Building a Model:Selecting a Grid Size

Selection of an appropriate grid size is important in successful modeling of a watershed and has been discussed in Section 4.2 of this manual. Key to successful modeling of processes at the cell level is that the cell size be smaller than the size of the essential feature of the landscape involved in the model. For example, Ogden et al. (2000) used a 30m grid size to model the Fort Collins, Colorado, flash flood of July 28, 1997. This small grid size was used because the urbanized Spring Creek catchment in Fort Collins has a number of small-scale landscape features such as: roads, parking lots, buildings. Had finer scale DEM data been available at the time, an even smaller grid cell may have been employed. Conversely, Doe and Saghafian (1992) successfully modeled the Taylor Arroyo watershed near Trinidad Colorado with a 300m grid. While the climates are similar, the Taylor Arroyo watershed is essentially completely undeveloped. 300m grid cells are not large compared to soil texture and vegetation complexes in the watershed. It’s possible that even larger grid cells could be used. The grid size also has an effect on slopes, which may be important for overland sediment transport calculations as discussed by Sanchez (2002). It is important to note that the grid size must be chosen in view of the data available to support providing input data. The grid size should never be smaller than the DEM resolution.