Stacia L. Davis Conger and Jeffrey S. Beasley

Louisiana’s hot, humid climate presents substantial opportunity to take advantage of natural rainfall, but factors such as rainfall frequency, variability and intensity can often reduce soil moisture retention within the shallow root zone of the landscape. Combined with increasingly intense and frequent short-term droughts affected by a changing climate, irrigation can aid in sustaining healthy and vibrant ornamental plant material by supplementing deficits in soil water status to maintain well-watered conditions when rainfall is insufficient.

In contrast, scheduling irrigation during rainy periods requires engagement; applying too much or too often can cause a variety of poor economic and environmental outcomes such as leaching of nutrients, erosion of topsoil, surface water runoff or creating areas of standing water that can breed mosquitoes and other pests. Thus, irrigation systems require responsible operation with consistent dynamic scheduling to be sustainable.

Each irrigation application must consider evapotranspiration in addition to rainfall. Evapotranspiration drives the water cycle as a representation of the water leaving the root zone from the combination of evaporation from the soil surface and transpiration through the plant material. It is calculated based on fluctuating weather variables such as temperature, relative humidity, solar radiation and wind speed. However, system owners rarely have the knowledge and resources to calculate the irrigation requirement themselves. Further, manually adjusting the irrigation schedule on a daily, monthly or even seasonal frequency is problematic for modern lifestyles when the time clock is inconveniently located outside or in a garage.

With irrigation of ornamental landscapes labeled as nonvital, some utility-driven water conservation programs have restricted or banned its use to help protect critical water supplies. Realizing the economic consequences, the irrigation industry responded by developing products termed “smart technologies” that can automatically adjust irrigation schedules by adapting to environmental changes. Weather-based irrigation controllers and soil moisture sensors are the two primary products available in today’s irrigation market.

The weather-based irrigation controllers technologies share the concept of using evapotranspiration estimations to adjust irrigation schedules, but the market remains diverse with proprietary differences in how evapotranspiration and rainfall are estimated and used for scheduling. The most simplistic version of weather-based irrigation controllers uses a seasonal adjustment approach to automatically adjust initially programmed runtimes that represent the maximum expected irrigation need for July or August to account for changes in weather throughout the year. These products have an external sensor that estimates weather parameters (e.g., air temperature) to scale the irrigation schedule. This option functions most similarly to a traditional time clock; the day-of-the-week schedule remains firm with real-time adjustments to the programmed runtime for each zone.

Other weather-based irrigation controllers use an estimated daily evapotranspiration value to calculate the irrigation requirement for each zone using the water balance approach. These devices maintain a running total of available water within the root zone by keeping track of evapotranspiration and rainfall. Irrigation is applied when the plant material is no longer well-watered. Each zone is operated independently under an individualized water balance to meet specific irrigation requirements. As the more advanced option, the ability to obtain reliable evapotranspiration and rainfall data is key to performance in a humid climate such as Louisiana. Initial versions required a simplified onsite weather station or cellular data subscription fee to obtain daily evapotranspiration and rainfall information; today, these products utilize existing Wi-Fi networks and utilize apps for remote access to the controller.

The soil moisture sensors technologies work in conjunction with an irrigation time clock to bypass user-programmed irrigation events when soil moisture is adequate. Each product includes a soil moisture sensor buried within the root zone of the landscape and a controlling device that handles communication between the sensor and the time clock. When the time clock initiates an irrigation event, the sensor reports the current soil moisture status to the controlling device. The controlling device is programmed with a moisture bypass threshold, resulting in the prevention of the event if the sensor measures above the threshold or allowance of the event if the sensor measures below the threshold. Like the seasonal adjustment approach for weather-based irrigation controllers, the time clock should be programmed to accommodate the irrigation requirement of a high evapotranspiration scenario. However, the schedule should be broken into multiple increments per day or week so that portions of the event can be skipped when soil moisture is sufficient for plant health.

Overall performance has been largely positive but with inconsistent water savings due to both technologies’ dependence on user programming and operation. In Florida, a similar rain-dominated climate, water savings in research settings averaged 43% by weather-based irrigation controllers and 56% by soil moisture sensors under drought conditions with up to 92% by soil moisture sensors under rainy conditions. When research transitioned to single-family residential landscapes, both technologies continued to significantly reduce irrigation application by 10%-50% when compared to their over-irrigating neighbors with complimentary landscapes.

Selecting between these two technologies references the current irrigation habits and needs of the irrigator. Specifically, weather-based irrigation controllers select runtimes for each zone and can increase irrigation applications of conservative irrigators. However, soil moisture sensors cannot increase irrigation beyond programmed runtimes but instead bypass unnecessary irrigation events. Alternatively, terrain or infrastructure obstacles may render soil moisture sensor installation too inconvenient, making weather-based irrigation controllers a better option. It is important that irrigators understand the technologies available so that we can meet the challenges of water use in Louisiana responsibly.

Stacia L. Davis Conger is an assistant professor at the LSU AgCenter Red River Research Station, and Jeffrey S. Beasley is a professor in the LSU School of Plant, Environmental and Soil Sciences in Baton Rouge.

Combined with increasingly intense and frequent short-term droughts affected by a changing climate, irrigation can aid in sustaining healthy and vibrant ornamental plant material by supplementing deficits in soil water status to maintain well-watered conditions when rainfall is insufficient. Photo courtesy of Institute of Food and Agricultural Sciences - University of Florida

An example of a weather-based irrigation controller, the Rachio obtains evapotranspiration and rainfall information regularly through the home Wi-Fi network. Interaction with the controller, including additional programming and setup of each zone, is accomplished through a smart phone app. Photo courtesy of Hunter Industries Incorporated.

Soil moisture sensors for home landscapes include the sensor itself (on right) that wires back to the closest solenoid valve and a sensor controller (on left) that interacts with the irrigation time clock. Photo courtesy of Hunter Industries Incorporated.