Laboratory Studies of the Effects of the Capillary Fringe on Streamflow Generation

Abstract

This paper describes laboratory experiments that were conducted to investigate the groundwatersurface water interactions during the process of streamflow generation. In particular, the experiments were designed to examine the role of the capillary fringe in the runoff processes and its influence on the groundwater response to precipitation. The experimental results clearly show that if the zone of tension saturation extends to, or near, ground surface, the application of a small amount of water can cause an immediate rise in the water table, and the magnitude of the rise is much greater than would be expected on the basis of normal specific yield values for sandy materials. Because of the sloping surface of the laboratory model the rising water table caused the rapid generation of hydraulic gradients directed towards the toe of the slope (the stream) resulting in the immediate generation of groundwater discharge to the stream. Two experiments using chloride as a tracer and having rainfall rates of 4.3 and 1.9 cm h−1 showed that the discharge of preevent water to the stream preceded event water and that at early times, preevent water was the predominant component of streamflow. In addition, the results of the two tracer experiments show that the significance of preevent water increases with decreasing rainfall rate and that for the 1.9 cm h−1 rainfall rate preevent water was the main component of the stream hydrograph. The proportions of preevent and event water in the streamflow after steady conditions were reached were determined largely by the extent of the seepage face. This in turn would be determined by the hydraulic conductivity of the medium, the surface slope, and the rainfall intensity.