In the finite difference formulation of the two-dimensional steady state turbulent diffusion equation for solving evaporation problems, two difficulties arise caused by the automatic satisfaction of one of the boundary conditions at the surface and by the infinite size of the solution domain. A general numerical scheme is developed to overcome these difficulties by the use of appropriate transformations. The results of some numerical experiments show that the longitudinal diffusion term is usually negligible and that with suitable parameters for roughness and stability power laws can be as useful for practical solutions as the more complicated logarithmic law. (Author)