What was done
The authors performed a number of statistical analyses of historical streamflow, climate and population data for 39 urbanizing and 21 nearby rural watershed basins in four regions of the United States in order to estimate the net effects of future potential changes in climate and urbanization on mean annual streamflow.

What was learned
In the words of the authors, "urbanization increased mean annual streamflow in rough proportion to average cumulative changes in population density on the basins, equivalent to an average flow increase of 103% with complete watershed urbanization."  They also report that urbanization reduced the sensitivity of mean annual streamflow to changes in temperature.  As an example, they noted that "predicted streamflow reductions with temperature increases were ~4.5 times greater on rural than on urbanizing basins."

What it means
The authors make the wise but generally neglected observation that "estimates of the impact of future climate change on mean annual streamflow should include consideration of the concurrent effects of population growth."  When this is done, as their real-world study demonstrates, global warming-induced decreases in annual streamflow may be significantly, or even totally, offset by the countervailing effects of concurrent increases in population density on the watershed feeding the stream in question.  And sometimes these effects can change predictions of streamflow decreases into predictions of streamflow increases.