During more intense melt years, supraglacial lakes on the Greenland Ice Sheet are found to fill more rapidly and to drain earlier in the melt season. They also develop at higher latitudes and elevations. Since the rapid drainage of lakes has been shown to enhance basal sliding and since the lake volume can play an important role in determining if and when it drains, understanding and modeling the processes which control lake filling rates forms a key development in successfully modeling the possible impacts of lake drainage events on ice motion. We have developed a surface water routing and lake filling model and applied it to a 100 km2area of the Paakitsoq region, west Greenland. The model takes the time series of calculated runoff (melt minus refreezing) over the area and calculates flow paths and water velocities over the snow-/ice-covered surface, routing the water into topographic depressions which can fill to form lakes. Runoff is calculated from a distributed, surface energy-balance model coupled to a subsurface model, which calculates changes in temperature, density and water content in the snow, firn and upper ice layers, and hence refreezing and therefore net runoff. The model is calibrated against field measurements of a filling lake in our study area during June 2011 and can be used to calculate the filling rate of the instrumented lake with a high degree of accuracy. The filling rate of the instrumented/modeled lake depends on melt and routing from the immediate lake catchment and from overflowing lakes in upstream catchments.