For a number of years the Extension Service has been recommending one inch of irrigation per week applied to St. Augustine grass in the summer. The results of the San Antonio ET project indicate that the 1 inch per week recommendation is unnecessarily high. In all cases the appearance of St. Augustine grass declined in mid-summer but rebounded to its original spring rating in the fall. Evapo-transpiration is the amount of water that is lost to a plant through evaporation from the soil and transpiration from the leaf surface.
Transpiration is calculated by multiplying PET values by an empirically derived co- efficient.
The experiment only included a few zoysia lawns. Soil depth and bulk density measured as dry weight per unit volume were also factored in the experiment. In the case of Bermuda grass and to a lesser extent buffalo grass soil depth was more important than for St. Augustine grass. After studying the 1997 and 1998 results the staff and ET Advisory Committee recommended that a major ET initiative be launched in 1999 and 2000. The program would give homeowners who choose to participate the option to divide their water applications into two parts over the week.