A first-order analysis of the climate change effect on flood frequencies in a subalpine watershed by means of a hydrological rainfall-runoff model

Abstract Scientific evidence indicates that a global climate change due to human activity is possible within a century. It is expected that doubling of CO 2 and increasing the amount of other greenhouse gases in the atmosphere would result in more severe weather, among other consequences. The purpose of the paper is to examine how flood frequencies and magnitudes in a mid-size subalpine watershed on the eastern slopes of the Rocky Mountains in Alberta, Canada, would change under 2×CO 2 conditions. Given the poor spatial resolution of present general circulation models (GCM) and their uncertain performance at the regional scale, a first-order analysis is carried on, in which only rainfall intensity changes are considered to have the most significant impact on future floods. Estimates of storm rainfall increases are based on the literature survey, GCM projections for the study area, and transposition of southern climatic conditions. Two scenarios of likely most severe changes were selected: first, a 25% increase in the mean and standard deviation of Gumbel distribution of rainfall depth for storm durations from 6 to 48 h; second, a 50% increase in the standard deviation only. The HEC-1 watershed model and the soil conservation service runoff curve method for abstractions were used in Monte Carlo simulation. Comparison of Monte Carlo derived flood frequency curves for the two scenarios with the present day curve shows that scenario 1 is more critical in terms of flood flow increases than scenario 2. Under scenario 1, the mean annual flood on the study watershed would increase by almost 80% and the 100-year flood would increase by 41%.