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

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Abstract
The uncertainty in mean discharge from two large South American rivers (the Amazonas at Obidos, Brazil, and the Paraná at Corrientes, Argentina) resulting 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 m3 s−1 uncertainty arising from rating curve variability amounted to 2256 m3 s−1 equivalent to about 16% of year-to-year variability. For the Paraná at Corrientes, where mean annual discharge over the 24-year period 1961–1985 was 17597 m3 s−1, the uncertainty arising from rating curve variability was 245.3 m3 s−1, equivalent to 4.4% of year-to-year variability in mean discharge. Where volumes of freshwater delivered to the oceans by large rivers must be estimated for climatemodelling or other purposes, ill-defined rating curves contribute to the uncertainty in mean annual discharges, but become of secondary importance where land-use change and other factors cause changes in discharge; and some evidence is advanced to suggest that such changes have occurred in the Parana basin since 1960.

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