This study examined interannual variations in runoff from forested catchments in Japan, corresponding to interannual variations in precipitation. We hypothesized that interannual variations in runoff are comparable to those in precipitation in Japan under the assumption of high annual precipitation (P) relative to annual potential evapotranspiration (E_p). We collated P and annual runoff (Q) data derived from 20 catchments in Japan and 37 catchments in other regions around the world (including Australia and the United States). We calculated a sensitivity index (s), which is defined by the slope of the relationship between P and Q, for each catchment. The majority of the data (70%) for Japan satisfied s ≥ 0.80. No data satisfied s < 0.40. These results contrast with those for other regions: only 35% of the data satisfied s ≥ 0.80 and there were several data satisfying s < 0.40. We also confirmed that high s values were observed for regions with high P relative to E_p and that P was higher than E_p for all catchments in Japan. Our results indicate that interannual variations in Q are generally comparable to those in P for forested catchments in Japan.

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Impacts of climate change may depend more on changes in mean daily minimum temperature (T_min) or maximum temperature (T_max) than on average temperature (T_avg). In this study, we apply a statistical downscaling model (SDSM) for estimating the average length of spells with temperature values greater than T_max = 30°C (ALS30), T_max, and diurnal temperature range (DTR) for the present period (1961–1990) and the future period (2071–2099) climate conditions. The outputs of two GCMS (HadCM3 and CGCM3) are used to show the potential applicability of SDSM. Scenarios A2, A1B and B2 are used by the SDSM to construct climate scenario information over the Shikoku region. The results showed that: (1) The SDSM model is able to capture the major part of the temperature change signals, with a plausible climatic regime for higher warming; (2) From June to August, the average DTR changes in northern Shikoku would be positive; but in southern Shikoku, the changes would be negative under A2, A1B, and B2 scenarios using HadCM3 and CGCM3. The most important finding is that the DTR trend will be different at the local scale and these results highlight the importance of separately considering projections for T_min and T_max, when evaluating climate change impact for hydrological and agricultural applications.

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Sediment replenishment below dams is an increasingly common practice in Japan to compensate sediment deficits downstream and improve habitat quality and ecological functions. We reviewed implementation methods in the context of sediment placement, and compared the design and implementation activities undertaken in the Nunome River of Japan and Trinity River of California. Nunome River was supplied with a maximum of 500 m³ of mixed sand-gravel to the stream channel from a single site through a high-flow stockpile method during a relatively small (80 m³/s) and short (less than 4 hours) peak flow. Trinity River was supplied through an in-channel stockpile, high-flow stockpile and high-flow direct injection in combination with mechanical rehabilitation with the aim of re-creating gravel bar features through fluvial processes. More (max 51,000 m³) and greater (gravel to boulder) sediment has been added from more than four sites along the downstream channels during a longer (max 5 days) and higher (up to 311 m³/s) peak flow regime that are designated depending on the water year types which are determined by systematic analyses in a given year. This comparative study provides the present programs in Japan with some recommendations that will inform proper methods corresponding to river-specific high-flow and sediment regimes.

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There is a strong pressing to clarify the effect of thinning coniferous plantations on components of the forest water cycle in Japan. This study evaluates changes in canopy transpiration (E) due to thinning of a Cryptomeria japonica plantation, the most common type of plantation in Japan. Using E derived with the sap-flux method, we modeled canopy conductance (G_c) for periods before thinning and after thinning with the input of the vapor pressure deficit separately. We hypothetically calculated E values using these G_c models, respectively, under the same meteorological conditions. The ratio of E estimated using the G_c model after thinning to that before thinning was 56%. This value was comparable to the ratio of the total sapwood area for the stand (A_s) before thinning to that after thinning (63%). This suggests the possibility of predicting the relative change in E due to thinning using data for the mean diameter at breast height and stem density for the target stand before and after thinning, which are readily available for most C. japonica plantations in Japan and are used to predict A_s.

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The objective of this research was to evaluate the applicability of the Asian Precipitation Highly Resolved Observational Data Integration Towards Evaluation of Water Resources (APHRODITE) gridded dataset to develop a spatially distributed extreme daily rainfall map in Southeast Asia. We compared the extreme daily rainfall values of the 5-year return period, estimated by a frequency analysis based on annual maximum rainfall during 1987–2006 from the APHRODITE dataset and rainfall observations from 185 gauges in Thailand, Laos, and Vietnam. It was found that the extreme rainfall values estimated from the APHRODITE dataset are approximately 33–38% less than those estimated from observations at rain gauge stations. When the APHRODITE data were corrected with an appropriate bias correction ratio and elevation factor and then used to estimate extreme daily rainfall, they showed a better match with the extreme daily rainfall estimations from the rain gauge observations. Moreover, our estimations were independently verified with the rainfall observations in the Mae Chaem Watershed (MCW) in Thailand. This study suggests that the APHRODITE dataset can be used to obtain an estimation of the extreme rainfall in the Mekong River countries after proper bias and orographic corrections are made.

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We developed a monitoring system for deriving outlines of mountain glaciers and ice caps (MG&IC) at a 30 m horizontal resolution from Landsat Thematic Mapper (TM) and Landsat Enhanced Thematic Mapper plus (ETM+). Location and area information at 30 m resolution was obtained using a band ratio (TM4/TM5) and a threshold value of TM3 with a 9 by 9 pixel average filter. The total area and number of MG&IC were 449482 km² and 414258, respectively. The glacier outlines were similar to previous satellite-derived products for different regions. Although the derived glacier area was similar to previous estimates at regional scales, it was overestimated in some parts of Scandinavia where available satellite images are limited and only snowy season images can be used, and was underestimated in the western Himalayas and Caucasus where the glacier outlines are derived with difficulty from satellite images because of the effect of debris cover. Our system to monitor MG&IC has potential application in global hydrological and land-surface models and estimates of global sea-level rise.

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