River discharge in a doubled carbon dioxide climate

The global climate model at the Max-Planck-Institute in Hamburg requires freshwater as an input variable for the coupling of the ocean and the atmosphere. Freshwater influx into the oceans from the continents is computed in two steps: first, infiltration excess is calculated from rainfall and snowmelt and the soil moisture deficit. Second, the infiltration excess (surface runoff) and a much slower component resulting from drainage of very wet soils are subjected to the scheme by Hagemann and Dumenil (1998). This scheme divides the continents into catchments and computes the river discharge for all land points according to the topography gradient between gridpoints. The scheme was validated using long time series of river discharge data from the Global Runoff Data Centre (Koblenz, Germany) for the 35 largest rivers worldwide. In addition to the simulation of present-day climate, the MPI coupled model ECHAM4-T42-OPYC has been applied to a transient scenario simulation for the climate of the next century (Roeckner et al., 1998). River runoff computed for a time-slice at the time of doubling of atmospheric carbon dioxide is compared to present-day values, both computed and measured. River discharge of large-scale catchments such as the Amazon river provides a useful tool to study the integrated regional impact of climate change on the hydrological cycle of a region. Regional analysis od the annual cycle of river discharge shows that for certain groups of rivers the discharge changes in a typical way depending on the characteristics of the present-day climate. Rivers which are dominated by snow melt in Arctic regions show earlier peaks due to earlier snow melt in the warmer climate, while the rivers fed by the Indian summer monsoon have increased discharge due to a higher average of monsoon precipitation in the north of the Indian subcontinent, while individual poor monsoon years are retained. Measured data provides an estimate of the interannual variability of the river discharge, thereby providing a measure of the significance of these results.