Science Outputs

Quantifying groundwater contribution to stream flow generation in a steep headwater catchment

Journal of Hydrology, Vol. 53 (1)

Silica (SiO2) concentration of water increases with contact time between the water and subsurface silica-bearing minerals. Consequently, SiO2 can be used as a tracer to investigate the water sources (reservoirs) and associated different flow paths through a catchment. By measuring flow and SiO2 concentration in a stream, and relating these data to the stream concentrations from near-surface (2.8 mg l-1 SiO2) and groundwater (23.0 mg l-1 SiO2), the contributions that each of these reservoirs make per rain event, on a monthly and annual basis can be estimated. Analysis of 23 rain events revealed an increase in stream flow (0.07 to 0.42 l s-1) can be explained by increase in groundwater discharge, without a contribution from ‘near-surface’ water. A regression equation that relates SiO2 concentrations to stream flow was used with a mixing model to determine the maximum groundwater discharge to stream flow (1.72 l s-1) occurs at 21.3 l/s which represents a contribution of 8.1% of the flow. On a monthly basis, the discharge from groundwater to stream flow is described by a logarithmic relationship, with maximum contribution of 1500 m3 month-1. Near-surface flow follows an exponential relationship and can reach 4000 m3 month-1. On an annual basis the contributions from both reservoirs are best described by linear models. Groundwater accounts for a minimum of 52 % of stream flow during the wettest of the nine years and a maximum of 74 % during the driest year. Concurrent measurements, suggests that SiO2 is better suited for separation of the sources of stream flow than 18O in this catchment.