"Smart Controllers" are controllers that automatically update the watering schedule to allow for changes in water needs throughout the year. So a smart controller will automatically reduce the watering times as the weather gets cooler and less water is needed. Then as the weather begins to warm up, the controller will add more watering time. The way this typically works is that you set the controller for a default maximum watering time, based on the hottest time of year. Then the controller reduces that time amount by a percentage value when less water is needed. There are several methods used by different controllers to determine how much to reduce the watering time. Some controllers may allow for use of more than one method.
Here's a list of the common methods used by smart controllers to determine the watering time:
Uses historical weather and water data for your area to determine what amount of water is required. Typically it only resets the time monthly. While the historic data is not perfect, it still gives significant water savings for most users. You will periodically need to manually override the automatic controller settings, especially if you have unusually hot weather for the month. To setup the controller on some models you simply enter your zip code and it accesses the historic data from it's memory. On others you have to initially key in the historic data from the user's manual or a website. Due to the lower cost of this type of smart controller, often it will give you the best financial return on your investment. This is especially true for a small residential irrigation system.
Historical with a Sensor
Uses historical data to determine an initial reduction in watering time, but then further adjusts the time based on a sensor. Typically a temperature sensor is used. If the daily high temperature is higher than the historical data says is normal, it adds more time, if the temperature is lower, it reduces the watering time. This gives more accuracy that the historic data alone will.
Uses water and/or weather data provided by a remote provider. The controller uses a radio, Internet, or phone connection to obtain the data from either a central data provider, or from a local weather station. Generally there is a subscription charge for the data service and there may also be charges for the telephone, Internet, or radio link. Accuracy is dependent on where the data is obtained from (garbage in, garbage out). If the data comes from a nearby weather station it can be very accurate. If it comes from a central data base of historic data that is expertly manipulated for current conditions it can be very accurate also. So with this type of smart controller you need to ask the provider exactly where the data would come from for a controller installed at your location.
This controller has it's own weather station that you install with it. It uses real-time data from the weather station to adjust the watering times.
A moisture sensor (often more than one) is placed under the irrigation system to measure the actual amount of moisture in the soil. The irrigation time is based on the amount of moisture present. There are several types of sensors available. Some require regular maintenance, others do not. There are advantages and disadvantages to each type of sensor. Do some research, as different types of sensors work better with some types of soils than they do with others. Be prepared to spend some time calibrating the sensors and making adjustments. With that said, this is a very accurate method of determining watering times.