Canadian Forest Hydrology Website

The Need for Hydrologic Classification

There has been increasing interest within the literature and from the resource management community for a national hydrologic classification. The main entity of interest within such a classification is a landscape unit within which the dominant hydrologic processes are similar.  A classification system should characterize the mechanisms of movement and storage of water in order to identify landscape units with similar hydrologic regimes.  Therefore, the resulting classification would identify areas that have similar hydrologic controls while preserving the variability within each region. For the hydrologic community a classification would provide an organizing principle, create a common language, guide modelling and measuring efforts, and assist in the estimate of disturbance impacts (McDonnell and Woods 2004).  For the resource management community it would provide a context for comparing landscapes and response to disturbance, and would help tailor hydrologic criteria and indicators.

Ecological land classifications based on ecological principles are common and have been completed for numerous areas around the globe including Canada (Marshall and Schutt 1999). Ecoregion classifications are a good starting point for hydrology because there is an overlap in the dominant controls of both (Chapman 1989). However, the ecoregion classification is not specific enough for many water resource issues.

Wolock et al. (2004) created a national scale hydrologic classification of catchments for the United States, based on the hydrologic landscapes framework of Winter (2001). The fundamental building block of this framework is the hydrologic landscape unit, which is defined on the basis of topography, geology, and climate. These characteristics were assumed to be the dominant controlling factors on the release of water from the catchment.

The foundation for a similar framework has been initiated in Canada by Devito et al (2005). They proposed a hydrologic response unit framework for hydrologic classification, based on a hierarchy of processes that control the hydrologic response.  The order of the hierarchy is: climate » bedrock geology » surficial geology » soil depth and type » topography and drainage network.  Buttle (2006) extends on the work by Wolock et al. (2004) and Devito et al. (2005) with the development of the T3 template that can be used to assess the interaction of fundamental controls on streamflow from catchments within a given hydrologic landscape region.  Buttle defines three first order controls on the characteristics of streamflow generated from a catchment under a specific hydroclimatic condition: typology, topography and topology.  Typology characterizes the ability of different surfaces (vegetation types, soils, geology etc.) to partition water inputs either vertically or laterally.  Topology is a measure of the hydrologic connectivity of the drainage network.  Topography is the relative role of hydraulic gradients, defined by the basin shape and form.

The HELP research team is attempting to bring these frameworks together and develop a system that works for all of Canada.  This will be challenging due to under-studied hydrologic systems like the boreal plain, the vast amounts of water stored as snow and ice, limited data in Northern parts of the country, and very coarse national datasets.  The classification should also be adaptive, taking into account changes in the dominant controls with scale and time.  Members of the HELP project team are working to overcome these challenges and anticipate that a hydrologic classification for Canada will be completed by the end of the project in the spring of 2009. For more information please see the above mentioned references, which can be found in the HELP library (coming soon), and read about our progress in the HELP newsletters.