Some times people think of temperature for guiding irrigation. The California Department of Water Resources (CIMIS) require that the temperature sensor be mounted in open fields, away from buildings, asphalt, concrete, etc. as shown in the photo.

The PBS program NOVA reports that while global temperatures are rising, evapotranspiration is falling. Watch video clips from this NOVA program.

AquaConserve and Weathermatic are 2 companies using temperature sensors. Will these temperature sensors be placed on poles in grassy areas like that shown to the right? While these controllers do save water, they do not use the best input to modify their historical averages and their temperature sensors are likely to placed where convenient, not where they will make useful measurements. See the CIMIS link above for proper location for temperature sensors.

The chart to the right uses data from CIMIS station #78 for ETo and adds CIMIS #78 temperature data to the chart. Three parts of this chart as discussed below and are identified by the 3 vertical lines.

The first two vertical lines show 2 instances where the CIMIS temperatures were the same, and yet ET dropped differently, offering a great opportunity for 50% more water conservation. SunFall™ measurement in first chart responded. Temperature did not respond properly to changes in ET.

After the third vertical line, temperature and ET go in opposite direction. The blue ET line goes in the opposite directions from the red Temperature line, first jumping and then radically dropping, offering great opportunity of water conservation. The temperature responded in the wrong direction whereas the SunFall™ measurements responded properly.

CIMIS does use temperature in its computations. But, CIMIS combines temperature with relative humidity to add 5% to ET. Wind speed contributes about 10% to ET. The single best, on-site measurement for water conservation is SunFall™. At least, 85% of the ETo computation comes directly from measurements of Solar Radiation which we call SunFall™. When the air is calm, the SunFall™ is 90-95% of ET. Our low cost SFR sensor is the most practical and most convenient method of on-site weather-tracking for automatic adjustment of irrigation....rain or shine.

ET and Temperature

Actual ET was 32% higher than Historical ET for one month.
Temperature and humidity sensors should be placed in open fields, not on buildings or near asphalt and concrete.
SunFall™ measurements are not affected by buildings, asphalt, concrete, etc

The site requirements for CIMIS ET stations is 20 acres of grass. Then the temperature and humidity sensor get meaningful measurements of the landscape.