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

Abstract

Silica (SiO₂) concentration of water increases with contact time between the water and subsurface silica-bearing minerals. SiO₂ can thus be used as a natural tracer to investigate the different water flow paths (and their associated reservoirs) that generate stream flow. This approach was applied to the Pukemanga stream, discharging from a small (3 ha) steep catchment near Hamilton in the North Island of New Zealand. By measuring flow and SiO₂ concentration in the stream, and relating these data to the stream concentrations originating entirely from ‘near-surface’ flow (2.8 mg l⁻¹ SiO₂) and groundwater discharge (23.0 mg l⁻¹ SiO₂), the contributions that each reservoir made per rain event, on a monthly and annual basis, to stream flow were estimated. Analysis of 23 rain events revealed that an increase in stream flow from 0.07 to 0.42 l s⁻¹ can be explained by increase in groundwater discharge alone, 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 groundwater discharge to stream flow. On a monthly basis, the discharge from groundwater to stream flow was described by a logarithmic relationship, with a maximum contribution of 1700 m3 month⁻¹. This groundwater contribution to the stream flow varied between 31% and 100% of the monthly stream flow. Near-surface flow followed a quadratic relationship and reached up to 4000 m3 month⁻¹. On an annual basis the contributions from both reservoirs are best described by linear models. Groundwater accounted for a minimum of 52% of stream flow during the wettest of the nine years and a maximum of 74% during the driest year, with an average of 66%. Concurrent measurements suggested that in this catchment SiO₂ is better suited for separation of the stream flow generating flow paths than ¹⁸O.