Research Spotlight: Interpreting parameters in water transit time distribution



[1] The distribution of the time it takes water or solute molecules to flow through a drainage basin, or catchment, can be useful for understanding variability, storage capacity, and pollutant transport in catchments and for comparing hydrology in different catchments. A statistical distribution called the gamma distribution has been shown to be a suitable representation of transit time distribution in hydrological tracer studies. However, the gamma distribution involves two parameters that have not had a clear physical interpretation when used as a catchment transit time distribution. To explore a physical interpretation for these parameters, Hrachowitz et al. analyzed long-term tracer and precipitation data in three different catchments in the Scottish Highlands. They found that one parameter had little variation in time and did not vary along with meteorological/climatic conditions such as precipitation intensities but instead was related to features of the catchment landscape such as the hydrological characteristics of the soils and drainage density. The other parameter was found to vary in time and was controlled largely by precipitation intensity. Using this information, the authors calculated time-variant transit time distributions, which they suggest provide more detailed and useful information about the timing and magnitude of tracer fluxes. This could be especially useful in representing the effects of contaminants in sensitive catchments. (Water Resources Research, doi:10.1029/2010WR009148, 2010)