Simultaneous Separation of Runoff Pathways and Storage Times via Coupled Electrical Conductivity Mass Balance and Nonlinear Storage‐Discharge Relationship: Theory and Application Testing

Abstract Streamflow can be separated into fast runoff and baseflow according to runoff pathways, and also into new and old water according to storage times. Typically, baseflow does not completely overlap with old water, nor does fast runoff completely overlap with new water. This imperfect overlap relationship divides total streamflow into four components. Separating the fractions of these components is an important prerequisite for understanding hydrological processes. However, the current baseflow separation methods are still inadequate to accomplish this task, and most of them can only separate two components. Therefore, we developed a new hydrograph separation method that couples mass balance and a nonlinear storage‐discharge relationship. This method can achieve simultaneous separation of the four components based on an inexpensive tracer electrical conductivity (EC) without the need for precipitation data. The applicability of the new method was tested at 51 gages in the continental United States, which have high‐quality streamflow and EC data and are widely hydrologically representative. The results show that the new method is applicable to small‐to medium‐scale natural watersheds across a wide range of climatic, vegetative, and geologic zones. The added consideration of the impacts of passive storage and water‐rock interaction significantly increases the reproducibility of streamflow EC and also leads to more similar baseflow and old water separation, which in fact have significant differences due to distinct components they contain. The new method has a well‐defined physical basis and can be expanded for several practical applications.