Researchers in countries outside of Japan have recently reported a height-related decline in stand water use in coniferous forests. These reports suggest that one of the main reasons for this decline is lower transpiration rates per unit leaf area (Q_l) for taller trees. However, it is unclear whether lower Q_l is recorded for taller trees in Japan because no studies have examined the relationship of tree transpiration with tree height. Thus, this study examined whether lower Q_l is recorded for taller Japanese cypress (Chamaecyparis obtusa) trees, one of the most common conifer species found in Japan. Three adjacently situated Japanese cypress stands with different tree heights were selected for this study. Here, each stand was named as S, M and L stand, respectively. Three sample trees were selected from each stand and sap flow measurements were performed to estimate transpiration rates. Consequently, Q_l was negatively-correlated to tree height in the range of 7.0 - 11.5 m (r = 0.97, P < 0.005), which corresponds to S - M stands. Q_lwas not significantly correlated to tree height in the range of 11.3 - 18.1 m (r = 0.40, P > 0.1), which corresponds to M - L stands. These results suggest that decline of Q_l was significant for smaller tree height and that it is less significant for greater tree height.

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To estimate the precipitation variation in the designated river basin, the pattern classification method and weather generation algorithm are combined to take the downscale approaches for outputs of Atmosphere-Ocean General Circulation Model. The simulated results in the Shiribetu River and the Gokase River in Japan, showed the great difference in the special and temporal precipitation distribution. The hydrological impacts due to global warming in the river basin scale were evaluated through the distributed runoff model with 1km and 1hour unit data.

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A new oscillating nozzle rainfall simulator, modified for use on steep hillslopes, was developed to generate simulated rainfall with large raindrops and raindrop kinetic energy. The modified simulator was based on the designs of existing rainfall simulators. A laboratory calibration experiment demonstrated that the modified rainfall simulator could generate large raindrops (> 2.5 mm in diameter) with high uniformity over the target plot; the raindrop kinetic energy of simulated rain was similar to that of the throughfall beneath hinoki (Japanese cypress; Chamaecyparis obtusa) canopy. This study measured the infiltration rate at devastated hinoki plantations by a series of field infiltration tests using the modified rainfall simulator. The measured standard final infiltration rates (FIR₁₈₀) ranged from 39.0 to 172.8 mm h^-1; these values are less than the infiltration rate (> 200 mm h^-1) reported by conventional infiltrometers that did not generate large raindrop impact. In addition, high correlation was found between the surface cover conditions and the maximum final infiltration rates (FIR_max). The result suggests that the magnitude and processes of decreased infiltration under devastated hinoki plantations can be evaluated reliably by simulating larger raindrop impact using the modified oscillating nozzle rainfall simulator.

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In the first paper, we reported that management of water resources could be more efficient than actual, by appropriate dam operation based on the comparison in advance between estimated quantity and the normal quantity in downstream observatories. If this operation is carried out under year-long targeted quantities of storage, management of water resources could be even more efficient. The year-long targeted quantities of storage can be replaced with necessary storage until early spring, because spring runoff of Japanese rivers is large enough to recover storage of dams. So, we propose to arrange several scenarios including year-long targeted quantities of storage, for planned operation, for dry season operation to reduce intake, or for high water season operation to discharge more than obligated quantity. And, we decide following dam-operation according to the most suitable scenario , where present storage is located closest above targeted quantities of storage. The proposed method here has been proved efficient and feasible by trial account performed in an actual low flow management.

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The 100-yr return levels of probable annual maximum daily precipitation were estimated with nonparametric method at 51 meteorological observatories in Japan, and compared with those estimated with the conventional parametric method. The results show that both levels of probable annual maximum daily precipitation are almost same with the correlation coefficient of 0.98 and the averaged ratio of 1.03. However, we find that the levels of probable annual maximum daily precipitation estimated with nonparametric method tend to be higher than those of the other at some observatories where extreme precipitation was observed. On the other hand, the standard deviation estimated from re-sampling based on the bootstrap method is found to be valuable as an index to show the uncertainty of the estimated probable annual maximum daily precipitation, because it is highly correlated with the observed extreme precipitation.

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