Trends in floods and low flows in the United States: impact of spatial correlation

Trends in flood and low flows in the US were evaluated using a regional average Kendall's S trend test at two spatial scales and over two timeframes. Field significance was assessed using a bootstrap methodology to account for the observed regional cross-correlation of streamflows. Using a 5% significance level, we found no evidence of trends in flood flows but did find evidence of upward trends in low flows at the larger scale in the Midwest and at the smaller scale in the Ohio, the north central and the upper Midwest regions. A dramatically different interpretation would have been achieved if regional cross-correlation had been ignored. In that case, statistically significant trends would have been found in all but two of the low flow analyses and in two-thirds of the flood flow analyses. We show that the cross-correlation of flow records dramatically reduces the effective number of samples available for trend assessment. We also found that low flow time series exhibit significant temporal persistence. Even when the serial correlation was removed from the time series, significant trends in low flow series were apparent, though the number of significant trends decreased.