A number of coupled surface and subsurface flow models include Richards' equation (RE) solutions to simulate flow through the vadose zone, but the question of the appropriate application of RE at the watershed scale remains unclear. Published applications of RE in watershed-scale modeling often use coarse vertical resolutions to decrease the computational burden. In contrast, investigations demonstrate that small vertical cell sizes, on the order of 1 cm, are required near the soil surface, to accurately simulate RE infiltration fluxes in watersheds. A compromise is sought to satisfy both the computational burden and accuracy issues within the confines of available hydrological data for this scale of problem. Research indicates that conditions commonly exist in watersheds wherein basic assumptions of RE applicability are violated or where RE is unable to reproduce unstable infiltration behavior. Thus, physical and computational factors motivate the search for alternatives to RE in simulating the quantity and rate of infiltration that occur in coupled surface and subsurface flow systems. Research is underway to identify a toolbox of infiltration models to evaluate appropriateness for the scale and scope of the physical processes being simulated. The selection of toolbox models must also be sensitive to the fidelity of available data and the site hydrologic, hydrogeologic, and topographic conditions. Results to date in the identification of the range of application of toolbox models will be presented.