PET: A Lawn Water Management Tool
Jul 3, 2010
PET is an acronym for "potential evapotranspiration". Potential evapotranspiration is a predicted value calculated from a weather station based on a cool season grass growing 4-inches tall in a deep soil under well-watered conditions. It is defined as the maximum amount of water (inches) lost from a vegetative surface through evaporation from the soil and transpiration from plants. Factors directly affecting the calculated value include air temperature, relative humidity, wind speed, and solar radiation.
PET can be a very useful tool in your water management program for your lawn. Research from Texas A&M University suggested that PET was a very good predictor of soil moisture loss in landscapes. PET values from weather stations offer you the maximum amount of soil water lost, but most landscapes can maintain a healthy condition with much less water. Hence, a multiplying factor called "crop coefficient" is used.
The coefficient for warm season turfgrasses is 0.6 (60%) and for cool season grasses is 0.8 (80%). An example would be if you were to find that PET for a 7-day period in your area was 1.5 inches. If you have St. Augustinegrass (warm season turfgrass), you would multiply 1.5 inches by 0.6. This means you would only need to apply 0.9 inches of water to your site in order to replace the water that was lost through evapotranspiration. Depending on what species you have in your entire landscape will dictate what coefficient value to use in each zoned area. You will need to determine this for your own site because all landscapes are different (i.e. shade, soil type, soil depth, etc.).
For most lawn grasses, you may be able to use an even lower coefficient value depending on the amount of stress you are willing to impose on your turf. PET can be a great tool for you to use in order to irrigate with the correct amount water. It most certainly can help conserve our greatest natural resourceŚWATER!Information Source: