrow - have differing management concerns.

For vegetable production, drip irrigation is best; it applies small, precise amounts of water directly to the root zone, where it is best utilized. Sprinkler systems put water in between the rows, where it doesn't help plant growth, and are more wasteful. Overhead watering can cause fungus and disease issues.

With any system, maintenance is required. Brass nozzles in sprinkler systems have a shelf life of about 7,000 hours. As they wear, the system distributes water unevenly, which decreases accuracy and thus efficiency. Drip system emitters can plug, causing pressure issues. Even in a system with the best scheduling, improper maintenance can cause issues.

"All malfunctioning components should be repaired or replaced, and all leaks should be fixed," Smeal said. "To do otherwise can upset the intended flow rate or pressure characteristics of the system, resulting in adverse effects on water application uniformity."

Irrigation systems can be adjusted to vary pressure, flow, run time or application rates. Sprinkler systems can be controlled for direction. Sprinkler and drip systems can be manually controlled, offer basic automation, or provide varying degrees of computerized, wireless remote control. Manual control panels allow some adjustments to be made, such as to pivot sprinkler systems, without having to physically adjust the machine, while high-tech systems offer graphics and programmable adjustments to fit any field's exact needs.

"In a properly designed sprinkler irrigation system, a near equal depth of water should be applied to every section of a cropped field during a given run time," Smeal said. "Likewise, each emitter of an efficiently designed drip zone that irrigates the same crop should have an equal flow rate."

Uniformity can be impacted by sprinklers that are improperly spaced or have improper nozzle configurations. To achieve equal pressure along the system, it's important to keep laterals as short as possible and laid out along the contour of any slopes.

"Sensor/valve combinations that can automatically turn off water when leaks are detected from increased flow rate or decreased pressure are also important in preventing water waste," Smeal added.

Surge irrigation, a type of furrow irrigation, is focused on water conservation. Surge irrigation utilizes a surge controller butterfly valve to cause irrigation water to be oscillated at intervals, rather than with a continuous flow. This reduces the water intake rate of the soil, results in uniformity of application, and ultimately reduces water use from traditional furrow irrigation systems. Surge valves have controllers that allow the grower to choose the duration of the irrigation oscillations from one side of the field to the other.

Energy efficiency

Conserving water is one part of irrigation efficiency. As the use of water in any system is decreased through appropriate irrigation scheduling, the energy used for irrigation in that system is likewise decreased. Beyond that, there are some other ways to maximize energy conservation while minimizing water waste.

Collecting runoff from rainwater and storing it to use in an irrigation system is one option; this may be more practical for smaller systems. Rainwater harvesting can augment groundwater supplies as well as reduce stormwater flow and contamination from runoff. Water from roofs is filtered, collected and stored, and the stored water is pumped from the tank into an irrigation system.

An option to decrease overall energy use is to run the irrigation system on solar power or a combination of solar and wind. There are a variety of solar-powered pump options. Good irrigation system design is essential to getting the best performance out of a solar irrigation pump.

No matter what choice you make for irrigating your crops, managing the system you have makes a difference. Intelligent use of irrigation, whether delivered by the most sophisticated system or the simplest, can have a major impact on water use.

"I can have the worst technology out there, and if I want to put the time and energy into managing it, it can be the most efficient," Schneekloth said. Even the best technology won't be effective without proper management.

Whether you opt for a high-tech, fully computerized mobile irrigation monitoring and management system or are seeking lower-cost options to better manage your irrigation scheduling, today's irrigation equipment can help you reduce water use. These tools also save you money by increasing plant health, reducing energy use and making every drop count.

The author is a freelance contributor based in New Jersey. Comment or question? Visit http://www.farmingforumsite.com and join in the discussions.