 This study examines the mechanisms behind the rapid release of stored water from macropores into streams. It uses a tile-drained field site to combine macroporous soils with controlled lower boundary conditions. Three experiments were conducted to measure the effects of different initial conditions on the amount of irrigation water needed to increase the tile drain discharge above the base flow level. Additionally, oxygen, 18, and deuterium data were collected to determine the contribution of pre-event soil water to the tile-drain event flow. Finally, mixing calculations were used to estimate the percentage of soil water that was irrigation water. The results show that the pre-event water in the tile drain was mobilized in 20 to 40 centimeters soil depth, where the macro-pore matrix interaction initiates macro-pore flow once the moisture threshold is exceeded. This article was authored by J. Klaus, EZ, M. Ellsner, and others.