Trends in precipitation and streamflow in the eastern U.S.: Paradox or perception?

Abstract

Many studies have reported that total precipitation is increasing across the United States with most of the increase resulting from a positive trend in the upper tail of the daily precipitation distribution. Other studies have found that low and moderate, but not high flows are also increasing across much of the United States. How can precipitation, especially that produced by intense events, increase without a corresponding increase in high flows? We analyzed trends in annual 7‐day low, average and high flows along with seasonal precipitation that is averaged over individual basins. Our findings suggest that statistically significant trends in both fall precipitation and 7‐day low flow are found in a large percentage of the basins in the upper Mississippi and Great Lakes regions of the country. A large fraction of the trends in annual precipitation can be explained by an increase in fall precipitation. By estimating trends in precipitation at the spatial scale of individual basins, we offer a simple explanation for the apparent paradox of lack of trends in high flows. At the spatial scale of individual basins, precipitation is increasing during the fall but not during the spring, the season when high flows are generally observed. The increase in fall precipitation appears to result in an increase in the low flows while the lack of trends in precipitation in spring explains the lack of widespread trends in the high flows.