Global trends in streamflow and water availability

The annual runoff fields from a total of 62 runs of the 20C3M experiment on 21 different models (one to nine runs per model) were integrated spatially over 165 river basins with long-term (28–99 years, median 59 years) streamflow measurements (see Methods). Climate-model runoff commonly does not reflect the time lag associated with storage in river basins. Although the missing time lag may not affect computed climates, it can strongly affect the temporal variability of streamflow, which is important for our analysis. Accordingly, the climate-model basin runoff time series were converted to equivalent streamflow time series by routing them through a model of a linear reservoir in such a way as to match the observed serial correlation (see Methods).