A shifting hydrological regime : a field investigation of snowmelt runoff processes and their connection to summer baseflow, Sunshine Coast, B.C.

The annual hydrographs in British Columbia rivers are either characterized by glacial, nival, pluvial or “hybrid” (both pluvial and nival) sources of runoff. Climate change scenarios for the 2050s indicate that snow-water-equivalent (SWE) will diminish by 50 to 80% in lower snowfed-dominated basins in the South Coastal region compared to historical values. This could trigger a shift from a hybrid to a pluvial regime for many creeks, including streams used as primary water supply such as Chapman Creek on the Sunshine Coast. It has been suggested in previous studies that this change in runoff regime will negatively impact summer low flows due to an earlier onset of snowmelt and a prolonged summer recession period. However, the connection between groundwater recharge during snowmelt and late-summer water yield remains unclear. A local headwater catchment (Stephen’s Creek) was instrumented and monitored from the fall of 2008 to the fall of 2009. A two- and a three-component isotopic hydrograph separation (2-, 3-IHS) method was developed by adapting the runoff-corrected model (runCE) to a semi-distributed environment in order to account for spatial variability in snowmelt and in isotopic release from the snowpack. IHSs results show that event water (snowmelt) and soil water composed most of the streamflow both at the headwater site (66 ± 19%) and at the mouth (62 ± 23%) during the peak of the freshet, while the contribution of event water to streamflow was significantly different in July (34 ± 11 % at the headwater site vs. 7 ± 4% at the mouth). Hydrometric, isotopic and geochemical data suggest that saturated throughflow was the predominant flow-path taken by melt water during freshet. Preliminary streamflow recession analysis revealed that the snowmelt-recharged headwater catchment can support a steadier summer baseflow than Robert’s Creek – a much larger, but rainfed-dominated watershed. It is concluded that the large input of melt water during the spring was sufficient to “over-turn” the shallow subsurface reservoir of the headwater catchment and recharge deeper flow-paths at a rate that can not be matched by rainfed-dominated systems. The results are of interest to water resource planning in the South Coastal region.Science, Faculty ofResources, Environment and Sustainability (IRES), Institute forGraduat