CONCENTRATION-DISCHARGE RELATIONSHIPS HIGHLIGHT DIFFERENCES IN STREAMFLOW GENERATION BETWEEN TWO TRIBUTARIES IN A HEADWATER CATCHMENT

Many studies have compared water chemistry and stream discharge to investigate runoff generation processes in headwater streams. Studying small stream systems through this lens, can help us to understand how runoff generation varies at small spatial scales. In this study, we compared three streams in the same 43 ha catchment. We collected water samples from each stream twice a week. We used the discharge that was recorded at a flume at the catchment outlet for the time the sample was collected. Water samples were filtered in the field and analyzed for Al, Ca, Mg, and Si using an ICP-OES. Mg concentrations did not change much between steams, but Si, Al, and Ca varied between the three streams and with discharge. We observed that Al had the greatest range of variability. Overall, we saw that concentrations decreased as discharge increased according to a power law relationship, but with varying slopes. Many factors can affect the slope of this relationship, such as chemical weathering and differing runoff generation processes. Since the parent material composition does not appear to vary across the catchment, different runoff generation processes may explain the different relationships between discharge and concentrations in these stream systems. One of the sampled sub-catchment streams contains a paved road, so runoff from this impervious surface could be one reason for the differing relationships observed. Additionally, previous work has suggested that shallower flow paths dominate in the same sub-catchment, which could also explain the different slope. This study suggests we may be able to detect variations in runoff generation at small spatial scales using concentration-discharge relationships.