Arjen Hoekstra (June 28, 1967 - November 18, 2019) was a professor at the University of Twente who pioneered the concept of the water footprint - a way of measuring the extent of water consumption. His work drew attention to the hidden water use associated with a range of activities, and continues to have a profound effect both on scholarship and on environmental policy and activism. He strongly supported open source science, and all his articles were published under a Creative Commons License.