Regression analysis is usually the statistical tool of choice in hydrologic
al studies when there is a strong correlation between two variables. Howeve
r, weak correlations can also be of interest if a region within the scatter
plot is data-free. This could direct attention to seeking some underlying
physical process that might create regions with low probability of generati
ng data points. A necessary prior requirement here is to verify that the da
ta-free area in the plot is sufficiently large to be a real effect and not
a visual illusion. This check can be most simply carried out in a hypothesi
s-testing framework. A permutation approach to hypothesis testing is sugges
ted for the particular case where a data-free region occupies one of the co
rners of a scatter plot, and a test statistic Delta is presented for testin
g the statistical significance of the size of this "empty corner". Applicat
ion to some rainfall data from southern Israel shows that the new test can
sometimes yield higher levels of statistical significance than linear regre
ssion when applied to the same data. (C) 1999 Elsevier Science B.V. All rig
hts reserved.