Quantifying the Glacial Meltwater Component of Streamflow in the Middle Fork Nooksack River, Whatcom County WA Using a Distributed Hydrology Model
Abstract
Glacial meltwater is a vital component of rivers and streams in glaciated regions such as the Pacific Northwest, and can be critical for municipal water supplies, power generation, and habitat issues. The Middle Fork of the Nooksack River is fed by meltwater from Deming Glacier on Mount Baker, WA. The City of Bellingham has been diverting water from the Middle Fork since 1962 to supplement the water supply, and to maintain water quality in Lake Whatcom, the water source for the city. Because of regulations, water is only diverted when the Middle Fork exceeds minimum acceptable streamflow. A concern for water resource managers in Whatcom County, WA, is that Deming Glacier is retreating. In this study, the Distributed Hydrology Soils Vegetation Model (DHSVM) is used to perform a detailed assessment of the hydrology in the Middle Fork basin, to quantify future meltwater contributions to the Middle Fork Nooksack River as Deming Glacier continues to retreat, and to evaluate streamflow contributions based on predicted climate change.
DHSVM is a physically based, spatially distributed hydrology model that simulates a water and energy balance at the pixel scale of a digital elevation model (DEM). DHSVM requires multiple GIS input grids to characterize the watershed including a DEM, soil type, soil thickness, vegetation, stream network, and watershed boundary. Required meteorological input includes an hourly time series of air temperature, relative humidity, incoming shortwave and longwave radiation, and wind speed. Meteorological data were compiled from historical records of lower-altitude weather stations. The model was calibrated to measured snow-water equivalent at the Middle Fork SNOTEL station and stream discharge at the USGS stream gauge on the Middle Fork using a 1-hour time step and 50m GIS grid resolution. Once calibrated, the model was applied to examine the effects of glacier size on streamflow. The model was also applied to simulate future streamflow based on predicted future climate change scenarios.
The estimated glacial meltwater component of late-summer streamflow as defined by the 2002 glacier coverage and present climate conditions was between 8.4% and 26.1%, depending on the climate of a given year (wet year vs. dry year). The late-summer glacial meltwater component was greater for drought simulations and predicted climate simulations, but less for increased precipitation simulations. DHSVM consistently simulated a smaller glacial meltwater component for progressively smaller glaciers. Simulation results suggest that late-summer streamflow in the Middle Fork could be reduced by as much as 8.6% as the direct result of glacier shrinkage predicted in the next fifty years, or by as much as 15.7% as the result of glacier shrinkage and predicted climate change for the same time period.
Glacier shrinkage could have significant implications for salmon habitat and migration during the late-summer, and may in turn compromise the feasibility of the Middle Fork Nooksack diversion. Further research is necessary to evaluate the effects of glacier shrinkage on the entire Nooksack watershed, particularly the North Fork.