Runoff characteristics of pipeflow and effects of pipeflow on rainfall-runoff phenomena in a mountainous watershed
Introduction
A chain of connected macropores, developed nearly parallel to the soil surface, is commonly found in slopes, ranging from subarctic wetland (Woo and diCenzo, 1988) to tropical rain forest (Walsh and Howells, 1988, Elsenbeer et al., 1995, Elsenbeer and Lack, 1996). These macropores are commonly referred to as ‘soil pipes’, and the water flowing through them is called ‘pipeflow’ (e.g. Jones, 1971, Beven and Germann, 1982). Numerous studies have demonstrated that a great quantity of pipeflow from soil pipes can be produced during rainfall (McCaig, 1983, Tsukamoto and Ohta, 1988, Kitahara and Nakai, 1992, Onda, 1994). Tracer experiments have indicated a velocity higher than Darcian groundwater flow (Mosely, 1982, Tanaka et al., 1988). The high velocity of pipeflow has also been indicated in studies that investigated the relationship between the runoff and soil water content or the groundwater level (McDonnell, 1990, Tani, 1997). In studies involving the chemistry of pipeflow, the role of pipeflow contents has been shown to be an important concern in stream water quality (Muscutt et al., 1990, Elsenbeer et al., 1995, Elsenbeer and Lack, 1996).
As suggested by Kirkby (1988), investigations such as those mentioned above indicate that pipeflow plays an important role in the lateral movement of water in forested hillslopes. However, Selby, 1993, Bryan and Jones, 1997, Jones, 1997 indicated the general lack of long term pipeflow measurements from a variety of catchments and climatic environments. Only in the Maesnant basin (mid-Wales), over 200 storms has been monitored (Jones and Crane, 1984, Jones, 1987, Jones, 1997). Moreover, very few studies have simultaneously measured the discharge rates of pipeflow and streamflow in natural mountainous watersheds. Accordingly, adequate information is lacking with regard to the effects of pipeflow on runoff from such watersheds.
The objectives of this study are to obtain an insight into the runoff characteristics of pipeflow and to evaluate the effects of pipeflow on water discharge from mountainous watershed through field measurements.
Section snippets
Site description
The observations were conducted at the Toinotani 0-order valley watershed of Kyoto University Forest in Ashiu. The watershed is located in the northeastern part of Kyoto Prefecture in the central part of Japan (Fig. 1). With an area of 0.64 ha and a mean hollow gradient of 35.8° (Fig. 2), the altitude of the watershed ranges from 730 to 830 m above sea level. The watershed is covered with a closed forest of predominately Cryptomeria japonica and Betula grossa, and is underlain by Paleozoic
General trends of observed hydrographs
Table 1 shows the number of monitored storm events and the number of storm events in which flow occurred in Pipes A and B1. Fig. 6, Fig. 7 show hyetographs and hydrographs of the flow from Pipe A from 1 July to 31 October 1993 and 1996, respectively. Fig. 6 also shows the hydrograph of Pipe B1 flow and Fig. 7 shows the flow duration of Pipe B1. Moreover, the streamflow hydrograph is also shown in Fig. 7. Water runoff from Pipe A occurred only during heavy rainfall storms above approximately 12 mm
Conclusions
Over a three-year period, the following measurements were made: precipitation and the amount of pipeflow and stream discharge, in order to assess the runoff characteristics of pipeflow, and the effects of pipeflow on the water discharge. The following conclusions were drawn:
- 1.
The maximum pipeflow was 0.1 mm h−1 (0.18 l s−1), mean lags in the start and peak of pipeflow in comparison with the start and end of rainfall were 11.0–12.1 and 1.6–3.7 h, respectively.
- 2.
The runoff characteristics of the ephemeral
Acknowledgements
The authors express their deep appreciation to Prof Sumiji Kobashi., Drs Masaharu Fujita and Nobuhito Ohte (all from Kyoto University) for their valuable and encouraging advice and suggestions.
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