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Sediment transported from debris‐flow initiation zones is typically stored in a topographic feature called a debris‐flow fan, the formation process of which governs secondary sediment transport further downstream. Although sediment transport from debris‐flow fans can impact sediment regimes and change landforms, the determinants of progressive fan formation are not well constrained. To identify such determinants, this study monitored debris flows and performed topographic surveys of debris‐flow fans in the Ichino‐sawa torrent (Japan) during 2016–2017. In this period, eight debris flows occurred, two of which eroded the existing fan and formed a new channel with a short recurrence interval (~40 days). Consequently, these two cases induced substantial sediment transport further downstream from the fan. The examined rainfall indices did not provide a threshold for diagnosing the occurrence of such sediment transport. Debris flows with a large flow depth and a long duration led to changes in the runout direction and subsequently formed new channels. Before these processes, the existing channel was backfilled and plugged by previous debris flows, forming a steep fan surface around the fan apex. The results suggest that increasing the magnitude and the duration of debris flows potentially triggers sediment transport from fans coupled with channel plugging. The annual sediment transport from the fan exceeded almost all sediment yields of the world rivers and was found comparable with that linked with volcanic eruptions and their aftermath. Thus, the fan‐formation process can induce substantial sediment transport, independent of volcanic perturbations and extreme climatic events, and is dependent on the sediment supply from repeated occurrence of debris flows in the initiation zones. [ABSTRACT FROM AUTHOR]

The long‐term behaviour of radiocaesium (137Cs) activity concentrations in forest ecosystems and their downstream impacts remain important issues in the deciduous broadleaf forests of Fukushima, Japan following the Fukushima Daiichi Nuclear Power Plant accident. To predict 137Cs cycling and discharge in the forest ecosystem, it is important to understand the spatial dynamics of the 137Cs inventory and transport along hillslopes. Therefore, we observed the spatial distribution of the 137Cs inventory and 137Cs transport via sediment and litter of a deciduous forest hillslope in Fukushima, Japan in 2016 and 2017 and examined how the spatial distribution of 137Cs inventory was formed using a mass balance model. In 2017, the 137Cs activity concentration was significantly greater in the downslope riparian area (455 kBq/m2) than in the upslope ridge area (179 kBq/m2). Annual 137Cs transport within litter and sediment contributed <0.5% to the current 137Cs inventory and cannot explain the current spatial variation of 137Cs inventory on the hillslope. The mass balance model results showed that if the initial 137Cs deposition was distributed uniformly in 2011, the spatial distribution of the hillslope 137Cs inventory was influenced mainly by the movement of leaf litter with a high 137Cs activity concentration. [ABSTRACT FROM AUTHOR]

Explosive volcanic eruptions can cause long‐term landscape change, leading to increased sediment discharge that continues after the cessation of the eruptions. During the period 1990–1995, eruptions of Mount Unzen, Japan, generated large amounts of pyroclastic material, resulting in 57 debris‐flow events during 1991–2018. To investigate changes in the relationships between rainfall characteristics and debris‐flow occurrence, we conducted the following: geometric analysis of two gullies (i.e., debris‐flow initiation zones) using LiDAR (light detection and ranging)‐generated 1 m DEMs (digital elevation models); rainfall analysis, based on the relationship between rainfall duration and mean intensity (i.e., considering the intensity–duration, or ID, threshold); and debris‐flow monitoring during 2016–2018. Since 1991, rainfall runoff has caused erosion of the supplied pyroclastic material, generating a channel network consisting of incised gullies. With sufficient rainfall, debris flows formed, accompanied by further gully erosion; this resulted in both vertical and lateral adjustments of the cross‐sectional geometry. In the two decades since the eruptions ceased, readily mobilized pyroclastic material has become scarce as the gullies have adjusted to local hydrographic conditions. At the same time, the infiltration capacity of the volcanic flank has increased, reducing the capacity for overland flow. As a result, since 2000, rainfall events with intensities above the ID threshold have occurred; however, the lack of sediment supplied by the gullies appears to have hindered the occurrence and development of debris flows. This suggests that debris flows in volcanically perturbed landscapes may occur at lower rainfall thresholds as long as the corresponding upland channels are evolving as a result of intense overland flow. However, as such channels evolve towards equilibrium geometries, the frequency of debris flows decreases in response to the reduction in sediment availability. [ABSTRACT FROM AUTHOR]

