Abstract:
Changes in timing and quantity of snowmelt-fed streamflow have great importance for water supply, flood management, and ecological processes, as well as, being a common indicator of climate change. Within this scope, a snowmelt runoff timing analysis covering the contiguous United States with the most recent data is needed. In this study, these changes were investigated by analyzing relatively undisturbed streamflow observations in 97 streamflow gauges for the period between 1957 and 2016. The proposed approach was applied to 6 zones (A, B, C, D, E, and F) from the northeast to southwest of the conterminous US. The annual snowmelt runoff timing shift was identified using ‘Center Time (CT)’ and ‘Spring Pulse Onset (SPO)’ methods, jointly with the analysis of monthly average and fractional streamflow from January to June, which is the time period covering later snow accumulation and snow melting. Since snowmelt-derived streamflow timing change is mainly induced by regional meteorological factors, such as air temperature and precipitation, their trends and relationship with CT were examined. Shifts toward earlier snowmelt runoff timing were found by both methods, CT (85.6% of the stations) and SPO (59.8% of the stations).
Widespread average trends in CT and SPO were 8.3 and 8.5 days toward earlier, respectively. Although the results of CT change are stronger than SPO change, both outcomes are mostly correlated, particularly in the central and northwestern parts of the country. Monthly average streamflow and monthly fractional streamflow trends support the outcomes of CT and SPO. In January, February, and especially March, a higher number of the stations indicated increasing trends in monthly fractional streamflow. In April, May, and June, their number decreased and the number of gages with diminishing trends rose sharply. The timing difference is highly related to temperature change. Annual average temperature and temperature in the melting period increases considerably. The annual average temperature is significantly negatively correlated with CT in the vast majority of the regions. Although precipitation is not as effective as the temperature, its trends have impacts on snowmelt runoff timing change depending on the region and elevation. These results demonstrate 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 contiguous United States.
