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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We compared median runoff (R) and precipitation (P) relationships over 25 years from 20 mesoscale (50 to 5,000 km2) catchments on the Boreal Plains, Alberta, Canada, to understand controls on water sink and source dynamics in water‐limited, low‐relief northern environments. Long‐term catchment R and runoff efficiency (RP−1) were low and varied spatially by over an order of magnitude (3 to 119 mm/year, 1 to 27%). Intercatchment differences were not associated with small variations in climate. The partitioning of P into evapotranspiration (ET) and R instead reflected the interplay between underlying glacial deposit texture, overlying soil‐vegetation land cover, and regional slope. Correlation and principal component analyses results show that peatland‐swamp wetlands were the major source areas of water. The lowest estimates of median annual catchment ET (321 to 395 mm) and greatest R (60 to 119 mm, 13 to 27% of P) were observed in low‐relief, peatland‐swamp dominated catchments, within both fine‐textured clay‐plain and coarse‐textured glacial deposits. In contrast, open‐water wetlands and deciduous‐mixedwood forest land covers acted as water sinks, and less catchment R was observed with increases in proportional coverage of these land covers. In catchments dominated by hummocky moraines, long‐term runoff was restricted to 10 mm/year, or 2% of P. This reflects the poor surface‐drainage networks and slightly greater regional slope of the fine‐textured glacial deposit, coupled with the large soil‐water and depression storage and higher actual ET of associated shallow open‐water marsh wetland and deciduous‐forest land covers. This intercatchment study enhances current conceptual frameworks for predicting water yield in the Boreal Plains based on the sink and source functions of glacial landforms and soil‐vegetation land covers. It offers the capability within this hydro‐geoclimatic region to design reclaimed catchments with desired hydrological functionality and associated tolerances to climate or land‐use changes and inform land management decisions based on effective catchment‐scale conceptual understanding.

Abstract:Wetlands in the Western Boreal Plain (WBP) of North Central Alberta exist within a moisture-deficit regime where evapotranspiration (ET) is the dominant hydrologic flux. As such these systems are extremely susceptible to the slightest climatic variability that may upset the balance between precipitation (P) and ET. Wetland ET is predominantly controlled by vegetation composition but may also vary due to moisture regimes and microclimatic factors. To address this variability in moisture regimes, ET was examined in a typical moraine-wetland-pond system of the WBP during the 2005 and 2006 snow-free seasons. Closed dynamic chamber measurements were used to gather data on plant community-scale actual evapotranspiration (ET) in an undisturbed natural bog with varying degrees of canopy cover surrounding a shallow groundwater-fed pond. For the purposes of scaling plant community ET contributions to those of the wetland, potential ET (PET EQ ) was measured using a Priestley-Taylor energy balance approach at three separate wetland sites with varying aspects surrounding the central pond, along with actual evapotranspiration using a roving eddy covariance (EC) tower. Growing season peak ET rates ranged from 0Ð2 mm/h to 0Ð6 mm/h depending on the location, vegetation composition and time period. Sphagnum contributions were the greatest early in the growing season, reaching peaks of 0Ð6 mm/h, while lichen sites exhibited the greatest late season rates at 0Ð4 mm/h. Thus, Sphagnum and other nonvascular wetland plant species control ET differently throughout the growing season and as such should be considered an integral part of the moisture and water balances within wetland environments at the sub-landscape unit scale.

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