A weather station records changes in temperature,
solar radiation, wind speed and relative humidity. All of these factors affect
the amount of water lost due to evaporation and transpiration. That information is used
to calculate "PET" or Potential Evapotranspiration.
Austin Water Conservation staff use the PET amounts to calculate the 5-day ET Watering Index.
The PET amounts for
each of the past five days are added together and multiplied by coefficients to account
for plant stress tolerance and sun exposure.
For Austin's, ET amount, we use a coefficient of 0.6 for moderate-to-full sun exposure,
and 0.6 for plant type and stress tolerance. This
approximates a St. Augustine lawn, which is probably the most common for our area.
After the coefficients have been applied to PET, the resulting number is
rounded to the nearest increment to determine the ET Index in inches. In
cases when PET data is not available, staff estimates the ET index based on
weather conditions and available data.
WHAT ABOUT RAIN?
The ET Index published by the City of Austin's Water Conservation Department does not account for rainfall. Since our water
service area is so large, rainfall distribution can vary widely. The east side of town might receive half an inch of rain in a five-day
period while the southwest remains dry. To make sure your lawn gets the water it needs, we recommend you use a rain gauge to measure
how much rainfall your lawn actually receives in a 5-day period.
SMART ET CONTROLLERS
New "smart" controllers can do the work for you!
Learn more about how they use ET data to adjust your irrigation schedule.
