Oxygen-18 studies of catchment runoff generation

Understanding how runnoff is generated in catchments is important for landuse management. The authors used the response of streams to inputs of rainfall and oxygen-18 to give insight into the process of generating runoff. At Maimai, a West Coast pine forest catchment converted from native beech, streams rise to high levels very quickly after rainfall. Oxygen-18 studies have shown that most of the water discharged during a rainfall event is actually from displaced soil water not current rainfall. The authors have identified the size of this large soil water store as well as the amounts of water flowing from the macropores (with rapid release) and micropores (slow release) within the soil. In contrast, at Glendhu in tussock grassland of upland Otago, streams continue to flow strongly even when there is no rain. The authors found that more baseflow is sources from the soil B-horizon (a compact loess layer) of the surrounding hillslopes and less from the extensive headwater wetlands than was previously thought. This suggests there may be adverse effects on the baseflow if the area is forested and B-horizon water is access by trees