Finite difference flow modelling of runoff on a terrain surface has usually been done using a regular grid. This has various disadvantages, as the regular pattern does not conform well to observed features such as watersheds, the runoff pattern is biased to the grid axes, and original data points are lost. We propose a flow modelling method using TIN models. A random Voronoi pattern is added to the original data. This avoids the issues of grid based methods, as there is no axis bias, points may be added anywhere and original data points may be retained. Our flow model simply requires a set of "buckets" to hold the water (the Voronoi cells) and slope information to provide the local runoff rate (the Delaunay edges).