Weather-based irrigation systems use automatic controllers based on computer technology to schedule watering according to the needs of the plants, historic weather patterns, temperature and the moisture in the soil. The extraordinary savings in water from using these systems has encouraged many municipal and county governments to give credits, rebates and other subsidies to encourage their use.
Automatic controllers can be added to hoses and pipes of sprinklers or drip irrigation systems for lawns, turf, gardens and fields. Irrigation systems based on the clock do not take into account wet weather or a hot spell. A weather-based system can take all the relevant facts into account, including the needs of different kinds of plants. Plants are watered when they need it and in the amounts they need, and water is not wasted. When plants are in a stage of growth where they need more water or the level of moisture in the soil is low, controllers add water. When the weather is cool in the winter and plants are dormant and need little water, controllers cut back on irrigation.
Data of weather historical weather patterns in a particular ZIP code help some controllers decide how much water is needed and when. Each month, these controllers adjust watering based on month-to-month needs in the past. Some controllers base the decision on historical data plus a temperature sensor. If the daily high temperatures are exceeding the historic normal, the controller adds more irrigation time; if the temperature is lower than the average, the controller cuts back on the water.
Some controllers adjust irrigation according to real-time weather data so that cloudy weather, heat waves, unexpected rain and storms may be taken into account. Some models have thermometers and rain sensors. More sophisticated models rely on weather stations and Internet services that charge a fee for beaming real-time data about current and anticipated weather via the Internet, telephone or radio. Data from local weather stations are ordinarily more accurate than national sources.
Controllers can also base irrigation time on the actual amount of moisture in the soil as recorded by special sensors. Many models take into account the soil characteristics, whether or not it drains well and the slope of the ground.
The Environmental Protection Agency has developed standards for a WaterSense label to identify those weather-based irrigation controllers that take care of plant needs without using too much water. Third parties test and certify controllers to see if they meet EPA standards. Qualifying controllers carry a WaterSense label. EPA WaterSense labels are also applied to toilets, bathroom faucets, shower heads, urinals and other household products that use water.