Research has shown that evapotranspiration (ET) is a substantial component of the water budget of a green roof. Over the course of six years, a green roof weighing lysimeter experiment at Villanova University has shown that approximately 70%-85% of rainfall from April to November each year goes to ET. Other studies have shown similar performance. Despite evidence that ET is significant, a model that can be easily and effectively used for planning and design is still elusive. In developing a planning model, incorporating a true physical representation of system processes must be balanced with ease of use, including procuring input data. The Hargreaves model is an empirical model used to predict potential evapotranspiration and is considered relatively accurate for longer time periods (several days) in the agricultural community. While other physical based models require many inputs, the Hargreaves model simply needs daily temperature, which is easily procured, as well as terrestrial radiation, which is calculated from latitude and time of year. Using water availability as a limiting factor, the Hargreaves model is adjusted to account for available soil moisture within the system. A simple accounting system that uses easily obtained design values (e.g. roof area and media characteristics), historical rainfall and calculated ET values is used to estimate the water availability of the system at any given point throughout the year as well as potential runoff from a storm event. Preliminary data from this modified Hargreaves model closely matched the six years of observed ET from the Villanova weighing lysimeter with an average percent error of 8%. The simplicity of this model could prove to be an effective planning tool for green roof designs in the future as well as an easier way to credit green roofs for their stormwater runoff reduction potential. © 2015 ASCE.