 Although attributed to Richards, it is established  that this equation was actually discovered 9 years earlier by Lewis Fry Richardson in his book "Weather prediction by numerical process" published in 1922 (p.108).Darcy's law was developed for saturated flow in porous media; to this Richardson applied a continuity requirement suggested by Edgar Buckingham and obtained a "general partial differential equation describing water movement in unsaturated non-swelling soils".The transient state form of this flow equation, known commonly as Richards' equation writes in one-dimension (vertical)