Evaluating the Role of Precipitation Pattern on the Temporal Changes in Streamflow and Baseflow

The Nebraska Department of Natural Resources has long been interested in assessing the long-term trends in streamflow and baseflow in major drainage areas across the state. One of the overall objectives of this effort is to examine the presence of temporal trends in precipitation and its impact on streamflow and baseflow. In order to isolate the impact of precipitation, a relatively undeveloped area in the Nebraska Sandhills was selected for this evaluation. Streamflow data for the Middle Loup River was obtained from USGS for the 1950 to 2014. Regional climate data at the Purdum, NE weather station was also provided by High Plains Climate Center (HPRCC) for the same time period. Simple linear regressions indicate an increasing trend in daily streamflow and baseflow over time. Meanwhile, the trend in daily precipitation is effectively zero. The regressions for the total annual values also indicate a robust positive trend in streamflow and baseflow. Further investigations show that the number of rainfall events have been steadily declining in the past few decades. A correlation matrix also reveals that streamflow has a stronger relationship with the number of rainfall events than with total precipitation. The declining number of wet days and the lack of trend in total rainfall suggest a potential increase in rainfall intensity, which could partially explain the increase in streamflow. It is important to note that other conditions that can increase streamflow, such as snowmelt rate, still need to be properly considered.