FINAL REPORT - International Joint Commission

FINAL REPORT
Sensitivity Analyses
During the course of the six “practice” decision workshops, the Study Board was able to isolate the
performance indicators that were critical to the decision. Because the estimated values of these indicators
were so important, the indicators were given extra scrutiny. But no amount of review will guarantee
perfect models of the future, so, in some cases, the Study Board asked that an analysis be conducted to
determine how sensitive the plan performance was to variations in modeling elements that could be
considered within the range of irreducible uncertainty. The investigations fell into three categories:
• Extreme conditions: Would a plan that scores well during periods of average hydrologic conditions
fail when water supplies are extremely dry, extremely wet, or sequenced differently?
• Climate change: Would a plan that performs well under the current climate fail in fifty years or so if
climate change alters the seasonal timing and amount of water supply and evaporation?
• Performance indicator error: Would plan performance change from acceptable to unsatisfactory if the
modeling assumptions behind the performance indicators driving the ranking change to a different but
defensible value?
Extreme Conditions Analysis
The Board used average annual benefits to conduct its initial evaluation of plans, but asked whether high
ranking plans also performed well in extreme conditions. The Plan Formulation and Evaluation Group
addressed this in two ways:
1. It compared the candidate plans under four particular centuries selected from the stochastically
generated 50,000-year sequence: the century with the most severe Lake Ontario supply drought (S1);
the century with the most severe wet Lake Ontario supply period, which also had the largest range
from wet to dry supplies (S2); a century with a similar range and average of supplies as the historical
(S3); and a century with the longest sustained Lake Ontario drought (S4). These four “centuries”
show differences among the plans during the most unusual water supply sequences in the 50,000-year
sample and in the case of average supplies in a different sequence from the historical. Because the
complete Integrated Ecological Response Model component of the Shared Vision Model could not
feasibly be adapted to run the full 50,000-year sequence, these four centuries give a sampling of how
the environmental performance of the plans might vary under extreme conditions. This is used in
conjunction with the Lake Ontario wetland model results that were run over the full 50,000-year
sequence to provide insights into how the environmental performance indicators respond to conditions
outside the historical range.
2. It developed frequency distributions for a few key water levels (Lake Ontario, Lake St. Lawrence,
Lac St. Louis, and Montreal) and releases. These graphs occasionally revealed changes in plan
behavior between average and extreme conditions.
Figure 42 shows a five-year moving average of the net total supply (NTS) for the four stochastic centuries
(S1-S4) and the historical net total supplies. S1 (pink line) has extremely dry supplies towards the middle
of the century. S2 (green line) starts with extremely wet supplies towards the beginning of the century,
then drops to low supplies towards the end of the century. S3 (blue line) has about the same length of
time between wet and dry periods as the historical case and its wet and dry sequences are never more
extreme than the historical case (black line). S4 (red line) has a very long drought that lasts over half
the century.
72 Options for Managing Lake Ontario and St. Lawrence River Water Levels and Flows