| In Chapter 1, trends in flood and low flows in the U.S. were evaluated using a regional average Kendall's S trend test at two spatial scales and over two timeframes. After accounting for spatial correlation, we found no evidence of trends in flood flows but did find evidence of upward trends in low flows in the Midwest. We show that ignoring spatial correlation leads to a dramatically different interpretation because spatial correlation 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 lag-1 serial correlation was removed from the time series, significant upward trends in low flow series were apparent, though the number of significant trends decreased.; In Chapter 2, we explore the effect of the temporal persistence in low flows on frequency and risk estimation. We first defined E( T) as the expected value of the time to the first failure and then we present a useful method for estimating E(T) and risk of failure, R, in the presence of persistence. We show that for observed ranges of persistence, E( T) can be nearly 100% greater and R more than 20% lower than conventional estimates. This implies that the expected design life of a system is longer when persistence is taken into account. By ignoring persistence, low flow quantiles may be underestimated by 50% or more. An evaluation of the effect of persistence on low flow risk estimation across the U.S. is presented.; In Chapter 3, we explore the behavior of the flood of record. We show that if annual floods arise from a generalized extreme values (GEV) distribution, then the flood of record also arises from a GEV distribution with different mean and variance but identical tail behavior. We investigate spatial and temporal relationships of floods of record at over 1,500 sites in the U.S. as a possible explanation for observed non-random behavior. We also develop a multivariate regression model for estimating the floods of record from geomorphologic, climatic, and even extraterrestrial (solar) information. |
