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A evaluating soil water drainage of a humid mountain forest site in southwestern British Columbia by two field techniques Cheng, Jie-Dar

Abstract

This study was based on the premise that watershed management on mountainous forested land generally, and in the Coast Mountains of British Columbia more specifically, will be benefitted by further knowledge of soil water drainage from the root zone of forested soils and by such development of methods of measuring soil water drainage as will make increased sampling more feasible. The study was concerned with (1) the development and application of two methods for evaluating soil water drainage; a tension lysimeter system and a method based on Darcy's equation, (2) the exploration of the relationship between rainfall, soil water drainage and streamflow. It was carried out at a forest site of Jamieson Creek Experimental Watershed in the upper Seymour River basin, southwestern B.C. near Vancouver. The tension lysimeter system incorporated a simple capability for manually regulating suction on the lysimeter plate in close conformity with the tension in surrounding soil and also a lysimeter plate that ensured satisfactory hydraulic contact between it and the soil. The application of Darcy's equation for calculating soil water drainage was based on field determinations of both soil hydraulic conductivity and hydraulic gradient at the study site. Soil water drainage rates measured by the tension lysimeter system and those calculated by Darcy's equation showed good agreement, although the former were consistently and slightly higher. It was concluded that each method can provide reasonable estimates of soil water drainage and may be particularly useful in developing countries where a cheap labor source permits extensive and frequent soil water drainage sampling within a watershed. Soil water drainage amounts obtained by these two methods during each of two drying periods were in good agreement with those estimated from a water balance equation. Possible sources of error associated with soil water drainage measurements by tension lysimeter and by the method of Darcy's equation are discussed and possible improvements suggested. This study also indicated that, in the humid coastal region of southwestern British Columbia, soil water drainage is a major component of water balance for the root zone of forested soil and deserves further study. The time trends of soil water drainage were found to be similar to that of streamflow from the small watershed containing the study site. This suggests that the geologic, edaphic, topographical, climatological conditions favor a large and direct contribution of root zone soil water drainage to streamflow.

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