Macropore flow of old water revisited: Experimental insights from a tile-drained hillslope

The mechanisms allowing the rapid release of stored water to streams are poorly understood. Here we use a tile-drained field site to combine macroporous soils at the hillslope scale with the advantage of at least partly controlled lower boundary conditions. We performed a series of three irrigation experiments combining hydrometric measurements with stable isotope and bromide tracers to better understand macropore–matrix interactions and stored water release processes at the hillslope scale. Stable isotope concentrations were monitored in the irrigation water, the tile-drain discharge and the soil water before and after the experiment. Bromide was measured every 5–15 min in the tile-drain hydrograph. Different initial conditions for each experiment were used to examine how these influenced flow and transport. Different amounts of irrigation water were necessary to increase tile-drain discharge above the baseflow level. Hydrograph separation based on bromide data revealed that irrigation water contributions to peak tile-drain discharge were on the order of 20%. Oxygen-18 and deuterium data were consistent with the bromide data and showed that pre-event soil water contributed significantly to the tile-drain event flow. However, the isotopic composition of soil water converged towards the isotopic composition of irrigation water through the course of the experiment. Mixing calculations revealed that by the end of the irrigation experiments 20% of the soil water in the entire profile was irrigation water. The isotopic data showed that the pre-event water in the tile drain was mobilized in 20–40 cm soil depth where the macropore–matrix interaction leads to an initiation of macropore flow after a moisture threshold is exceeded