A New Framework for Exploring Process Controls of Flow Duration Curves

The flow duration curve (FDC) is effectively the cumulative distribution function of streamflow. For a long time, hydrologists have sought deeper understanding of the process controls on the shape of FDC, which has been a challenge due to contrasting processes controlling the fast flow and slow flow components of streamflow and their interactions. In this paper, we outline a new framework for exploring the process controls of the FDC, which involves studying fast and slow flow components of FDC separately and combining them statistically, explicitly accounting for their dependence. We illustrate the potential of the framework by constructing empirical fast flow duration (FFDC) and slow flow duration (SFDC) curves from the flow components obtained by traditional baseflow separation. Streamflow time series data from 245 MOPEX catchments across the continental United States are used. The dependence of FFDC and SFDC components is captured by the Gumbel copula, the strength of which varies regionally. In catchments where FFDC and SFDC are independent, FDC can be approximated by a simple convolution of FFDC and SFDC. The proposed conceptual framework opens the way for future modeling studies to explore process controls of fast and slow components of streamflow separately, their dependence, and their relative contributions to the shape of the FDC.