Multi-millennial streamflow dynamics in two forested watersheds on Vancouver Island, Canada

Abstract

a b s t r a c tHolocene streamflow was reconstructed for two rivers on Vancouver Island, British Columbia, Canada in 500-yr intervals. The San Juan River watershed is located on the wetter western side of the island, whereas the Koksilah River watershed is positioned on the drier eastern side. Both watersheds are forested. To reconstruct streamflow, temporal changes in precipitation (estimated using a pollen-based transfer function) and evapotranspiration were established for each watershed and integrated into a water balance model, calibrated using modern data. While seasonal streamflow variability was maintained throughout the Holocene, with greater flow in the winter relative to the summer, the amount of discharge has changed markedly through time. Lowest simulated flow occurred in the earliest Holocene, with low-flow conditions beginning earlier in the year and extending later into the fall. Such conditions may have inhibited salmon from using many of the smaller rivers in the region. Streamflow steadily increased throughout the early Holocene so that by ca. 6500 cal yr before present nearmodern flow regimes were established. As climate changes in the future, the San Juan and Koksilah watersheds are expected to remain as pluvial hydroclimatic regimes, though with an extended season of low flow similar to conditions during the early Holocene.Crown

Long-term climate, vegetation and fire regime change in a managed municipal water supply area, British Columbia, Canada

“…Adding an additional data point (0.55 DWHI and 1386 mm) from the topmost sample of the Begbie Lake core to the existing DWHI-annual precipitation regression (Brown and Schoups, 2015) did not profoundly change the regression. Following data transformation, the best fit parameters of the new linear regression are a = 737 and b = 1.63, with a RMSE (root mean square error) = 534 mm, which are similar to the previous regression (744, 1.64 and 535 mm, respectively).…”

“…The water balance models for the Rithet and Judge catchments were forced with reconstructed Holocene precipitation using the DWHI regression, resulting in reconstructed monthly discharge (Figure 5). The model parameters for both catchments were the same as those obtained by Brown and Schoups (2015) for the nearby Koksilah River basin, located north of SLW. The water balance models correctly reproduce historical discharge data for Rithet and Judge creeks, with RMSE values of 13.9 and 13.3 mm month −1 , respectively (Figure 5a and 5b).…”

“…In addition to pollen percentage, pollen accumulation rate (grains cm −2 yr −1 ) was calculated by multiplying pollen concentration (grains cm −3 ), determined using the number of Lycopodium spores encountered, by the vertical accretion rate (cm yr −1 ). A previously developed Pseudotsuga (Douglas-fir) to T. heterophylla (western hemlock) pollen index (DWHI; Brown and Schoups, 2015; Brown et al, 2006) was used to quantitatively reconstruct changes in Holocene precipitation. The upper DWHI value from the Begbie Lake core provided an additional data point that was added to the existing regression model (Brown and Schoups, 2015), slightly changing the regression parameters.…”

“…A previously developed Pseudotsuga (Douglas-fir) to T. heterophylla (western hemlock) pollen index (DWHI; Brown and Schoups, 2015; Brown et al, 2006) was used to quantitatively reconstruct changes in Holocene precipitation. The upper DWHI value from the Begbie Lake core provided an additional data point that was added to the existing regression model (Brown and Schoups, 2015), slightly changing the regression parameters. The DWHI ratio is based on the fact that Pseudotsuga favours xeric settings whereas T. heterophylla prefers hydric environments.…”

“…Temperature drift corrections were automatically applied every five samples. Holocene streamflow was reconstructed for Rithet and Judge creeks using a calibrated water balance model for each catchment, with model development, validation and assumptions described in detail in Brown and Schoups (2015). Briefly, the model accounts for temporal changes in annual precipitation by applying the DWHI-precipitation transfer function to down-core changes in DWHI values from the Begbie Lake core.…”