Uncertainties in mean discharges from two large South American rivers due to rating curve variability

The uncertainty in mean discharge from two large South American rivers (the Amazonas at Obidos, Brazil, and the Parana at Corrientes, Argentina) resul ting from variation in the data used to fit rating curves at the two sites is evaluated. For the Amazonas at Obidos, where mean annual discharge over the 27-year period 1970-1996 was 164 200 m(3) s(-1), uncertainty arising fr om rating curve variability amounted to 2256 m(3) s(-1), equivalent to abou t 16% of year-to-year variability. For the Parana at Corrientes, where mean annual discharge over the 24-year period 1961-1985 was 17597 m(3) s(-1), t he uncertainty arising from rating curve variability was 245.3 m(3) s(-1), equivalent to 4.4% of year-to-year variability in mean discharge. Where vol umes freshwater delivered to the oceans by large rivers must be estimated f or climate-modelling or other purposes, ill-defined rating curves contribut e to the uncertainty in mean annual discharges, but become of secondary imp ortance where land-use change and other factors cause changes in discharge; and some evidence is advanced to suggest that such changes have occurred i n the Parana basin since 1960.