Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 3 1 1999 10.5194/hess-3-137-1999 http://www.hydrol-earth-syst-sci.net/3/137/1999/ http://www.hydrol-earth-syst-sci.net/3/137/1999/hess-3-137-1999.html http://www.hydrol-earth-syst-sci.net/3/137/1999/hess-3-137-1999.pdf 137 149 0000-00-00 The UP modelling system for large scale hydrology: simulation of the Arkansas-Red River basin C. G. Kilsby J. Ewen W. T. Sloan A. Burton C. S. Fallows P. E. O'Connell Water Resource Systems Research Laboratory, Department of Civil Engineering, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK. email: c.g.kilsby@ncl.ac.uk The UP (Upscaled Physically-based) hydrological modelling system to the Arkansas-Red River basin (USA) is designed for macro-scale simulations of land surface processes, and aims for a physical basis and, avoids the use of discharge records in the direct calibration of parameters. This is achieved in a two stage process: in the first stage parametrizations are derived from detailed modelling of selected representative small and then used in a second stage in which a simple distributed model is used to simulate the dynamic behaviour of the whole basin. The first stage of the process is described in a companion paper (Ewen <i>et al.</i>, this issue), and the second stage of this process is described here. The model operated at an hourly time-step on 17-km grid squares for a two year simulation period, and represents all the important hydrological processes including regional aquifer recharge, groundwater discharge, infiltration- and saturation-excess runoff, evapotranspiration, snowmelt, overland and channel flow. Outputs from the model are discussed, and include river discharge at gauging stations and space-time fields of evaporation and soil moisture. Whilst the model efficiency assessed by comparison of simulated and observed discharge records is not as good as could be achieved with a model calibrated against discharge, there are considerable advantages in retaining a physical basis in applications to ungauged river basins and assessments of impacts of land use or climate change.