ABSTRACT

Potential climate changes have the capacity to alter the magnitudes and frequencies of extreme hydrologic events. The main objective of this study was to investigate climate change impacts on floods in a medium sized watershed, namely James River watershed in south western Alberta, by means of semi-distributed modeling.

A flood simulation technique has been developed by initial derivation of a regional dimensionless hydrograph, RDH. The RDH facilitates synthesis of unit hydrographs, UGs, for sub-basins of the watershed. The UGs are then implemented in calibration of the HEC-1 hydrologic model using the Natural Resources Conservation Service Curve Number method which transforms excess rainfall to direct runoff. Due to data constraints, regional hydraulic equations for south western Alberta watersheds are developed and combined with a triangulated irregular network model of the watershed to estimate parameters for numerical kinematic wave routing equation.

Probability models are fitted to hydro-climatic variables and linked to constitute a Monte Carlo Simulator (MCS). Five-day rainfall, a surrogate of moisture conditions, is found to fit well to 2-parameter Gamma distribution, while discharge at start of direct runoff during a flood event is adequately described by Log-Pearson Type III distribution. The MCS/HEC-1 interface offers an efficient, fast and automated system for realistic flood hydrographs simulations under different climatic scenarios.

Climatic scenarios are then designed by 1) a hypothetical method based on elasticity theory, 2) statistical method (SM) based on coupling canonical correlation analysis and a modified kriging estimation (MKE) using the Canadian General Circulation Model outputs, 3) temporal analog method and 4) spatial analog method. The MKE is found to have less prediction error compared to classical ordinary kriging.

The scenarios for the doubling of atmospheric carbon dioxide indicate significant increase in storm rainfall ranging between 14.7 and 26.3 % in the mean and 17.7 to 41.08 % in the standard deviation. The curve number reduces by 0.7 % but is associated with significant shift in the ecological zones.

Results reveal an overall shift towards more frequent occurrence of heavy floods e.g. the 200-year flood becomes 70-year flood in the SM scenario while the time to peak shortens by an average of 0.2%. Risk analysis suggests increased vulnerability to climate change for structures with short to medium design lives. However, the simulated hydrographs are significantly dependent on the scenario applied. In general the SM scenario gives estimations in the mid range of other scenarios.
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