It is not practical to directly measure ET. So various state agencies have researched the correlation between weather data and ET. The picture below shows a weather station specifically designed to collect weather data for the purpose of estimating ET. This ET weather instrument is part of the California Irrigation Management Information Service (CIMIS).
- use of historical monthly averages of ET;
- broadcasting of ET measurements; and
- use of on-site sensors to track ET.
WeatherSet's ET controllers use on-site measurements of solar radiation, which is the dominant factor (85%) in ET computations, and on-site measurements of rainfall to adjust irrigation automatically to track weather....rain or shine !!! WeatherSet's ET controllers measure the SunFall™ and rainfall every second on your property which is as "real time" and "on-site" as it gets. There are no signal fees with WeatherSet products.
Use of historical ET to automatically change irrigation will save water. But, experience has shown that unusual weather will easily raise ET significantly above historical ET and stress the plants. Water is conserved, but plant stress is likely. AquaConserve and Weathermatic's SmartLines are ET controllers which use historical averages of ET for their watering decisions. AquaConserve and Weathermatic offer a temperature sensor to adjust these historical averages ET. Weathermatic calls its temperature sensor a "Weather Monitor" which may be shrinking the meaning of "weather." Temperature and humidity are the least important factors in ET.
Broadcasting or webcasting "real-time" ET data to remote controllers for a monthly fee are methods used by other controller. Controllers that use this method include
- WeatherTrak controller from HydroPoint,
- Intelli-Sense controller from Toro,
- SmartDial controller from Irritrol, and
- Weather Reach from IrriSoft.
The business model of these companies is similar to cable companies which insist on a monthly fee for the rest of your life for their broadcast service. These ET controllers require continuous monthly fees for proper functioning.
Business questions to consider before signing a contract to pay fees for ET signals:
- Will the signal be available over the life of your project? Business failures or acquisitions can result in loss of ET signals. Also, these private broadcasting companies can simply decide to pursue another line of business and cease broadcasting. The recent years have seen many such changes in American business.
- Look in their contracts and see whether there are "no fault" or "hold harmless" clauses to protect themselves if they end broadcasts of their ET signals to your controllers.
- solar radiation
- wind speed
- temperature
- humidity
On average, 85% of ET is solar radiation and 10% is wind. When there is no wind, then solar radiation is 90-95% of ET.
ET EveryWhere™ is trademark of HydroPoint Data Systems and SunFall™ is a trademark of Accurate WeatherSet.
