IntroductionClearly defined hydrologic response units (HRUs) that incorporate unifying concepts in hydrology-the complete hydrologic cycle and conservation of mass (Dooge, 1986)-are required to direct and integrate local, regional and continental scales of hydrologic research and management. The topographically defined watershed or catchment has been championed as the basic HRU (Dooge, 1968). However, catchment studies reveal large complexity and heterogeneity of runoff behaviour, resulting in a multitude of conceptual and numerical model structures. Recent reviews argue that a broad-scale classification of catchments is required to generalize dominant hydrologic processes, direct field methodologies, and apply hydrologic model structure (Sivapalan, 2003; McDonnell and Woods, 2004). However, protocols on defining such areas are presently lacking.Traditionally, researchers have disregarded large portions of the landscape in favour of areas amenable to 'hydrologic study', by relying on catchments where hydrologic boundaries can be easily defined. These catchments are often small and homogeneous, to 'control' for climatic and geologic features, which may have misled non-catchment-hydrologists (or up-and-coming hydrologists and managers) to believe that the first variable to consider in predicting hydrologic response is topography. This approach may provide a false sense of security about the effectiveness of topographically defined catchments as an approach to conduct research, assess regional hydrology, and generalize results to broad landscape scales. Recent reviews clearly illustrate the need for a thorough integration of surface water and groundwater processes (Winter, with respect to dominant hydrologic cycling and mass balance. We believe that asserting the topographically defined catchment as a standard hydrologic unit, or by assuming that the water table conforms to topography, is a methodological approach that has been overstated in importance for regional to national scales of water management.2001a Effective Delineation of a Catchment Using Dominant HRUs: a Boreal Plain ExampleThe impetus for this commentary comes from an interest in understanding hydrology on the subhumid glaciated plains of the western Boreal Forest, and our realization that traditional approaches for hydrologic research may actually serve to limit insights into hydrologic function in this region. Ongoing research at our Utikuma Research Study Area (URSA), Alberta, Canada, reveals that glaciated regions, such as the Boreal Plain, with deep glaciated substrates arguably result in some of the most complex surface and groundwater interactions (e.g. Winter, 1999Winter, , 2001a The difference in a hydrologist's perception of the effective catchment area determined by first considering topography, rather than climate and geology, is illustrated in the example in Figure 1 (Mink Lake, Alberta). From the data provided and the scale of the example, similar runoff contribution per unit area would often be assumed, and the hydrologic response time...

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The efficiency with which riparian zones remove nitrate (NO−3) from contaminated ground water can vary with landscape setting. This study was conducted to determine the influence of flood plain geometry, lithology, hydrologic flow path, and nitrate transport on mechanisms of nitrate depletion of contaminated ground water. Patterns of NO−3−N, chloride, and dissolved organic carbon (DOC) concentrations and δ15N‐NO−3 and δ18O‐NO−3 values in combination with detailed piezometric head measurements were investigated in a river floodplain connected to a large upland sand aquifer in an agricultural region near Alliston, Ontario, Canada. Ground water discharging to the forested floodplain from the sand aquifer exhibited large spatial variability in NO−3−N concentrations (10–50 mg/L). The transport and depletion of NO−3 was strongly influenced by floodplain geometry and lithology. Little ground water flow occurred through the low‐conductivity matrix of peat in the floodplain. Plumes of NO−3‐rich ground water passed beneath the riparian wetland peat and flowed laterally in a 2‐ to 4‐m‐thick zone of permeable sands across the floodplain to the river. Analyses of the distribution of the NO−3−N concentrations, isotopes, and DOC within the floodplain indicate that denitrification occurred within the sand aquifer near the river where nitrate‐rich ground water interacted with buried channel sediments and surface water recharged from peat to the deeper sands. This study shows that the depth of permeable riparian sediments, ground water flow path, and the location of organic‐rich subsurface deposits may be more important than the width of vegetated strips in influencing the ability of riparian zones to remove nitrate.

Abstract:The water balance and runoff regime of a 55 ha aspen-forested headwater catchment located on the Boreal Plain, Alberta, Canada (55Ð1°N, 113Ð8°W) were determined for 5 years following a partial timber harvest. Variability in precipitation provided the opportunity to contrast catchment water balances in relatively dry (<350 mm year 1 ), wet (>500 mm year 1 ), and average precipitation years. In most years, the catchment water balance was dominated by soil water storage, evapotranspiration losses, and vertical recharge. In 1997, despite near-average annual precipitation (486 mm), there was significant runoff (250 mm year 1 ) with a runoff coefficient of 52%. A wet summer and autumn in the preceding year (1996) and large snow accumulation in the spring (1997) reduced the soil water storage potential, and large runoff occurred in response to a substantial July rainfall event. Maps of the surface saturated areas indicated that runoff was generated from the uplands, ephemeral draws, and valley-bottom wetlands. Following 1997, evapotranspiration exceeded precipitation and large soil water storage potentials developed, resulting in a reduction in surface runoff to 11 mm in 1998, and <2 mm in 1999-2001. During this time, the uplands were hydrologically disconnected from ephemeral draws and valley-bottom wetlands. Interannual variability was influenced by the degree of saturation and connectivity of ephemeral draws and valley wetlands. Variability in runoff from tributaries within the catchment was influenced by the soil water storage capacity as defined by the depth to the confining layer. An analysis of the regional water balance over the past 30 years indicated that the potential to exceed upland soil water storage capacity, to connect uplands to low-lying areas, and to generate significant runoff may only occur about once every 20 years. The spatial and temporal variability of soil water storage capacity in relation to evaporation and precipitation deficits complicates interpretation of forest harvesting studies, and low runoff responses may mask the impacts of harvesting of aspen headwater areas on surface runoff in subhumid climates of the Boreal Plain.

The higher mid-latitudes of the Northern Hemisphere are particularly sensitive to climate change as small differences in temperature determine frozen ground status, precipitation phase, and the magnitude and timing of snow accumulation and melt. An international inter-catchment comparison program, North-Watch, seeks to improve our understanding of the sensitivity of northern catchments to climate change by examining their hydrological and biogeochemical responses. The catchments are located in Sweden (Krycklan), Scotland (Mharcaidh, Girnock and Strontian), the United States (Sleepers River, Hubbard Brook and HJ Andrews) and Canada (Catamaran, Dorset and Wolf Creek). This briefing presents the initial stage of the North-Watch program, which focuses on how these catchments collect, store and release water and identify 'types' of hydro-climatic catchment response. At most sites, a 10-year data of daily precipitation, discharge and temperature were compiled and evaporation and storage were calculated. Inter-annual and seasonal patterns of hydrological processes were assessed via normalized fluxes and standard flow metrics. At the annual-scale, relations between temperature, precipitation and discharge were compared, highlighting the role of seasonality, wetness and snow/frozen ground. The seasonal pattern and synchronicity of fluxes at the monthly scale provided insight into system memory and the role of storage. We identified types of catchments that rapidly translate precipitation into runoff and others that more readily store water for delayed release. Synchronicity and variance of rainfall-runoff patterns were characterized by the coefficient of variation (cv ) of monthly fluxes and correlation coefficients. Principal component analysis (PCA) revealed clustering among like catchments in terms of functioning, largely controlled by two components that (i) reflect temperature and precipitation gradients and the correlation of monthly precipitation and discharge and (ii) the seasonality of precipitation and storage. By advancing the ecological concepts of resistance and resilience for catchment functioning, results provided a conceptual framework for understanding susceptibility to hydrological change across northern catchments.