Changes in US streamflow and western US snowpack

Hydroclimatological records are increasingly examined for evidence of trends and shifts that may assist in prediction of future climate change scenarios. This study investigates the trend and step changes in U.S. streamflow over a 52-year period (1951-2002) using data from 639 unimpaired streamflow stations categorized according to the hydrologic unit codes. This is particularly relevant since the issue of climate change is of interest to many, and studies have indicated an abrupt change in climate around the year 1976/77. Trends were evaluated using three statistical tests: Spearman's rho, Mann-Kendall, and linear regression, and step changes were evaluated using the rank sum and student's t test. The temporal resolution used for the study included water year (Oct-Sept), autumn-winter (Oct-Mar), and spring-summer (Apr-Sept) periods. Additionally, April I snow-water equivalent (SWE) data for 121 SNOTEL stations for the period 1941 to 2004 were used to test for the trends in the western U.S. The multiple statistical tests provided robust results for regions with significant changes. Results indicated that the Mississippi and Missouri regions have an increasing trend in streamflow quantity. The Pacific Northwest and South Atlantic-Gulf regions have streamflow decreasing due to a step change in climate. Decreasing trends for the SWE were noted for a number of stations in the states of Oregon and Utah.