REPRESENTATIVE ELEMENTARY WATERSHED (REW) APPROACH, A NEW BLUEPRINT FOR DISTRIBUTED HYDROLOGICAL MODELLING AT THE CATCHMENT SCALE: DEVELOPMENT OF CLOSURE RELATIONS Haksu Lee 1 , Murugesu Sivapalan 1 , Erwin Zehe 2 1 Centre for Water Research, The University of Western Australia Crawley, WA 6009, Australia 2 Institute of Geoecology, University of Potsdam, Germany ABSTRACT Reggiani et al. (1998, 1999) derived, from first principles and in a general manner, the balance equations for mass, momentum and energy at what they called the Representative Elementary Watershed (or REW) scale and also developed an associated constitutive theory. It has been suggested that the REW approach and the associated balance equations can be the basis for a new blueprint for the development of a new generation of distributed hydrological models with the REWs as building blocks. However, the balance equations of Reggiani et al. contain numerous terms representing mass exchange fluxes between different sub-regions of individual REWs and between different REWs. Developing physically reasonable closure relations for these mass exchange flux terms is a crucial pre-requisite for the success of the approach. In this paper, we utilize several methodologies to establish a number of these closure relationships, expressing mass exchange fluxes as functions of relevant state variables in a physically reasonable way, and effectively parameterizing the effects of sub-grid or sub-REW heterogeneity of catchment physiographic properties on these mass flux exchanges. For example, closure relations for infiltration, exfiltration and groundwater recharge were derived analytically, or on theoretical grounds, while numerical experiments with a detailed fine-scale distributed model, CATFLOW, were used to obtain the closure relationship for seepage outflow. The detailed model, CATFLOW, was also used to derive REW scale pressure-saturation (water retention curve), and hydraulic conductivity- saturation relationships for the unsaturated zone. Closure relations for concentrated overland flow and saturated overland flow were derived using both theoretical arguments and simpler process models. In addition to these, to 165 Prediction in Ungauged Basins: Approaches for Canada’s Cold Regions