Runoff characteristics of pipeflow and effects of pipeflow on rainfall-runoff phenomena in a mountainous watershed

Abstract In order to assess the runoff characteristics of pipeflow in mountainous watersheds, precipitation and amount of pipeflow were measured for 3 years at a forested mountainous watershed in the central part of Japan. Similar changes in runoff characteristics were observed for two pipes over the study period; the maximum discharge rate of pipeflow increased, the recession hydrograph of pipeflow became steep, and the duration of pipeflow discharge became shorter. However, the changes of runoff characteristics had no significant effect on relationships between total rainfall and total discharge from pipes. Based on observations of sediment discharge from soil pipes, it was suggested that the sediment discharge from these soil pipes caused the changes of runoff characteristics of pipeflow. Moreover, in order to clarify the effects of pipeflow on streamflow, simultaneous measurements of pipeflow and streamflow were conducted at the same watershed. Results showed that the water discharge phenomena from this watershed can be classified into three stages based on the soil pipes which contribute the water discharge: (1) under small amounts of rainfall, water discharge occurs only at the spring situated at the watershed exit; (2) as the magnitude of rainfall increases, water comes out from a pipe located at 6 m upstream from the watershed exit; and (3) under the heaviest rainfall, additional discharge occurs from a group of pipes located 10 m upstream from the watershed exit.

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