Water Renewal Efficiency of Watershed and Lake Combinations in the ELA Region of the Precambrian Shield

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Abstract
Individual components of lakewater budgets in the Experimental Lakes Area (ELA) are highly variable through time. Direct field measurements combined with empirical relationships for unmeasurable components are required to achieve balanced lakewater budgets over short intervals of several hours or days. However, annual variations in the hydrological regime are related to a few simple characteristics of the lake and surrounding watershed. In the 1969–77 period the annual ratios of terrestrial evapotranspiration/precipitation and lake evaporation/precipitation showed little variation and were of a similar magnitude. As a result, the potential for water renewal or replacement in the lake basins annually can be characterized by only four factors: the annual precipitation, the tributary drainage basin area, the lake area, and the lake volume. The potential has been expressed as the Annual Watershed Efficiency (AWE), the reciprocal of the water renewal time, to obtain a linear relationship between the four variables. The AWE i a measure of the extent to which a particular watershed and lake combination can renew the lake water under various annual precipitation conditions. A general graphical relationship for the AWE has been prepared for small headwater lakes similar to those of the ELA for a range of annual precipitations.Key words: watershed hydrology, water renewal, water residence, precipitation/runoff ratios, water budgets, evaporation, watershed efficiency

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