Quantifying the effects of climate change and land use on streamflow and lake levels in the Lesser Slave Watershed

We know that what we do on our landscape has an effect on the water in our streams and lake. The LSWC received funding support from Alberta Ecotrust to work with WaterSMART Solutions to examine how changing our water use, forest cover, and wetland cover would change the water flows in our rivers and the lake level on Lesser Slave Lake. The model was also used to predict lake levels and stream flows under 2 climate change scenarios. The results of this work have been referenced in the Lesser Slave Integrated Watershed Management Plan and strengthen many of the plans recommendations with the results of the model outputs. 

Introduction 

This project used hydrological modelling to quantify the effects of climate change, land use, and water allocations on the hydrology of the Lesser Slave Watershed and Lesser Slave Lake. This work used the Athabasca Integrated Regional Model (AIRM). AIRM was designed as a high-level model of the entire Athabasca River Basin (ARB), which required that the “Lesser Slave" sub-model first be refined to ensure it best represented the hydrology of the Lesser Slave Watershed, including wetland processes and Lesser Slave Lake storage and flow attenuation.

Once the model was verified to perform well within the watershed, results were obtained under observed historical conditions (1986 – 2015); these results included daily streamflow, lake levels, and average annual runoff by sub-basin. A tracer routine was used to track average daily contributions to streamflow from wetlands for each sub-basin. Second, the effects of water allocations were explored by simulating streamflow and lake levels under historical, doubled, and no water allocation scenarios. Third, the effects of forest disturbance and wetland degradation were quantified by simulating streamflow under land use scenarios where varying percentages of each sub-basin were modified, and under a final scenario where both forest disturbance and wetland degradation were simultaneously simulated. Finally, the effects of climate change were explored by running the model under various historical climate normals and future climate scenarios. These simulations allowed the range of natural variability in streamflow and lake levels to be estimated in the watershed, as well as how climate change could alter the hydrology of the Lesser Slave Watershed.

Summary of findings

To quantify the effects of climate change, land use, and water allocations on the hydrology of the Lesser Slave Watershed and Lesser Slave Lake, the hydrology of the region was simulated using a refined version of the hydrological model designed as part of the Athabasca Integrated Regional Model. Simulated results show that runoff was greater in higher elevation sub-basins where winter snowpack was larger and less prone to episodic winter melts. Simulated water levels in Lesser Slave Lake have risen only modestly over the last century. However, much larger increases are expected in the coming century due to increased precipitation.

Land use scenarios demonstrate that both forest disturbance and wetland degradation increase streamflow on an annual timescale. The magnitude and timing of these effects depends on the location and type of the disturbance. Sub-basins with large areas of forest or wetland are more prone to changes, since a larger proportion of the sub-basin can be altered. Additionally, forest disturbances typically lead to larger changes in spring snowmelt and freshet and modest increases in low flows, while wetland degradation typically results faster streamflow response to storm events and small increases or decreases in baseflows. Current water allocations in the watershed have a relatively minor effect on water quantity and any discernible effect was limited to the South Heart River, Lesser Slave Lake, and Lesser Slave River. In these sub-basins, a doubling of usage leads to annual changes in streamflow of approximately 1%.

Wetland contributions were both seasonally and annually variable. Wetlands predominantly contributed to streamflow during the low flow periods, suggesting they play important roles in sustaining baseflows during periods in which water shortages are typically observed. On an annual scale, the contributions of wetlands to streamflow vary by sub-basin, mainly as a function of the relative percentage of the sub-basin covered in wetlands. In high relief sub-basins, which are primarily forested, wetlands contribute little to streamflow, while low relief sub-basins with high wetland coverage are very reliant on them for water production.

Changes in climate are likely to drive the most significant alterations to the regional hydrology. Under all tested climate scenarios, the projected changes were large enough to not only increase flows, but also to change the hydrologic regime of much of the watershed. For instance, while many of the sub-basins, particularly those with higher relief, currently do not experience frequent winter warm spells that melt much of the snowpack, these events are projected to occur more often and be more severe in the future. These changes, in turn, are likely to affect the reliability of spring freshet and alter the flood risk. Overall, while changes to land use and water allocations will have important local effects on sub-basin hydrology, changes in climate are likely to be the primary driver of future hydrologic change by disrupting the timing and reliability of flows over the entire watershed.

Figure 14: Lesser Slave Lake average annual lake levels by overlapping 30-year normals from 1901 to 2100. Observed period is referenced as 2000 and encompasses 1986 – 2015. Shaded areas correspond to 10 and 90% quantiles of annual average lake levels.

Summary of Key Findings

Hydrologic Regime

• Runoff is higher in high elevation sub-basins.

• High elevation sub-basins follow a largely snowmelt-dominated streamflow pattern, while low elevation sub-basins are less seasonally variable.

Water Usage

• Water use accounts for less than 1% of annual runoff in the Lesser Slave Watershed

• Usage is primarily concentrated in the lower Lesser Slave and South Heart Rivers. 

 Land Use Change

• Forest disturbances lead to an increase in runoff. The effects of this are larger in higher elevation basins, where precipitation and winter snowpacks are greater. Streamflow increases are most pronounced during spring runoff due to higher winter snowpacks in non-forested areas.

• Wetland degradation leads to an increase in runoff and makes Lesser Slave Lake more prone to high water events. The effects of this are larger in lower elevation basins, near Lesser Slave Lake, where most wetlands are located. Streamflow increases substantially immediately following rain or snowmelt events, but there are minimal increases or decreases in baseflows.

• For both wetland degradation and forest disturbance, increasingly large levels of disturbance lead to linearly proportional increases in runoff at an annual timescale. For low relief sub-basins, wetland degradation has a larger effect, while conversely forest disturbance has a larger effect in high relief forested sub-basins.

Wetland Contributions to Streamflow

• Streamflow contributions from wetlands are highest in low relief and low elevation sub-basins.

• Streamflow from wetlands are highest during low-flow periods, predominantly during the winter months, suggesting they are effective at attenuating flows and storing water to be subsequently released later in the year.

Climate Change

• Future climate conditions are projected to be wetter and warmer under the scenarios used, particularly during the winter months.

• Simulated water levels for Lesser Slave Lake show only modest changes, and a relatively low range of natural variability. Streamflow in sub-basins occurred earlier and was larger relative to the current-day baseline.

• Under all future climate scenarios, projections indicate large increases in water levels and streamflow, well above the range of natural variability.

• Freshet is projected to occur earlier in the spring and winter flows are projected to increase under all climate scenarios.

• Runoff, average annual flow, and peak flows are all projected to increase under future climate scenarios.

Overall, while changes to land use and water allocations will have important local effects on sub-basin hydrology, changes in climate are likely to disrupt the timing and reliability of flows over the entire watershed to a more substantial degree.

For the full report click HERE

Other relevant information from Alberta WaterSMART
Athabasca River Basin Initiative - Review of hydrologic, river systems and land models

Sustainable Water Management in the Athabasca River Basin Initiative Webpage

Alberta WaterSMART website

The South Saskatchewan Water Project 

Thank you to Alberta Ecotrust for project funding.