Evolution of the spatial and temporal characteristics of the isotope hydrology of a montane river basin

Abstract

Precipitation and streamwater were analysed weekly for δ18O in seven tributaries and five main stem sites of a 2100 km2catchment; >60% of it is upland in character. Precipitation δ18O followed seasonal patterns ranging from -20‰ in winter to -4‰ in summer. Seasonality was also evident in stream waters, though much more damped. Mean transit times (MTTs) were estimated using δ18O input-output relationships in a convolution integral with a gamma distribution. The MTTs were relatively similar (528-830 days): tributaries exhibited a greater range, being shorter in catchments with montane topography and hydrologically responsive soils, and longer where catchments have significant water storage. Along the main stem, MTTs increased modestly from 621 to 741 days. This indicates that montane headwaters are the dominant sources of runoff along the main stem of the river system. Models suggest that around 10% of runoff has transit times of less than two weeks during higher flows whilst older (>10-year old) water sustains low flows contributing around 5% of runoff. © 2011 IAHS Press.