Parameter values and grid size in the distributed hydrological modeling for a small mountain basin

A finite difference distributed model for the continuous simulation of the coupled groundwater-channel flow has been applied to a 4.56-km2 vegetated mountain basin. The effects of the numerical dissipation of the difference schemes adopted are investigated using different space and time-step sizes. Saturated subsurface flow is simplified to a 2-D horizontal representation by integrating over the soil depth. Surface flow is simplified to a kinematic 1-D representation along the channel network. Groundwater and stream channel components of the hydrological cycle are simulated with the mass and momentum conservation equations. The comparison of the discharge hydrographs for four grid-step sizes has highlighted that the truncation error of the higher order terms in the finite difference approximation of groundwater flow equation, produces a numerical dissipation which grows as the space-step size grows. This numerical dissipation can be compensated adopting new soil hydraulic conductivity values, greater than physically meaningful values, which considerably increase proportionally to the grid-step size. The conductivity parameter can then take on field measured values only if a small space-step is used.