Reference Evapotranspiration
Weather data is used to calculate Reference Evapotranspiration (ETo), also known as Potential Evapotranspiration (PET), which is an estimate of the maximum potential water use for a well-watered reference crop under the given climate conditions. Daily ETo rates of the reference crop are adjusted for grapevines by multiplying by a crop coefficient (Kc) for grapevines. A simple calculation provides an estimate of grapevine water use as a guideline for irrigation scheduling.
Estimated daily water use = (daily ETo) X (Kc)
Grapevine crop coefficients for Texas are currently under development. Preliminary Kc values are presented in the Irrigation Scheduling Guidelines Research Progress Report, and a method for determining your own crop coefficients is described.
Additional information on the use of evapotranspiration data is available at the following links on the Texas ET Network website. 
What is ET?
Grower's Guide: Using PET for Determining Crop Water Requirements and Irrigation Scheduling
The following websites provide daily ET estimates for locations in Texas.
Texas ET Network
South Plains Evapotranspiration Network
North Plains PET Network
