Quantifying the Principal Components of Stormflow in a Forested and Developed Headwater Catchment with End-Member Mixing Analysis and Continuous Water Quality Measurements

Several studies have been conducted to quantify the components of stormflow in forested headwater catchments. There appears to be a lack of information, however, describing stormflow components in heavily developed catchments. We have monitored stream water quality and flow every 15 minutes during the summer of 2001 at two watersheds - one forested and one developed - in the rapidly-developing north shore of Massachusetts. Observations from this data, particularly electrical conductivity, suggest that hydrologic flowpaths differ significantly between the two catchments during stormflow events. In the heavily developed 3.7 km2 watershed, low conductivity (typically <30 iS/cm) precipitation appeared to be the dominant component of stormflow, with a consistent but short-duration (less than 1 hour) pulse of higher conductivity water from surface runoff (i.e. roads, parking areas) during the rising limb of the hydrograph. Peak stormflow conductivity was approximately 25-95 % lower than pre-event baseflow conductivity with higher magnitude events causing the greatest dilution. Conductivity typically remained 60-80 % below pre-event baseflow values throughout the entire falling limb. Conversely, conductivity in the 4.7 km2 forested catchment stream was only slightly diluted (10-15 % lower than pre-event baseflow) or even increased during the rising limb of the hydrograph. Conductivity values were also approximately 1.5 - 2 times higher than pre-event baseflow values throughout the duration of the falling limb of the hydrograph. This implies that higher conductivity pre-event water (i.e. soil water and groundwater) is likely the principal component of stormflow in the forested catchment. We will present a multi-component end-member mixing analysis model to describe principle stormflow components from these two catchments. In addition, the model will be used to determine if continuous stream water quality measurements can be used as a simple tool for monitoring contributing areas during stormflow events.