The litter layer on a forest floor can influence both short-term runoff and long-term water balance through modification of various hydrological processes. In this study, we have quantified the watershed-scale effects of the litter layer on runoff and evapotranspiration using a paired watershed method. The removal of the litter layer in a forested watershed with an area of 1.19 ha was conducted annually over the latter half of a 6-year experimental period. An adjacent forested watershed with an area of 1.42 ha was preserved as a control. Our results indicated that litter removal increased the 3-year runoff by 80.3 mm during the post-treatment period. Furthermore, when the peak flow range in the control watershed was 0.4–1.0 mm/h and >1.0 mm/h, peak runoff during flood events was about 1.5 and 1.4 times greater than that observed before litter layer removal, respectively. These data suggest that litter layer removal can decrease litter layer interception and, hence, increase peak flow, particularly during relatively large flood runoff events.
Materials and Methods
A paired watershed method was applied in this study to identify how a litter layer controls runoff and evapotranspiration. Approximately 4.4 × 10 kg of litter was removed from the forest floor of an entire watershed for three consecutive years, which increased the 3-year runoff by a total of 80.3 mm. Results showed that the peak runoff during a large flood event was ~1.4–1.5 times greater due to removal of the litter layer; indeed, increased runoff due to litter layer removal occurred mainly during such flood events. If a litter layer exists in a plantation forest, its removal may have a negative impact on flood mitigation, but a positive impact on water resource management. Conversely, in a plantation forest from which a litter layer has been lost, its recovery may have a positive impact on flood mitigation, but a negative impact on water resource management. Further observational and modeling studies are necessary to clarify the role of a litter layer on the processes and mechanisms of runoff during flood events, and to evaluate the impact of the increase or decrease of litter volume on flood mitigation measures and water resources management.
We thank Yoshie Gomyo (project specialist) and Junko Kato (project academic support staff) for their support of this study. The authors express their appreciation to the Ecohydrology Research Institute, The University of Tokyo Forests, and the Graduate School of Agricultural and Life Sciences at The University of Tokyo for their continuous support of our activities. We acknowledge the two anonymous reviewers for providing helpful comments on an initial version of this manuscript. We would like to thank Editage () for English language editing.
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