In this peer-reviewed article, we examine environmental controls on the ratio of actual evapotranspiration to potential evapotranspiration, also known as the crop coefficient. The crop coefficient has been traditionally used to estimate crop water demand but has not been widely applied in other types of ecosystems, including forests. In this study, we used data from the global network of eddy flux towers to develop a series of linear regression models that help to explain the seasonal dynamics of the crop coefficient for seven different land cover types. Using commonly available datasets on precipitation and leaf area index, we have developed a practical tool for estimating the seasonal dynamics of forest ecosystem water use and stress and projecting ecohydrological responses to climate variability and change.