Abstract

The transit time of water is an important indicator of catchment functioning and affects many biological and geochemical processes. Water entering a catchment at one point in time is composed of water molecules that will spend different amounts of time in the catchment before exiting. The next water input pulse can exhibit a totally different distribution of transit times. The distribution of water transit times is thus best characterized by a time-variable probability density function. It is often assumed, however, that the variability of the transit time distribution is negligible and that catchments can be characterized with a unique transit time distribution. In many cases this assumption is not valid due to variations in precipitation, evapotranspiration, and in catchment water storage and associated (de-)activation of dominant flow paths. This paper presents a general method to estimate the time-variable transit time distribution of catchment waters. Application of the method using several years of rainfall-runoff and stable water isotope data yields an ensemble of transit time distributions with different moments. The combined probability density function represents the master transit time distribution and characterizes the intra- and inter-annual variability of catchment storage and flow paths. Comparing the derived master transit time distributions of two research catchments (one humid and one semi-arid) reveals differences in dominant hydrologic processes and dynamic water storage behavior - with the semi-arid catchment generally reacting slower to precipitation events and containing a lower fraction of pre-event water in the immediate hydrologic response.

Citation

Heidbüchel I., Troch P.A., Lyon S.W., and Weiler M. (2012): The master transit time distribution of variable flow systems. Water Resources Research 48(6): W06520. DOI: 10.1029/2011WR011293

This Paper/Book acknowledges NSF CZO grant support.