Simplifications to the depth-integrated soil water transport equation lead to an Ito stochastic differential equation where the evaporation is related to the stored water by the nonlinear soil water diffusivity function. From the derived equation the diffusivity function was estimated from daily stored water measurements obtained at the field scale using nonlinear filtering theory for a period of 100 days. Comparisons with daily evaporation measured with a sensitive 50-ton lysimeter indicated that the proposed method may be used to determine soil water diffusivity functions at the field scale under natural conditions when applied water and evaporation are the primary controlling physical mechanisms in the hydrologic budget. ¿ American Geophysical Union 1993