Temporal variation of river water mean transit times and hydrological pathway contributions in Waikato catchments

Abstract

Bayesian chemistry-assisted hydrograph separation (BACH, Woodward and Stenger, 2018) assumes that river hydrographs can be explained by temporally varying flow contributions by three main pathways connecting the land with the monitoring site (very young near-surface water, young shallow groundwater, and old deep groundwater). Temporal variation in pathway contributions should consequently also result in the mean age of the river water varying with flow. It was the aim of this study to ascertain how dynamic flow contributions and tritium-based mean transit times (MTTs) are in a diverse group of 19 Waikato rivers, and whether the MTT dynamics matches the pathway dynamics.