Evaporation from finite areas with constant sources is determined by simulating the mass transfer equation. The equation of mass transfer in a turbulent atmosphere describes the movement of mass, water vapor, and momentum. The process of transferring water vapor by the evaporation phenomena from a constant source can be analyzed by the solution of this diffusion equation. A hybrid computer simulation method to solve the three‐dimensional nonuniform diffusion equation is described. The proposed technique can handle any geometric configuration of lakes and reservoirs. Using finite difference method, the partial differential equation is transformed into a set of algebraic equations. The analogy between these algebraic equations and the node equations of a passive resistance network plane makes the hybrid computer simulation technique applicable to solve problems of this type. A vertical plane, perpendicular to the direction of the wind, is simulated by a passive resistance network. The movement in the direction of the wind is discretized with resistances simulating the step size. The problem is solved by moving the vertical plane in the direction of the wind. Obtaining the solution using a simulation technique involves a repetitive process of observing voltages on the simulation network. The entire process is performed in the hybrid computer system using the digital computer as the control. This then is a general method available to analyze the evaporation phenomena using a mathematical solution of the diffusion equation.