Understanding changes in evapotranspiration during forest regrowth is essential to predict changes of stream runoff and recovery after forest cutting. Canopy interception (Ic) is an important component of evapotranspiration, however Ic changes and the impact on stream runoff during regrowth after cutting remains unclear due to limited observations. The objective of this study was to examine the effects of Ic changes on long‐term stream runoff in a regrowth Japanese cedar and Japanese cypress forest following clear‐cutting. This study was conducted in two 1‐ha paired headwater catchments at Fukuroyamasawa Experimental Watershed in Japan. The catchments were 100% covered by Japanese coniferous plantation forest, one of which was 100% clear‐cut in 1999 when the forest was 70 years old. In the treated catchment, annual runoff increased by 301 mm/year (14% of precipitation) the year following clear‐cutting, and remained 185 mm/year (7.9% of precipitation) higher in the young regrowth forest for 12–14 years compared to the estimated runoff assuming no clear‐cutting. The Ic change was −358 mm/year (17% of precipitation) after cutting and was −168 mm/year (6.7% of precipitation) in the 12–14 years old regrowth forest compared to the observed Ic during the pre‐cutting period. Stream runoff increased in all seasons, and the Ic change was the main fraction of evapotranspiration change in all seasons throughout the observation period. These results suggest that the change in Ic accounted for most of the runoff response following forest cutting and the subsequent runoff recovery in this coniferous forest. [ABSTRACT FROM AUTHOR]

Although we know that rainfall interception (the rain caught, stored, and evaporated from aboveground vegetative surfaces and ground litter) is affected by rain and throughfall drop size, what was unknown until now is the relative proportion of each throughfall type (free throughfall, splash throughfall, canopy drip) beneath coniferous and broadleaved trees. Based on a multinational data set of >120 million throughfall drops, we found that the type, number, and volume of throughfall drops are different between coniferous and broadleaved tree species, leaf states, and timing within rain events. Compared with leafed broadleaved trees, conifers had a lower percentage of canopy drip (51% vs. 69% with respect to total throughfall volume) and slightly smaller diameter splash throughfall and canopy drip. Canopy drip from leafless broadleaved trees consisted of fewer and smaller diameter drops (D50_DR, 50th cumulative drop volume percentile for canopy drip, of 2.24 mm) than leafed broadleaved trees (D50_DR of 4.32 mm). Canopy drip was much larger in diameter under woody drip points (D50_DR of 5.92 mm) than leafed broadleaved trees. Based on throughfall volume, the percentage of canopy drip was significantly different between conifers, leafed broadleaved trees, leafless broadleaved trees, and woody surface drip points (p ranged from <0.001 to 0.005). These findings are partly attributable to differences in canopy structure and plant surface characteristics between plant functional types and canopy state (leaf, leafless), among other factors. Hence, our results demonstrating the importance of drop‐size‐dependent partitioning between coniferous and broadleaved tree species could be useful to those requiring more detailed information on throughfall fluxes to the forest floor. [ABSTRACT FROM AUTHOR]

The article presents an analysis of suspended sediment yields after low impact forest harvesting. The authors state that disturbances to forest catchments have profound effects on the environment of headwater streams and have an impact on suspended sediment (SS) management. Forest harvesting is key in increasing SS yields. The study presented examines how harvesting influenced SS yields in a steep forested area. Soil surface disturbance was avoided as much as possible by using skyline logging treatments and piling branches at selected locations in the watershed.

