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Hydrologic properties and water balance of the forest floor of a Canadian west coast watershed Plamondon, André P.


The importance of the forest floor in the prevention of erosion has been well established; however, its role in the control of the amount and timing of water yield and in plant growth has been only partially investigated. The need to determine the role of the forest floor in watershed hydrology is especially important where it is several centimeters thick as in the humid, steeply sloping forests of the Canadian West Coast. The objective of this study was to quantitatively describe the processes controlling the amount of water absorbed by the forest floor during precipitation and the amount of water lost by drainage, evaporation, and transpiration. In addition, a survey of the spatial variation of the depth and the physical and hydrologic properties of the forest floor was undertaken. Chapter I. Depth of forest floor and altitude, slope, aspect, and forest basal area were systematically measured over four representative areas within a Coast Mountain watershed. Multiple regression was used to develop an equation that predicts the average forest floor depth of a small plot from physiographic factors. Bulk density, saturation capacity and field moisture capacity were determined in the laboratory from samples collected in the field. Bulk density was not related to depth or to the physiographic factors. Saturation and field moisture capacities were linearly related to the forest floor depth. Chapter II. Estimates of the evaporation from the forest floor using the energy balance method were compared with measurements made by a small, sensitive weighing lysimeter. Evaporation was well estimated by the net radiation minus the soil heat flux, indicating a small, downward,sensible heat flux. Results suggest that the similarity principle was not applicable under the canopy. For much of the time, evaporation from the forest floor was a capillary flow limited, rather than an energy limited, process. Chapter III. The role of the forest floor in watershed hydrology was investigated by measuring the components of its water balance on a 30° slope and by determining its water retention and hydraulic conductivity characteristics in the laboratory. The hydraulic conductivity varied by about three orders of magnitude over a range of matric potentials between -0.01 and -0.1 bars. When the forest floor had reached its maximum water content during rainfall, the drainage rate through the matrix accounted for approximately 0.5% of the rainfall rate. The amount of water absorbed during rainfall was largely a function of the initial water content and hydraulic conductivity. It appears that the forest floor contributes to delayed storm- flow, stores a significant amount of available water for plants, does not significantly contribute to base flow, or affect streamflow peaks. Chapter IV. The procedures previously used to measure the hydraulic conductivity characteristics of porous material are briefly reviewed. A simple steady-state method of measuring the hydraulic conductivity of an undisturbed sample of forest floor material in the laboratory is described. The main features of the method are that the water is applied at a constant rate at the top of the sample by a chromatography micropump while the water content within the sample is controlled by hanging a variable - length water column from a porous plate at the bottom of the forest floor core. An advantage of the method is that a small matric potential gradient can be maintained in the sample by adjusting the length of the hanging water column.

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