Studying catchment storm response using event- and pre-event-water volumes as fractions of precipitation rather than discharge
Authors: von Freyberg Jana , Studer Bjørn , Rinderer Michael , Kirchner James W. 
Published: November 2018

Abstract. Catchment response to precipitation is often investigated using
two-component isotope-based hydrograph separation, which quantifies the
contribution of precipitation (i.e., event water Qe) or water
from storage (i.e., pre-event water Qpe) to total discharge (Q)
during storm events. In order to better understand streamflow-generating
mechanisms, two-component hydrograph separation studies often seek to relate
the event-water fraction Qe∕Q to storm characteristics or
antecedent wetness conditions. However, these relationships may be obscured
because the same factors that influence Qe also necessarily
influence total discharge Q as well. Here we propose that the fractions of
event water and pre-event water relative to total precipitation
(Qe∕P and Qpe∕P), instead of total discharge,
provide useful alternative tools for studying catchment storm responses.
These two quantities separate the well-known runoff coefficient (Q∕P, i.e.,
the ratio between total discharge and precipitation volumes over the event
timescale) into its contributions from event water and pre-event water.
Whereas the runoff coefficient Q∕P quantifies how strongly precipitation
inputs affect streamflow, the fractions Qe∕P and
Qpe∕P track the sources of this streamflow response. We use high-frequency measurements of stable water isotopes for 24 storm
events at a steep headwater catchment (Erlenbach, central Switzerland) to
compare the storm-to-storm variations in Qe/Q,Qe/P
and Qpe∕P. Our analysis explores how storm characteristics and
antecedent wetness conditions affect the mobilization of event water and
pre-event water at the catchment scale. Isotopic hydrograph separation shows
that catchment outflow was typically dominated by pre-event water, although
event water exceeded 50 % of discharge for several storms. No clear
relationships were found linking either storm characteristics or antecedent
wetness conditions with the volumes of event water or pre-event water
(Qe, Qpe), or with event water as a fraction of
discharge (Qe∕Q), beyond the unsurprising correlation of larger
storms with greater Qe and greater total Q. By contrast, event
water as a fraction of precipitation (Qe∕P) was strongly
correlated with storm volume and intensity but not with antecedent wetness,
implying that the volume of event water that is transmitted to streamflow
increases more than proportionally with storm size under both wet and dry
conditions. Conversely, pre-event water as a fraction of precipitation
(Qpe∕P) was strongly correlated with all measures of antecedent
wetness but not with storm characteristics, implying that wet conditions
primarily facilitate the mobilization of old (pre-event) water, rather than
the fast transmission of new (event) water to streamflow, even at a catchment
where runoff coefficients can be large. Thus, expressing event- and pre-event-water volumes as fractions of
precipitation rather than discharge was more insightful for investigating the
Erlenbach catchment's hydrological behaviour. If Qe∕P and
Qpe∕P exhibit similar relationships with storm characteristics
and antecedent wetness conditions in other catchments, we suggest that these
patterns may potentially be useful as diagnostic “fingerprints” of
catchment storm response.