An improved methodology for predicting the daily hydrologic response of ungauged catchments

In order to model fluxes of water from the land surface to the atmosphere, and from one grid cell to another in climate models, predictions of hydrologic response are required for catchments where hydrologic data are not available. A methodology has been presented previously that has the capability of producing estimates of catchment scale hydrologic response for ungauged catchments on a daily timestep (Post and Jakeman, 1998, Ecol. Mod. submitted). In the present paper, it is demonstrated that these daily predictions of hydrologic response can be improved by incorporating information about the hydrologic response of the catchment on a longer timestep. This is because the influence of large scale phenomena such as climate and vegetation may produce a similar water yield in nearby catchments, even though their daily hydrologic response may be different, due for example, to differences in drainage density. Thus, the water yield of an ungauged catchment is inferred on an inter-annual timestep, and this information is used to balance the water budget of a daily timestep rainfall-runoff model. It was found that using tree stocking densities to predict water yields for small experimental catchments in the Maroondah region of Victoria produced better results than those obtained by inferring the water balance parameter of a daily timestep rainfall-runoff model from channel gradient and catchment elongation. Good predictions of inter-annual water yield were also obtained for small experimental catchments in the H. J. Andrews, Hubbard Brook, and Coweeta long term ecological research (LTER) sites in the United States, indicating that it may be possible to produce high quality predictions of daily hydrologic response for ungauged catchments in these regions also.