Role of simulation time in grid based distributed hydrologic simulations

Digital Elevation Model (DEM) grid size affects estimation of topographic parameters like slope, flow direction and flow path, thereby affecting hydrographs derived for a watershed. This study investigates the role of DEM grid size in determining simulation time-step of a hydrologic model. A simple grid-based hydrologic model, developed in a GIS environment using storage-release with cell travel time concept, named GIS and Hydrologic Information System Modeling Object (GHISMO) is used for the investigation. In any grid based hydrologic model, the output of the model is sensitive to grid resolution, where finer resolution DEMs produce lower peaks and coarser DEMS produce higher peaks. In this study, a series of simulations using synthetic data and observed data for Cedar Creek in north east Indiana show that there is a threshold time step at which a given DEM resolution yields hydrographs that are consistent with observed hydrographs. This threshold time step is called the Critical minimum simulation time step (CMST), and is equal to the time required for water movement from one cell to another cell for a given DEM resolution. The CMST depends largely on DEM resolution, but is also affected by surface roughness, watershed slope and rainfall intensity. Estimation of CMST for different study areas in Indiana, and its significance in grid based hydrologic simulations will be presented.