The problem of determining the level of groundwater near reservoirs, taking into account evaporation from the surface of groundwater, is reduced to a boundary value problem with an unknown boundary for a parabolic equation. The condition for the problem (motion) to be self-similar is established. A problem with an unknown boundary is distinguished and investigated, which differs from the well-known problems of Stefan, Verigin, Florin. The problem under consideration describes the filtration process near new canals and reservoirs, taking into account evaporation, which is a nonlinear finite function of time and groundwater level.