The mechanisms of catchment flow processes using natural variations in deuterium and oxygen-18

Abstract

Stable isotope compositions and chloride concentrations in rainfall, deep and shallow groundwaters and streamflow have been used to investigate the flow processes through a small catchment in southwest Western Australia. In addition to the stable isotope and chloride data, daily rainfall, streamflow rate, streamflow chloride concentrations and shallow and deep groundwater hydrographs were used in an integrated approach to determine the mechanisms of streamflow generation. The range in δ 18O and δ 2H of rainfall events incident to the catchment during 1985 was 9.0 and 60‰, respectively, and was large enough to be useful in tracing flows generated from them through the catchment. During the season, four separate rainfall events were analysed using stream hydrograph separation methods based on δ 18O, δ 2H and chloride. Results showed that between 60 and 95% of the streamflow generated from the respective events had originated from pre-event shallow groundwater within the catchment. The seasonal pattern in the streamflow isotopic composition during 1985 was also analysed. Most significant rainfall events during the season generated an almost immediate response in the streamflow isotopic composition. The response to individual rainfall events was strongest early in the season but diminished as the season progressed. This attenuation with time indicated isotopic mixing and dilution of individual rainfall events with the increasing storage of shallow groundwater as the season progressed. Kalman filtering methods were used to investigate the function linking rainfall volume and streamflow and also the average time lag between rain and flow, using δ values in rainfall as time markers. The analysis showed a range of “ages” for the streamflow from about 20 to about 50 days lag after rainfall, tending to decrease after rainfall events and then increase again. The estimated value of the lag was always significantly greater than zero. Deep and shallow groundwaters in the 1985 season were readily distinguished from each other in δ 18O- δ 2H space. Streamflow isotopic composition matched that of the shallow groundwater system for almost the entire season. Thus the isotopic evidence was in accord with chemical evidence that streamflow originated primarily from preceding rainfall after a comparatively short residence time in the shallow groundwater system.