Statistical Properties of Hydrographs in Minerotrophic Fens and Small Lakes in Mid-Latitude Québec, Canada

Minerotrophic fens cover a large proportion of the land in mid-latitude Québec. Since the last century, they have been subjected to an increase in mean water levels, which translates over a long period into an increase in the fraction of area covered by water-filled hollows, hypothetically slowly transforming them into shallow lakes as the hollows coalesce in larger ponds (aqualysis). This phenomenon progressively changes the hydrological reaction of aqualysed fens to rain events. Four sites (two fens and two shallow lakes), were monitored for rainfall, water table levels and surface runoff during two years in the La Grande River basin. Summer and fall hydrographs for rain generated events as well as relation between in situ water table and outlet surface runoff were compared, respectively via shape statistics and analyses of covariance. Depending on the hydrological property, results show some differences between sites, but not always systematically between fens and lakes. Fens had fewer runoff events than lakes but the events were of greater magnitude and duration. Four of the six hydrographs shape statistics (shape mean and variance, rising and falling slopes) were found to be significantly different between some sites, lakes (contrary to fens) being always in the same category. These results also indicate that the location and shape of individual ponds may play an important role in runoff generation. Concerning the relation between water table level and outlet runoff, regression slopes of fens were found to be steeper than those of lakes, especially in wet conditions. Climate change impact studies results suggest increases in annual runoff in this region in the future; this paper gives some insight about future hydrologic response of fens.

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