Mechanisms Affecting Streamflow and Stream Water Quality: An Approach via Stable Isotope, Hydrogeochemical,and Time Series Analysis

Abstract

Stable isotope (deuterium) and hydrogeochemical (chloride, nitrate) data in rainfall, streamflow and deep and shallow groundwaters have been applied to develop an understanding of the mechanisms of streamflow generation and nutrient discharge from a 27 km2 rural catchment near Perth, Western Australia. Intensive sampling of the stable isotopic composition of rainfall, groundwater and streamflow during the winter flow periods of 1987 and 1988 identified several significant rainfall events that were up to 90‰ depleted in deuterium relative to deep and shallow groundwaters in the catchment. Isotopic responses in streamflow to the depleted rainfall could be discerned on three time scales: (1) the rainfall events themselves (i.e., 1 to 2 days); (2) isotopic relaxation periods lasting up to 6 weeks as streamflow δ values slowly returned to their preevent values; and (3) a longer seasonal scale response of 2 to 3 months caused by a seasonal isotopic trend in rainfall composition and an increase in the proportion of groundwater in streamflow toward the end of the flow period. The sharp isotopic responses in streamflow observed in this study were in marked contrast to the attenuated responses observed in a previous study on a 1 km2 catchment under native forest in the same region. This contrast reflects the different roles of the shallow groundwater system in streamflow generation between the two catchments under different land use. Kalman filtering techniques were applied to investigate the numerical functions linking rainfall amount and streamflow volume and their respective stable isotopic compositions. The purpose of the numerical analysis was to estimate the average lag between rainfall and streamflow, thereby quantifying the time scales of streamflow generation. Analysis of streamflow water quality has been shown that the bulk of nutrients and solutes are discharged from the catchment at the onset of flow, in response to the first major rainfall events of the winter period. Integration of the isotopic, hydrogeochemical and numerical analysis presents a consistent view that surface and subsurface storm flow are important streamflow generating mechanisms over the time scale of a rainfall event. Postevent periods of isotopic relaxation in streamflow are due to discharge of groundwater to streamflow from an ephemeral perched aquifer. There is evidence for an increasing component of deep groundwater in streamflow as streamflow declines toward the end of the flow period in October and November.