Towards closing the vertical water balance in Canadian atmospheric models:Coupling of the Land Surface Scheme CLASS with the distributed hydrological model WATFLOOD

Second generation land surface schemes are the subject of much development activity among atmospheric modellers. This work is aimed at, among other th ings, improving the representation of the soil water balance in order to si mulate, more properly, exchanges with the atmosphere and to permit the use of model output to generate streamflow for model validation. The Canadian d evelopment program is centred on CLASS, the Canadian Land Surface Scheme, d eveloped at Environment Canada. This paper focuses on the improvement of hy drology in CLASS. This was accomplished by designing a two-way interface to WATFLOOD, a distributed hydrologic model developed at the University of Wa terloo. The two models share many features, which facilitated the coupling procedure. The interface retains the three-layer vertical moisture budget representati on in CLASS but adds three horizontal runoff possibilities. Runoff from the surface waterfollows Manning's equation for overland flow. Interflow, is g enerated from the near-surface soil layer using a parametrization of Richar d's equation and base flow is produced by Darcian flow from the bottom of l ayer 3. An approximation of the internal topography of grid elements is use d to supply horizontal gradients for the runoff components. Tests are in progress in four Canadian study areas. Initial results are pre sented for the summer of 1993 for the Saugeen River in southwestern Ontario . The new scheme produces realistic hydrographs, whereas the old scheme did not. Bare ground evaporation is reduced by about 17% as a consequence of r educed water availability in layer 1. Evapotranspiration is not affected be cause the rooting depth extends into layer 3, in which soil moisture does n ot change appreciably with the new scheme. These results suggest that the n ew scheme improves the representation of streamflow in WATFLOOD/CLASS and o f the soil moisture budget in CLASS. Work is in progress to validate this r esult over basins, such as the BOREAS study watersheds, where both runoff a nd evapotranspiration measurements are available.