What was done
For the period 1895-1999, the authors evaluated and analyzed annual values of (1) precipitation minus annual potential evapotranspiration, (2) surplus water that eventually becomes streamflow, and (3) the water deficit that must be supplied by irrigation to grow vegetation at an optimum rate.

What was learned
It was determined that for the conterminous United States as a whole, there was a statistically significant (at the 95% confidence level) increase in the first two of the three moisture conditions studied, while for the third there was no change.

What it means
In introducing the rationale for their study, the authors correctly report "there is concern that increasing concentrations of atmospheric carbon dioxide and other radiatively active gases may cause global warming and ... adversely affect water resources."  The results of their analyses, however, suggest that over the past century of significant - but neither unusual nor unnatural - global warming, just the opposite has occurred, at least within the conterminous United States.  Moisture has become more available, while there has been no increase in that required for optimum plant growth.