Rainfall-runoff trends in the south-eastern USA: 1938-2005

An extensive dataset (230 precipitation gauges and 79 stream gauges) was used to analyse rainfall-runoff relationships in 10 subregions of a 482000 km(2) area in the south-eastern USA (Maryland, Virginia, North Carolina, South Carolina and Georgia). The average annual rainfall and runoff for this study area between 1938 and 2005 were 1201 and 439 mm, respectively. Average runoff/rainfall ratios during this period varied between 0.24 in the southern most Coastal Plain subregion to 0.64 in the Blue Rid-e Province. Watershed elevation and relief are the principal determinants governing the conversion of rainfall to runoff. Temporal rainfall variation throughout the south-eastern USA ranges from similar to 40% above and below normal while the variation for runoff is higher, from -75% to +100%. In any given year there can exist a +/- 25-50% error in predicted runoff deviation using the annual rainfall-runoff regression. Fast Fourier Transform and autoregressive spectral analysis revealed dominant cyclicities for rainfall and runoff between 14 and 17 years. Secondary periodicities were typically between 6-7 and 10-12 years. The inferred cyclicity may be related to ENSO and/or Central North Pacific atmospheric phenomena. Mann-Kendall analyses indicate that there were no consistent statistically significant temporal trends with respect to Southeastern US rainfall and runoff during the Study period. The results of U-tests similarly indicated that rainfall between 1996 and 2005 was not statistically higher or lower than during earlier in the Study period. Copyright (C) 2008 John Wiley & Sons, Ltd.