Surface conductance Gs is a significant parameter for indicating the evaporative and photosynthetic properties of a vegetated surface. When comparing Gs values between different observation sites, some studies have used Gsmax and others have used Ĝsmax (where Gsmax is the maximum Gs value measured during the measurement period, and Ĝsmax is the maximum Gs value obtained with a vapour pressure deficit (VPD) of ≥ 1.0 kPa during the measurement period). In this study, we demonstrate a clear justification for using Gsmax instead of Gsmax when comparing Gs values between different sites. We examined whether both Ĝsmax and Gsmax lead to the same conclusions in classifying vegetated sites. Komatsu (2003b) [Hydrological Processes 17: 2503-2512] reported a clear relationship between canopy height h and Ĝsmax for coniferous forests with a projected leaf area index (LAI) of ≥ 3.0. We examined not only the relationship between h and Ĝsmax butalso the relationship between h and Gsmax for coniferous forests with a projected LAI of ≥ 3.0. Both Ĝsmax and Gsmax decreased with increasing h. However, the relationship between h and Gsmax was less well defined than the relationship between h and Ĝsmax because of biased Gsmax data. Consequently, we conclude that Ĝsmax is a more appropriate index than Gsmax to represent Gs for sites with different vegetation. [ABSTRACT FROM AUTHOR]

To our knowledge, there is no information on the canopy conductance of Cryptomeria japonica plantations, although such forests are very common in Japan. We examined the canopy conductance Gc of a C. japonica plantation based on 1-year sap-flow data using the simplified Penman-Monteith equation. After selecting data based on the criterion proposed in part 1 of this study (Komatsu et al., 2006. Hydrological Processes 20: this issue), we now examine Gc responses to meteorological factors. Gc was expressed using functions of solar radiation S (W m-2), the vapour pressure deficit D (kPa), and air temperature T (°C). The resulting functions were f1(S) = [(1000 + 30)S]/[1000(S + 30)], f2(D) = exp(-0.54D)/exp(-0.54), and f3(T) = {[(30 - 5) + 14](T - 5)}/{(30 - 5)[(T - 5) + 14]), where f1(S), f2(D), and f3(T) are functions expressing the Gc responses to S, D, and T respectively. The functions were characterized by comparing general functions for coniferous forests, which showed that it was necessary to consider f3(T) to reproduce the 1-year transpiration at the site, and that f1(S) and f2(D) determined here were not very different from the general functions. This paper is the first to determine Gc responses to meteorological factors for a C. japonica plantation. When predicting transpiration from a C. japonica plantation using hydrological or ecological models, the functions determined here should be used instead of the default functions of the models. [ABSTRACT FROM AUTHOR]

To our knowledge, no one has examined the canopy conductance of a Cryptomeria japonica plantation, although such forests are very common in Japan. We plan to examine the canopy conductance of a Cryptomeria japonica plantation (canopy height 8 m) based on 1-year sap-flow data using the simplified Penman-Monteith equation. This study examines the effect of the aerodynamic control of transpiration before calculating canopy conductance using the simplified Penman-Monteith equation, because the equation assumes that the effect of aerodynamic control is negligible. To examine the effect of aerodynamic control, we examined the dependency of the relationship between the vapour pressure deficit (VPD) and transpiration on wind speed, when solar radiation, air temperature, and soil water content do not limit transpiration greatly. The relationship between VPD and the heat pulse velocity measured 11 m above the ground was dependent on wind speed only when the wind speed was < 0.7 m s-1. This suggests that the effect of aerodynamic control was negligible when the wind speed was < 0.7 m s-1, but that the effect was not negligible when the wind speed was < 0.7 m s-1. Therefore, we should use data from when the wind speed was ≥ 0.7 m s-1 for calculating canopy conductance Gc, based on the simplified Penman-Monteith equation. Although this criterion is site specific, the methods used to develop it are applicable to other sites. [ABSTRACT FROM AUTHOR]