Forest Investment Account (FIA) - Forest Science Program
FIA Project Y093074

    Hydrologic Indicators For Watershed Sensitivity to Peak Flow Changes in Small Watersheds
Project lead: Hassan, Marwan (University of British Columbia)
Contributing Authors: Hassan, Marwan A.; Brayshaw, Drew
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
One of the major challenges in predicting or mitigating the impacts of disturbance on hydrologic systems is to link changes in hydrology to changes in sediment delivery and transport. Because of the complexity of modelling, usually only one system is considered in isolation, with any potential changes in the corresponding system inferred. For instance, a study of a small watershed might consider changes to peak flows or to sediment delivery to the channel, but not alteration in channel pattern caused by those changes. Linking our understanding of expected changes in hydrology and sediment transport is therefore important for improving land use management. In order to improve this understanding, the development of models and concepts linking hydrologic change to geomorphic change, and vice versa, is necessary.
Channel and reach parameters (such as width, depth, slope, and channel pattern) reflect the adjustment of the stream channel to inputs of water, wood and sediment from upstream and upslope. Therefore, channel parameters can be used as indicators which synthesize the hydrologic and geomorphic processes occurring in a watershed (Goodwin et al, 1998). Two parameters which are particularly relevant are the bankfull discharge and the effective discharge.
Bankfull discharge (Wolman and Leopold, 1957) is defined as the discharge at which the stream channel is full to the top of its banks, but not flooding over the bank. Effective discharge (Wolman and Miller, 1960) is defined as the discharge that, averaged over time, transports the most sediment. Estimating the frequency, magnitude, and duration of bankfull and effective discharge in a single stream reach provides an indication of the stream channelís stability and the frequency with which geomorphically effective events occur in the watershed upstream. Determining the bankfull and effective discharge for multiple streams across a region enables regionalization, consideration of scaling relationships, and evaluation of the relative importance of individual factors, such as stream channel gradient, or availability of large woody debris, which contribute to the stream channel stability.
Factors governing the frequency, magnitude, and duration of bankfull and effective discharge for large, alluvial rivers are generally well described in the scientific literature. By contrast, for small streams, the situation is not well understood, and some of the results published to date have been contradictory. Small streams and headwater basins cover the majority of the landscape in mountainous terrain such as British Columbia and the Pacific Northwest. Evaluating regional and scaling relationships for bankfull and effective discharge in small streams in British Columbia is therefore important for issues of forest management and response to climate change.
British Columbia and the Pacific Northwest are currently facing unprecedented changes in forest cover and streamflow regimes, driven by climate change and forest parasite outbreaks, most significantly the mountain pine beetle (MPB) (CFS, 2007). Parasite outbreaks and associated salvage logging are causing the temporary deforestation of thousands of small watersheds in British Columbia. Climate change is altering the proportion of precipitation falling as snow, changing the timing of snowmelt, and increasing the magnitude of storms. Together, these effects are significantly changing the streamflow regime. A recent study of Baker Creek near Quesnel (BC Forest Practices Board, 2007) found that what had been the 20-year flood before the MPB outbreak and salvage logging occurred, had now become the 3-year flood.
Faced with such unprecedented changes to forest cover and streamflow regimes, land managers urgently require improved linkages between geomorphology and hydrology, in order to use science to guide decision making. Because small watersheds occupy the majority of the forested landscape (eg. Cleaves, 2003), the impact of changes to climate, forest cover and streamflow will be concentrated therein. It is likely that some small watersheds will be highly susceptible to changes in the peak flow regime while other watersheds will be much less sensitive, but there is no current method of predicting such sensitivity, making a coordinated management response impossible. Understanding the variability of bankfull and effective discharge for small forested streams will enable land managers to identify those streams most susceptible to the effects of changes in forest cover and streamflow regime and to prioritize their responses accordingly.
Such research as has been conducted on this subject in small mountain streams in other areas is likely inapplicable to British Columbia and to the Pacific Northwest because of the regionally unique combination of biogeoclimatic zones, repeated recent glaciations, and history of forest resource exploitation. Therefore locally developed science is required to guide regional land management.
Related projects:  FSP_Y071074FSP_Y082074

Executive summary (65Kb)

Updated August 16, 2010 

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