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WEATHER

WHAT IS ET? ET stands for "evapotranspiration" and is a combination of evaporation from soil and exposed surfaces and transpiration, which is the normal release of moisture during everyday metabolism of plants. ET for the purposes of use in the Big Bear Valley, tends to be lowest in spring and fall and highest in mid- to late summer. People tend to use fixed run-times (15, 20, 25 minutes, etc.) when setting their irrigation controllers (usually turned on in spring, off in fall). Thus, even if the run times are set perfectly for peak summer ET's (which they rarely are), the amount of wasted water during the cooler spring and fall is considerable. The following is a summary of the amount of over-watering associated with typical run-times compared to ET run-times estimated for turf using weather variables collected at DWP's weather station during the spring, summer, and fall of 2003. Typical Sprinkler Run-times 15 min 20 min 25 min 30 min Percent Over-watered Compared to ET-based Run-times 36.3% 81.7% 127.1% 172.5% ET controllers are designed to water efficiently by providing plants with only the water lost during the previous day's evapotranspiration, which is estimated by the previous day's weather or by seasonal averages (different ET controller systems use slightly different estimates of ET). Thus, substantial water savings are possible using ET irrigation systems and the DWP will be promoting their use during the upcoming summer. This page contains an estimate of the previous day's ET along with weather variables used to predict ET. If you would like more information about ET or any of the DWP's water conservation programs, please contact us at (909) 866-5050.