Data of terrestrial evapotranspiration over large areas?
Millimeters per year measurements have an area component built into them, they're actually in cubic millimeters per square millimeter. So one millimeter of rainfall is enough to cover the given area that rainfall is for to an average depth of one millimeter. For example if the area is a 100 square kilometer catchment then 1mm of rainfall is 100,000,000 cubic millimeters (also millilitres), or 100,000 litres of water. Evapotranspiration is measured using exactly the same unit conventions. Hope that helps.