2.5 Double Mass Curve
The double mass curve is a simple, visual, and practical method, and it is widely used to study the consistency and long-term trend test of hydrometeorological data (Wei and Zhang, 2010; Gao et al. , 2011; Aryal and Zhu, 2020). The double-mass curve theory is based on the fact that a plot of the two cumulative quantities during the same period exhibits a straight line so long as the proportionality between the two remains unchanged. The slope of the line represents the proportionality. This method can smooth a time series and suppress random elements in the series, thus showing the time series’s main trends (Gao et al. , 2010).
In this study, double-mass curves of summer precipitation vs. suspended sediment discharge are plotted for the two different periods to estimate changes in regression slope (proportionality) to quantify the impact of rainfall on SSD before and after transition years. We used TerraClimate (Abatzoglou et al. , 2018) precipitation sums for June-August that were spatially averaged over every river basin.