SIMPLE WATER-BALANCE MODEL FOR ESTIMATING RUNOFF AT DIFFERENT SPATIALAND TEMPORAL SCALES

A parametric water balance model was developed based on statistical av eraging of the main hydrological processes. The model has a two-layer structure with both a physical and statistical basis for the model par ameters. It was developed to fill a need for models with a small numbe r of parameters and of intermediate complexity between a one-parameter simple bucket and more complex hydrologically oriented models with ma ny parameters such as the Sacramento model. The focus was to improve t he representation of runoff relative to the simple bucket without intr oducing the full complexity of the Sacramento model. The model was des igned to operate over a range of time steps to facilitate coupling to an atmospheric model. The model can be used for catchment scale simula tions in hydrological applications and for simple representation of ru noff in coupled atmospheric/hydrological models. An important role for the simple water balance (SWB) model is to assist in understanding ho w much complexity in representing land surface processes is needed and can be supported with available data to estimate model parameters. Th e model is tested using rainfall, runoff, and surface meteorological d ata for three catchments from different climate regimes. Model perform ance is compared to performance of a simple bucket model, the Sacramen to model, and the Oregon State University land surface model. Finally, a series of tests were conducted to evaluate the sensitivity of SWB p erformance when it is operated at time steps different from the time s tep for which it was calibrated.