The Energy Budget of Canadian Shield Subarctic Terrain and Its Impact on Hillslope Hydrological Processes

Abstract

The objective of the study is to determine the local processes that control the partitioning of the energy budget of shield terrain in the western Canadian subarctic. The magnitude of the spring snowmelt and its potential to flood exposed bedrock portions of the landscape control the energy budget in the early part of the summer. In wet years, Bowen ratios are low and increase over the growing season. The high latent heat fluxes early in the year are promoted by water pooled in bedrock depressions and stored in the shallow soil. The high evaporation rates deplete moisture storage by the end of July after which latent heat fluxes decrease so that Bowen ratios exceed unity until the end of the growing season. This regime differs from other subarctic terrain types with similar vegetation. Exposed and shallow Precambrian bedrock keeps water close to the topographic surface and available for evaporation. The low surface resistance of ponded water on the bedrock surface and high vapor pressure deficits are possible causes for the high evaporation rates in June and July. However, in drier years, when ponded snowmelt water is minimal, evaporation is small and sensible heat dominates the early summer energy budget to a much larger degree than is observed elsewhere in the subarctic. It then becomes a very arid landscape. High evaporation-to-precipitation ratios throughout the summer are an important feature of the western Canadian Shield subarctic region, and this feature has significant hydrological implications. It is normal for a moisture deficit to be created each summer. The magnitude of this moisture deficit is the primary control on hillslope runoff response to precipitation later in the summer and autumn.