Texas A&M University Agricultural Research & Extension Center at Overton

Potential Evapotranspiration

Potential evapotranspiration, or PET, represents the combined loss of water through:  1) the plant's process of transpiration via it's vascular system, and 2) evaporation of water from the earth's surface.  Both are influenced by temperature, humidity, sunlight, and wind.  PET values indicate the amount of water that has been lost, and thus needs to be replaced, through irrigation and/or rainfall. 

PET data is used to calculate the water requirements of crops and landscape plants, and adjust irrigation schedules in a way that encourages efficient water use, reduces waste, and saves money.  Reduction in over-watering also reduces pollution from fertilizer and chemical run-off into waterways.  With projected increases in population and water demand, water conservation is rapidly becoming an issue in all parts of Texas, not just regions of low rainfall.  Conservation methods must be implemented now, as one method of addressing future demands.

Tip:  PET values will be lower on cloudy, cool, or rainy days compared to sunny, warm days with low humidity.  Wind will increase PET values, because evapotranspiration rates are higher.

For more info on being "Water Smart",
check out this guide to Outdoor Water Use.

WaterWise Landscape Irrigation Calculator

Historical PET Data at Overton

For example, if you water your yard
once a week, add up the PET values
for the past seven days.  This is the
amount of water you need to put out
when irrigating.  If it has rained during
those seven days, subtract the total
amount of rainfall from the PET total
for the same time period.  This will be
your correct PET value, adjusted for
rainfall.  View historical PET data here.
 

NOTE:  The PET table on the right displays
two PET columns, TxET and Ovt.  These two
sets have been derived from the same
data - collected here at the Overton Center -
but calculated using two different versions
of the PET formula.  While the TxET formula
is more current (and also used on the
 TexasET website), we provide the
Ovt values for comparison.  This may be
useful for times when the TxET data set
is unavailable. 
 

February 2010