Principal Investigator Tony Jakeman Project r52

Centre for Resource and Environmental Studies Machine VP

Co-Investigators Robert J Oglesby, David P Hansen and David A Post

Department of Earth and Atmospheric Science, Purdue University and

Centre for Resource and Environmental Studies

Coupling Surface Hydrology Models to the CCM

The project aims to incorporate a surface hydrology model, IHACRES, into a Global Climate Model (GCM) to improve the description of water exchange between the land surface and the atmosphere. IHACRES is an acronym for Identification of unit Hydrographs And Component flows from Rainfall, Evaporation and Streamflow data. Initially, it had been planned to use the CCM1 version of the USA National Center for Atmospheric Research GCM. Subsequently, a more advanced version, CCM2, has become available to us. CCM2 is advantageous compared to CCM1 because it has a much higher spatial resolution which means that it can be better coupled to IHACRES, which is most applicable at the basin or catchment scale. A strategy has been developed whereby IHACRES will be tuned to selected basins and then used with CCM2 to generate stream runoff and evaporation for all (model) basins. The modelled stream discharges will be compared to available observations, while the IHACRES-supplied evaporation will be compared to that computed directly by CCM2. The goal is to predict runoff and evapotranspiration responses to climatic inputs (changes or variability) on a diurnal basis. The model should also be applicable to assessing effects of prescribed vegetation changes on these water balance response terms.

What are the basic questions addressed?

Current models of the water exchange between the land surface and the atmosphere are overly simplistic in terms of their physics (e.g. bucket models) or overly complicated and parameterised (e.g. biosphere-atmosphere transfer schemes). IHACRES has proven to be a relatively simple but effective model, incorporating the basic physics and having around six parameters depending on the climatological regime, that has been shown to be applicable in a wide variety of hydroclimatologies. Its computational simplicity means it does not hamper the performance speed of GCMs and its accuracy compares extremely favourably with all other, including more complex models. The aim of the project is to make this development available and to show the improvement attainable with its use.

What are the results to date and the future of the work?

In the last six months IHACRES has been ported to the CM-5 (see report for project s31), thus allowing for fast parameterisation of catchments. Furthermore, CCM2 has been ported to the VP (CCM2 was ported to the VP by Phillip Chen at Fujitsu America Inc. and then ported to the VP here at ANU) and is currently being run over a 20 year period. This run will be important in assessing the improvements within the model once IHACRES has been coupled to CCM2. It is anticipated that IHACRES will be incorporated into the GCM once the aforementioned run is complete and that this will lead to a very accurate coupled model of water and energy feedbacks to the atmosphere. Considerable work has also been done to collect the necessary global stream discharge and precipitation observational datasets

What computational techniques are used and why is a supercomputer required?

In climate models a range of computational techniques are used including forward numerical solutions of differential equations at grid points over the entire globe. The hydrological model uses a transfer function or discrete difference equation at each grid cell. A supercomputer is needed because of the large amounts of computation, in time and space, and the mass storage required.