Typically, 50-75% of the water use at a home in the arid southwest goes to outdoor landscape irrigation. Therefore, it is very important that irrigation water is applied efficiently to support much needed water conservation.
Let's examine the science and the weather-based irrigation controller.
What Is Evapotranspiration?
Evapotranspiration (ET) is the sum of evaporation of water from the soil and transpiration of water through plant foliage into the atmosphere. ETo is the reference ET and is calculated using weather/climate data for specific regions and is available on a daily basis from various agencies. Daily ETo data for 28 locations across Arizona is available on-line at the University of Arizona (UA) Arizona Meteorological Network (AZMET).
How Does ETo Vary During The Year?
ETo varies widely throughout the year, especially in the Sonoran Desert with more ET occurring in the hot summer months. This chart (right) was developed by the UA Cooperative Extension Service and shows a monthly plot of ETo in Tucson. This ETo graph is for turf grass which is widely used as the ETo standard for crop and landscape irrigation.
The higher the ETo, the more water is required for irrigation. For Tucson, ETo in the summer is 5 times that in the winter months. For example, if an irrigation controller is set to irrigate a zone for 60 minutes in July, it only needs to be set for 12 minutes in January. But are all irrigation controllers being adjusted for ETo throughout the year? Unlikely!
Follow the ETo Curve & Save Our Precious Water Supply
Let’s look at the numbers. A typical Marana home uses about 98,000 gallons per year. If we assume that 50% of this water is used for outdoor landscape irrigation, that equates to 49,000 gallons per year. If a standard irrigation controller is set for summer watering and not adjusted for ETo throughout the year, it can result in wasting 25%-50% of the outdoor water. If we assume 25%, that means that 12,250 gallons per year per home is wasted. If just 200 homes installed weather-based irrigation controllers, that would save 2.5 million gallons of water per year. Amazing!
What Are Weather-Based Irrigation Controllers?
Many newer irrigation controllers have the ability to automatically control irrigation valve operating times based on weather data (ETo and precipitation). The weather data can be obtained three ways; (1) dedicated weather station for the controller, (2) ETo curve built into the controller for the region or (3) on-line internet weather data. The software/hardware in the irrigation controller utilizes the weather data from one of the sources and adjusts the watering times automatically. Some irrigation controllers will actually turn zones off for a specified number of days if it rains. There are many different manufacturers and types of weather-based irrigation controllers but one of the most popular and versatile units is described next.
Rain Bird ESP-TM2 Irrigation Controller & LNK2 WiFi Module
Many of the newer homes built in Marana have a Rain Bird ESP-TM2 Irrigation Controller (right) to control irrigation to front and backyard landscaping. This controller is unique in that you can install a small device called the LNK2 WiFi Module (left) for about $100 that will link the controller via a 2.4 Ghz WiFi internet connection to a Rain Bird Application (App) on your smart phone. You simply set the watering times at the irrigation controller for the highest summer irrigation and the Rain Bird App automatically adjusts the watering times throughout the year based on the weather data for that location. You can also turn watering off from your smartphone and monitor each zone. It really is amazing. This Rain Bird ESP-TM2 LNK2 WiFi Module Video shows how it all works.
Final Thoughts
Marana Water and Tucson Water should be pushing for the building codes to be updated to require weather-based irrigation controllers for all new construction. In addition, Marana Water and Tucson Water should offer incentives like rebates to replace or upgrade existing controllers to weather-based controllers. This should have been done years ago!
But don't wait for them, if you do not have a weather-based irrigation controller, consider buying and installing one like the Rain Bird ESP-TM2 ($120) and/or the LNK2 WiFi Module ($100) and follow the ETo curve and save our precious water resources.