Over 15 years ago, Morton (1994) summarized the state of evapotranspiration (ET) research pessimistically: 'There have been few significant advances in our knowledge of evaporation on an environmental scale over the past four decades, a state of affairs linked to the current sterility of hydrology and related environmental sciences. Furthermore, almost none of the advances have been used successfully in practice.' He did not foresee the rapid progress in the ensuing years. These advances can be attributed largely to three convergent themes: 1) technical innovation; 2) synergy between disciplines; and 3) expressed need. The papers in this special issue address all of these three themes on remote sensing methods for ET estimation.
The papers in this special issue reflect the technological progress and interdisciplinary and synergistic state of ET research. The first theme addressed is technical innovation. Since 1990, there has been rapid development of methods for measuring ET unobtrusively: eddy covariance flux towers (over 500 from AmeriFlux, OzFlux, FluxNet), Bowen ratio flux towers, sap flow sensors, stable isotopes, new satellites and sensor systems, operational remote sensing methods for ET in near-real time at local, regional and global scales of measurement, and automated micro-meteorological stations deployed in irrigation districts worldwide, to name just a few. The second theme is synergy between disciplines. Rather than being just a specialty field in atmospheric physics, ET research is now carried out by agronomists, meteorologists, ecophysiologists, remote sensing specialists, and climate change scientists, often working together in teams, as illustrated by the lists of authors and institutions in the papers in this special issue. The third and most important theme is expressed need. There is now wide-spread recognition that the world is running short of high-quality fresh water. There is growing competition for water for agriculture, municipalities, people and the environment. Wise use of water supplies must begin with knowledge of how much water crops or natural ecosystems actually use through ET and how that will change with changing climate patterns.
The rationale for this topic as a special issue in Hydrological Processes is to explain the principles of estimating ET from remote sensing to those outside the field who may not fully understand these applications or what the present understanding of their limitations are, and