Changes in of Snowmelt Runoff Timing in the United States between 1957 and 2017

Changes in timing and quantity of snowmelt-related streamflow have great importance for water supply, flood management, and ecological processes as well as being a common indicator of climate change. In the past decades, earlier spring runoff has been observed in many rivers in the United States due to changes in snowmelt regimes. In this study, an investigation of these changes is conducted by analyzing relatively undisturbed streamflow observations in 235 streamflow gauges for the period between 1957 and 2017. The timing shift is identified using `center time' and `spring pulse onset' methods jointly with the analysis of monthly average streamflow from January to June, when snowmelt processes are observed in the US. Snowmelt derived streamflow timing change is induced by regional meteorological factors. After pairing streamflow gauges and meteorological stations, temperature and precipitation trends will also be examined. Linear trend analysis and Mann-Kendall test are applied to identify spatial and temporal regimes of trends in snowmelt runoff timing, streamflow, temperature, and precipitation. As a result of the trend analyses, the correlation between the timing and location of snowmelt streamflow, as well as elevation, and meteorological parameters (i.e. temperature and precipitation), will be examined. Preliminary results indicate that the timing of snowmelt runoff shifted towards an earlier period at the majority of the observed stations. This characteristic is related to climatic changes in the past few decades, especially due to the observed temperature increase. It is also anticipated that the significance of this temporal shift is correlated to elevation and location. More changes towards earlier dates are observed in the northwestern, southwestern, and northeastern US, and Alaska while less is observed in the central northern part of the country. Finally, the present study stands out for its importance towards water resources management and engineering, by describing the importance of the impacts of the changes in snowmelt runoff timing due to global warming on the regional and large scale hydrology in the United States.