NEW LIGHT ON STREAMWATER SOURCES IN THE GLENDHU EXPERIMENTAL CATCHMENTS, EAST OTAGO, NEW ZEALAND

Abstract: The east Otago uplands of New Zealand’s South Island have long been studied because of the environmental consequences of converting native tussock grasslands to other land covers, notably forestry and pasture for stock grazing. Early studies showed that afforestation substantially reduced annual water yield, stream peak flows, and 7–day low flows, mainly as a consequence of increased interception. Hydrometric measurements revealed two rates of recession with a sharp break between them, a rapid rate during the early part of the recession and a slow rate during the latter part. Oxygen-18 measurements showed that most storms in the moderate to small size range were dominated by ‘old’ (stored) water, especially early in the storm, and also in the later phase of base flow recession. It was suggested that the numerous small wetlands in the headwaters of the larger catchments acted as temporary storage areas supplying old water, although later work indicates that the wetlands merely serve to link the surrounding hillslopes with the streams that drain them. Interflow was identified as the source of storm runoff, and drainage from deep loess horizons on the surrounding hillslopes the source of the extended base flow. The present s tudy has used tritium and CFC concentrations to determine the residence times of water contributing to the second part of the recession. Tritium measurements (supported by CFC measurements) indicated that surprisingly old water is present in the (GH5) wetlan