Ó£»¨Ó°ÊÓ

Topic

Modelling the impact of water and land use on low flows in Xwulqw'selu Sta'lo', a coniferous rain-dominated watershed on Vancouver Island

Department of Civil Engineering

Date & location

• Tuesday, May 20, 2025

• 9:00 A.M.

• Engineering Computer Science Building

• Room 468

Reviewers

Supervisory Committee

• Dr. Tom Gleeson, Department of Civil Engineering, Ó£»¨Ó°ÊÓ (Supervisor)

• Dr. Tara Troy, Department of Civil Engineering, UVic (Member)

• Dr. Kristian Dubrawski, Department of Civil Engineering, UVic (Member) 

External Examiner

• Dr. Rajesh Shrestha, Water Resources and Climate Change, Environment Climate Change Canada 

Chair of Oral Examination

• Dr. Erica Woodin, Department of Psychology, UVic

   

Abstract

What is the influence of land use and water use on summer low flows in Xwulqw'se Sta'lo', a coniferous rain-dominated watershed on Vancouver Island, within the traditional territories of the Cowichan Tribes? Low flows have declined in recent decades due to climate change, water use, and potentially land use. This study develops a robust hydrological model with the most comprehensive available data on watershed characteristics, water use and land use. A generalized conceptual model of the watershed was first developed as a basis for numerical modeling. Then SWAT-MODFLOW, a widely used computer program for simulating surface and subsurface hydrology along with land and water use, was selected since it integrates the semi-distributed SWAT model for land surface components with the fully distributed MODFLOW groundwater model. Input variables include topography, geology, soils, and climate (precipitation, temperature, solar radiation, humidity, wind speed), while outputs include streamflow, evapotranspiration, infiltration, surface runoff, baseflow and groundwater recharge. The Xwulqw'selu SWAT-MODFLOW model simulates watershed processes across wet and dry seasons and years and all relevant watershed processes including forestry, tree growth, surface water use and groundwater use. Our focus is on summer low flows, while putting low flows in the context of other watershed processes.

The model is calibrated using recent (2013–2022) streamflow and groundwater levels and then compared to monitoring data from across the whole watershed. The calibrated model was further refined into a ‘baseline model’ by improving Douglas Fir growth parameters, and updating water use data. This baseline model represents current conditions and serves as a reference for evaluating alternative land and water use scenarios, which are versions of the baseline model that change water use or land use but keep all other variables the same so that we can see the possible impacts water use and land use. Water use scenarios include changing total 4 water use, decreasing groundwater or surface water use and changing the timing of water use restrictions. Land use scenarios by representing forests of different ages across the watershed including a mature forest scenario, and mature and immature forest scenario. 

The baseline model and scenarios could be useful for ongoing watershed sustainability planning but the model and our approach has limitations. First, the model relies on the best available Western scientific data and methods rather than Indigenous knowledge and methodologies. Second, focusing on recent conditions limits the ability to investigate long-term low-flow drivers including climate change or represent pre-impact watershed conditions, before extensive water withdrawals or extensive old-growth logging. Third, our analysis focuses on one aspect of low flows and land use as represented by age of forests but watershed health and management need to be much more holistic. For example, this study focuses on the age of forests in different land use scenarios, but forests are much more complex, and have significant cultural, ecological, and economic implications, particularly for biodiversity, ecological integrity, and cultural practices. Additionally, low flows can be characterized by flow rate, duration, or timing but herein we focus exclusively on flow rate. Fourth, water used for irrigation is removed from the watershed in the model even though some of it will return to the aquifers and streams. Fifth, although wet and dry years are included in the model, we do not specifically assess the impact of climate variability. Overall, it is important not to over-interpret the results of this study that focuses on a singular hydrologic metric (the volume of summer low flows) and put this in the context of multiple management priorities as well as Quw'utsun Mustimuhw teachings, knowledge and priorities.