where is the moisture tension.
References
Barry, A. "A Class of Exact Solutions for Richards' Equation." J. Hydrology 1993.
Corless, R. M.; Gonnet, G. H.; Hare, D. E. G.; Jeffrey, D. J.; and Knuth, D. E. "On the Lambert W Function." Adv. Comput. Math. 5, 329-359, 1996.
