Climate Change in Canadian Floodplain Mapping Assessments
Date
2020
Authors
Rajulapati, Chandra Rupa
Tesemma, Zelalem
Shook, Kevin
Papalexiou, Simon Michael
Pomeroy, John W.
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Centre for Hydrology, University of Saskatchewan, Saskatoon, Saskatchewan
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Technical Report
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Abstract
In the recent decades, precipitation patterns and corresponding streamflow responses in
many cold regions catchments have changed considerably due to warming. Understanding
historical changes and predicting future responses are of great importance for planning and
management of water resources systems. Regional climate simulations using convention-
permitting models are helpful in representing the fine-scale cloud and mesoscale processes,
which are critical for understanding the physical mechanisms that cause in convective
precipitation. From a hydrological perspective, these fine resolution simulations are helpful
in understanding the runoff generation mechanisms, particularly for mountainous
watersheds, which have high spatial variation in precipitation due to large differences in
elevation over small distances.
The sister-study of this report, the Bow River Basin Study (BRBS), used a physically based
hydrological land surface scheme along with a water management model, coupled with a
high resolution convention- permitting atmospheric regional model (Weather Research and
Forecasting, WRF) to understand the streamflow generating mechanisms and identify the
changes in streamflow responses of the Bow and Elbow River Basins. The coupled model
appears to provide a large improvement in predictability, with minimal calibration of
parameters and without bias correction of forcing from the atmospheric model. The model4
was able to provide reliable estimates of streamflows, despite the complex topography in the
catchment. Using the WRF Pseudo Global Warming (PGW) scenario, estimated future
streamflows simulated were then used to develop projected flow exceedance curves. The
uncertainty in the simulations is extremely helpful in the risk assessment for downstream
flood inundations. However, the uncertainty in streamflows cannot be assessed as the WRF-
PGW dataset was only available for a single realization, because of the high computational
cost.
The research presented in this report focusses instead on using the highly efficient
hydrological model developed and verified in BRBS whilst assessing uncertainty using
another regional climate model, the CanRCM4, where many realizations are available for
different boundary conditions. Since the CanRCM4 simulations have a relatively low
resolution, a novel methodology was developed to adjust regional climate model outputs
using the WRF-PGW data. An ensemble of 15 CanRCM4 simulations was used to force the
Bow River basin model to determine a measure of the uncertainty in the simulated
streamflows, and the projected streamflow exceedance probability curves. These curves are
extremely useful for risk assessment for downstream flood inundations. Given the
importance of understanding how much extreme precipitation will change in urban areas of
the basin, where short duration high intensity events cause flash flooding, frequency analysis
of these events was carried out for Calgary and Intensity Duration Frequency (IDF) curves
were developed. A ready-to-use empirical form of IDF curve has been proposed from this
analysis for the City of Calgary.
The results from the WRF-PGW modelling indicated that future high flow, low frequency
(exceedances less than 10%) streamflow events will decrease compared to those under the
current climate condition by 4, 9 and 1.6 m3/s for the Bow River at Banff and Calgary and
Elbow River at Sarcee Bridge respectively. The average of the 15 new CanRCM4-WRF-PGW
results supports the above result with some greater decreases in streamflow of 9, 16 and 4
m3/s for Bow River at Banff and Calgary and Elbow River at Sarcee Bridge respectively.
However, there were some CanRCM4-WRF-PGW realisations that suggested substantial
increases in future low frequency streamflow from those indicated by the average CanRCM4-
WRF-PGW-drive MESH model. The below average, high frequency (exceedances greater than
30%) future streamflows will increase modestly in all gauging locations by from 1 to 12.5
m3/s.
The results of the extreme precipitation analysis at Calgary indicated an increase in future
extreme precipitation events of all duration and return periods. On an average an increase
of 1.5 times is noted for short return periods (=2, 5), and an increase of 4 times for long
return periods (=500, 1000).
Description
Keywords
Floodplain mapping, Hydrological modelling, Climate change, Canada, Bow River, Elbow River, Precipitation, streamflow forecasts
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Part Of
Centre for Hydrology Report #17