Quoted from: Kouwen, Nick, Ric Soulis, Frank Seglenieks, Allyson Bingeman, and Bruce Davison. "An Introduction to WATFLOOD and WATCLASS." University of Waterloo Hydrology Lab, Waterloo, Ontario (2011). https://www.usask.ca/geography/MAGS/Events/Workshops/Model/WATFLOOD-WATCLASS.pdf 

WATCLASS is a coupled model that incorporates WATFLOOD into the Canadian Land Surface Scheme (CLASS). The coupling of these two models allows for the horizontal and vertical modeling of the water cycle using more physically based concepts (Figure 9). The energy cycle, however, is still only modeled in the vertical direction. In terms of the water cycle, the major improvement has been to move water in both the vertical and horizontal directions. In CLASS, water only moves in the vertical direction, and so excess water tends to pond on the surface, resulting in too much water being available for evaporation. To help facilitate the vertical movement of water in CLASS, an internal slope was added to each grid square to provide the gradient forcing the movement of water out of the cell (Soulis, et al. 2000).