**1. Introduction**

Previous studies have reported that the Hortonian overland flow (HOF) generation seldom occurs in forest soils that are generally rich in macropore-containing organic matter, but in recent years, HOF generation has been observed in part of the unmanaged forests.

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This overland flow might occur when the infiltration rate decreases with increasing uncovered forest floor [1–3]. Moreover, the heavy rain events have an effect on increasing the risk of floods [4]. Infiltration has been traditionally studied using a flood-type or misttype rainfall simulator [5, 6]. Because a flood-type rainfall simulator supplies an excessive amount of water and as the mist-type simulator cannot reproduce raindrop impacts, the conventional method using these simulators does not accurately reproduce natural rainfall. For accurate reproduction of rainfall, Onda et al. [7] presented a series of large-scale sprinkling experiments in which water was sprinkled from the upper canopy. It is presumed in this investigation that infiltration rate observed in previous experiments should be about one order of magnitude more than the actual value. A large-capacity tank truck was installed in this investigation, but a small portable oscillating nozzle rainfall simulator recently developed by Kato et al. [8] was used for the experiments. The experiments showed a positive correlation between surface cover materials and infiltration rate [8–10]. A similar experiment was carried out in the semi-arid region, and obtained results indicated a noticeable positive correlation between these two variables [11, 12]. Previous studies showed that the effect of understories and leaf litters on infiltration rate is significant, which suggests that the surface cover materials reduce raindrop impact on the soil surface, and therefore the formation of surface crusts and HOF are restricted [9, 13–16]. A related finding of these studies is that fragmentation process of *Chamaecyparis obtusa* leaf litter progresses rapidly, and thus the *C. obtusa* forests are likely to become uncovered due to soil loss from steep slopes [17]. Fragmentation of Hiba arborvitae leaves is less likely to occur than those of *C. obtusa*, and the fragmentation of cedar leaves is mostly unlikely to occur. Therefore, the Japanese cedar and Hiba arborvitae forests have larger effects on soil conservation than *C. obtusa* forests [18]. The case studies using the recently developed method does not, however, allow for the sufficient examination of infiltration rate in Japanese cedar and Hiba arborvitae forests. Thus, an additional investigation should be carried out in these forests using a new method. Furthermore, Miyazaki et al. [19] argue that the effect of leaf shape on soil conservation is not clear. Further consideration will be needed on this viewpoint. The results from previously published studies have revealed that the hydraulic conductivity of forest floor and soil surface was greater than several hundred mm/h [3, 20, 21]. A correlation was observed between the hydraulic conductivity and infiltration rate [5, 22], as well as surface cover materials and infiltration rate. However, according to the observation results of plot runoff due to rainfall, there was no obvious correlation between infiltration rate and hydraulic conductivity in *C. obtusa* forests, suggesting that soil detachment by raindrops has a potentially significant impact on the infiltration rate [3]. It is usually assumed that the hydraulic conductivities of surface cover materials and surface soil are the significant factors impacting infiltration rate in forests, but more detailed consideration needs to be given to explain this assumption. A portable oscillating nozzle rainfall simulator has been introduced by Refs. [8, 9] as a new method mainly in *C. obtusa* forests. Therefore, field measurement of infiltration rate using the new method was conducted in Japanese cedar and Hiba arborvitae forests to examine the relation between infiltration rate, surface cover materials and hydraulic conductivity of forest soil through Ishikawa Forest Environmental Tax project [10].

**2. Background of Ishikawa Forest Environmental Tax project**

Japan is a country with many flood disasters. Muroto Typhoon in 1934, Makurazaki Typhoon in 1945 and Isewan Typhoon in 1959 were the three major typhoons in the Showa era, which caused massive damage. In the Makurazaki Typhoon, sediment-related disasters occurred frequently in

Relation between Infiltration Rate, Cover Materials and Hydraulic Conductivity of Forest Soils...

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Forestry Agency [23] showed that Japanese government had been promoting the creation of artificial forest for Japanese cypress, Japanese cedar and more species to be restored in the site, which was located in the logged forest after World War II and during the high economic growth period. Changing forest economic conditions in Japan since World War II have caused many plantation forests to be poorly managed. Since the 1980s, wood prices in Japan have been on a declining trend, but the profitability of forestry management has deteriorated drastically since the management costs of employment cost and materials had increased. For this reason, forest owners' willingness to management had declined, and forestry production activities have stagnated. Moreover, mountainous areas where many forestry workers and

In 1998, severe natural disasters occurred due to 24-h rainfall of an unprecedented 900 mm in Kochi Prefecture, Japan [24]. The area seemed to have many artificial forests of Japanese cypress and landslides occurred under the heavy rain. With such a background, in 2003, Kochi Prefecture became the first local government to introduce forest environment tax before the whole country in order to improve the forests in the reservoir area. Sediment disasters have been caused by typhoons in recent years, and the importance of countermeasures at basin

Further, reports on previous flood disasters are available at the homepage of the Ministry of Land, Infrastructure, Transport and Tourism [25] and Japan Society of Civil Engineers [26].

Recently, 'forest environmental tax' as individualistic tax was considered and introduced in each prefecture of Japan. In the background of the introduction of 'forest environment tax', there is a move towards centralization of national economy that the decentralization law was enacted in April 2000. As a result, the Local Tax Act was revised in an effort to expand local taxation autonomy, and conventional local discretionary taxes were changed from approval system by government to prior consultation system. In addition, the establishment of special taxes for specific purposes by the prior consultation system was also conducted, possibly to implement the political management taking advantage of the characteristics of the region [27]. Yamaguchi Prefecture started to collect taxes for the purpose of forest protection called 'Yamaguchi Forest Management Prefectural Residence Tax', from April 2005 [28]. A detailed report on the introduction and introduction effect of forest environmental tax has been reported [29, 30].

the logged mountains area that was weakened by heavy rains, taking a heavy toll of life.

forest owners live have problems of depopulation and the rapidly aging society.

scale including forest management has been reported.

**2.2. What is forest environmental tax?**

**2.1. Sediment and flood damage in Japan**
