Combining Hydrometric and Hydrochemical Data Sets for Investigating Runoff Generation Processes: Tautologies, Inconsistencies and Possible Explanations

The rapid increase of catchment runoff after the onset of rainstorms and the associated short-term changes of stream-water solute concentration are still poorly understood. These factors are dependent on the flowpaths that the water follows to the stream, and the length of time from rainwater infiltration to discharge. To study these processes, different approaches are used. Hydrometric data on, for example, discharge, groundwater level, soil matrix potential and soil moisture data, have a high spatial and temporal resolution. In contrast, hydrochemical data for solutes, isotopes and temperature, are spatial and temporal integrals. This article provides an overview of the different approaches that are used to investigate stormflow runoff, and critically examines apparent and actual tautologies and inconsistencies in them. Some of the inconsistencies can be traced back to a failure of the basic assumptions, whereas others remain to challenge our understanding of the runoff generation process. Finally, suggestions for future avenues of research in this field are made.