The isotopic composition of the remaining water depends on the temperature and the isotopic signiture of the air moisture because of possible back diffusion.
My best guess:
Gat, Joel R. (2010): Isotope hydrology. A study of the water cycle. London: Imperial College Press (Series on environmental science and management, 6).
Majoube, M. (1971), Fractionannement en oxygene-18 et deuterium entre
l’eau et sa vapeur, J. Chim. Phys., 197, 1423–1436.