H.3.1 Changes in Rainfall Amounts and Intensities
The effects of precipitation on flood hazard vary over a wide range of temporal and spatial scales, from the cumulative effects of seasonal rainfall to the intensities encountered during a single storm. The projected approximately 10% increase in winter precipitation, combined with predicted higher temperatures during this same period, will influence the extent of winter snowpack and the timing and rate of melt. Increased temperatures may also influence the intensity of summer convectional showers and the frequency of strong southwesterly flows bringing particularly heavy rainfall to the coast in winter (the so-called pineapple express). For the practitioner, these changes have potential bearing on long-term estimates of the timing and magnitude of winter storms, including rain-on-snow events, the spring freshet, soil water balance, and effects of antecedent moisture on debris flow and debris flood triggering. At shorter (e.g., sub 72-hour) time scales, IDF curves are a standard method to estimate the probability that a given average rainfall intensity will occur at various event return periods. They are routinely used in water management and form the basis for urban stormwater drainage calculations and sizing of culverts, drain pipes and other waste-water infrastructure. Much of this infrastructure is designed to function for a half a century or more, a time scale comparable with that over which measurable changes in precipitation characteristics are expected.
IDF curves are based on historic precipitation at a particular climate station and depend on the statistical principle of data stationarity: that the mean and variance of data will not change significantly over time so that past precipitation patterns can be used to predict future events. However, given that such data stationarity is not expected to hold, IDF curves based on past conditions should be interpreted with caution when used as design inputs for long-term (>30- year design life) infrastructure. For flood assessments, a precautionary sensitivity allowance for climate change is recommended. The basis of such sensitivity analysis would likely be ensemble projections from regional climate models.
Currently, the short-term precipitation data required to construct IDF curves cannot be discerned by regional climate models, which typically report results at monthly or longer time scales. This poses a challenge for workers tasked with estimating rainfall intensities in a changing climate. Prodanovic and Simonovic (2007) generated simulated IDF curves for London, Ontario, based on existing, drier, and wetter climate scenarios. These authors used non-parametric weather generators to produce short duration rainfall predictions. The weather generator combines historic information with Global Circulation Model output and produces climate information based on perturbation algorithms. A basis for adjusting IDF curves is presented by Burn et al. (2011) in an analysis of rainfall totals for 1-12 hours for long-term recording stations in BC.
H.3.2 Changes in Snowcover and Glacial Ice Cover
Warmer winters will raise winter snowline (Cohen et al., 2012). However, high level snowpack may increase, given the expectation for wetter winters. Glaciers, which sustain mid and late- summer runoff in a significant number of BC mid-size drainage basins, are generally in retreat because of recent warm summers (Bolch et al., 2010). Changes are regionally variable: in northwestern BC, glaciers have dominantly been thinning, leading to increased summer runoff
Professional Practice Guidelines - Legislated Flood
APEGBC June 2012 Assessments in a Changing Climate in BC 125
and sediment influx into streams, whereas in central and southern BC, glaciers have been in frontal retreat so that reduced area has led to lower late-summer flows (Moore et al., 2009). High elevation snowpacks may be expected eventually to sustain many of these glaciers in a new equilibrium with reduced area. So long as climate continues to change, however, glaciers will continue to change; the larger ones more slowly than small ones because of their longer adjustment times to reach equilibrium with the prevailing climate.
H.3.3 Changes in Land Use, Insect Infestations and Wildfires
Population in BC, in comparison with land area, is light. Whilst population will continue to increase substantially, it is not expected to produce land use changes as severe as those experienced between 1850 and about 1980, except around the main foci of settlement. Urban land conversion will continue to be relatively rapid in the Lower Mainland, lower Vancouver Island and the Okanagan Valley, with the first being largely urban by late in the century. This implies strongly changed patterns of runoff and streamflow in relatively small drainage basins in and immediately around these focal points of settlement. Stormwater management in small urban watersheds will be sufficiently important to merit concerted study at provincial scale. Forest condition and forest hydrology are impacted over significant areas by fungal and insect infestations and by fire. The recent mountain pine beetle infestation demonstrates this. A future changed climate will induce ecological disequilibrium in many respects, including shifting the ranges of both forest species and their pests. The latter being more mobile, an increased incidence of infestation might reasonably be expected with a transient time scale of order a century (or more). This will influence runoff and the incidence of flooding in small to medium-sized drainage basins. The pine beetle history provides valuable experience for anticipating such events. Pike et al. (2010) present an authoritative review of forest hydrology for BC (see, in particular, chs. 6: Hydrologic Processes and Watershed Response, and 19:
Climate Change Effects on Watershed Processes in British Columbia).
Increases in temperature and summer droughts will augment the potential for forest fires. An increased incidence of severe summer convectional storms will raise the incidence and severity of lightning strikes, hence the incidence of forest and grassland fire. Particularly hot (stand-replacing) forest fires can lead to formation of hydrophobic (water repellent) soils that can increase runoff and increase the probability of debris flows even at relatively minor (1-5 year) rainfall return periods for various intensities (e.g., Cannon and Gartner, 2005).
H.3.4 Changes in Runoff
The net result of the above factors is that runoff and flood flows will change in BC through the 21st century. Salient features include the following:
An increased incidence of winter flooding in coastal BC, with the possibility for more extreme flows than in the past, due both to the increased proportion of winter precipitation that will fall as rain and a possible increased persistence of warm southwesterly flows that deliver particularly heavy and often long-duration rainfall.
Spring floods associated with seasonal snowmelt may become more severe because of more rapid snowmelt, or when a major warm storm occurs over a rapidly melting
snowpack. Possible increases of order 10% in extreme spring flood flows are envisaged. Increased likelihood of severe summer convectional showers inducing extreme floods in
small to medium drainage basins. This applies everywhere in the province but is of greatest concern in the Interior.
Increased precipitation intensity leading to the need for enhanced stormwater management measures in urban areas and along major communication routes.
Increased probability of forest fires due to more intense droughts and more pest-afflicted forests will lead to higher runoff and increase probability of debris floods and debris flows in affected watersheds.
The foregoing circumstances need to be factored into analyses of flood hazard that forecast likely conditions for more than a decade ahead.