Regional analysis of trend and step changes observed in hydroclimatic variables around the Colorado River Basin

Recent research has suggested that changes in temperature and precipitation events due to climate change have had a significant impact on the availability and timing of streamflow. In this study, monthly temperature and precipitation data collected over 29 climate divisions covering the entire Colorado River basin and monthly natural flow data from 29 U.S. Geological Survey (USGS) gauge locations along the Colorado River are investigated for trend or step changes using parametric and nonparametric statistical tests. Temperature increases are persistent (at least 10 climate divisions over 6 months in trend analysis) throughout the year over the Colorado River basin, whereas precipitation only notably increased over 17 climate divisions (during trend analysis) during February and remained relatively unchanged otherwise. These results correspond with changes in naturalized streamflow throughout the year. Streamflow increases are recorded between November and February but exhibit a decreasing trend over the traditional peak runoff season (April through July). Under trend analysis, 18 flow stations exhibited increasing trends in January and 19 flow stations exhibited decreasing trends in June. It is likely that increasing temperature trends have affected the character of precipitation in the Colorado River basin, causing a change in the timing of runoff events.