Testing a GCM land surface scheme against catchment-scale runoff data

Abstract

A GCM land surface scheme was used, in off-line mode, to simulate the runoff, latent and sensible heat fluxes for two distinct Australian catchments using observed atmospheric forcing. The tropical Jardine River catchment is 2500 km2 and has an annual rainfall of 1700 mm y–1 while the Canning River catchment is 540 km2, has a Mediterranean climate (annual rainfall of 800 mm y–1) and is ephemeral for half the year. It was found that the standard version of a land surface scheme developed for a GCM, and initialised as for incorporation into a GCM, simulated similar latent and sensible heat fluxes compared to a basin-scale hydrological model (MODHYDROLOG) which was calibrated for each catchment. However, the standard version of the land surface scheme grossly overestimated the observed peak runoff in the wet Jardine River catchment at the expense of runoff later in the season. Increasing the soil water storage permitted the land surface scheme to simulate observed runoff quite well, but led to a different simulation of latent and sensible heat compared to MODHYDROLOG. It is concluded that this 2-layer land surface scheme was unable to simulate both catchments realistically. The land surface scheme was then extended to a three-layer model. In terms of runoff, the resulting control simulations with soil depths chosen as for the GCM were better than the best simulations obtained with the two-layer model. The three-layer model simulated similar latent and sensible heat for both catchments compared to MODHYDROLOG. Unfortunately, for the ephemeral Canning River catchment, the land surface scheme was unable to time the observed runoff peak correctly. A tentative conclusion would be that this GCM land surface scheme may be able to simulate the present day state of some larger and wetter catchments but not catchments with peaky hydrographs and zero flows for part of the year. This conclusion requires examination with a range of GCM land surface schemes against a range of catchments.