The quantity of water evaporated per unit area, per unit time from a continuous, uniform area of grass with a fixed height of 0. 12 m, an albedo of 0. 23, and a surface resistance of 69 s m⁻¹; approximated by the rate of evaporation for an extensive area of 0. 08 to 0. 15 m high green grass cover of uniform height, actively growing, completely shading the ground, and not short of water.