We describe an approach for reconstructing snowfall that combines satellite observations of the snow disappearance date (SDD) with a snow model for two mountainous areas in Japan having distinct snow climatology. This approach allows assessment of the distribution of snow and topographical effects on snowfall within a catchment. We also evaluated how the reconstructed snowfall affects the catchment snow hydrology.
Validation at observation sites demonstrated that a combination of the snow model and snowfall reconstructions successfully estimated the seasonal changes of snow water equivalent (SWE). In Japan, the dependence of snowfall on elevation is stronger in mountainous areas along the coast on the Sea of Japan side of Japan’s central mountain spine (where snowfall triples with every 1000 m increase in elevation; C_sf = 0.002 m⁻¹) compared with inland locations of the same region (where snowfall doubles with every 1000 m increase; C_sf = 0.001 m⁻¹). Moreover, the reconstructed snowfall improved the estimation of maximum catchment SWE. Maximum total SWE, estimated with reconstructed snowfall, was 3.8 × 10⁸ m³ in Kurobe catchment (along the coast on the Sea of Japan side of Japan’s central mountain spine), while that, estimated by convectional method with the spatially-constant C_sf = 0.001 m⁻¹, was 2.0 × 10⁸ m³. As a result, estimations of the snow disappearance date and of the catchment snowmelt were also improved. These results suggest that it is useful to estimate the spatial snow distribution, especially where steep topography causes large gradients of snowfall amount with respect to elevation.

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In snowy regions, the seasonal runoff pattern is greatly influenced by winter season temperature and precipitation. Our objective is to forecast the change in seasonal and monthly runoff volumes resulting from regional climate change scenarios, especially concerning spring snowmelt runoff. Snowmelt Runoff Model (SRM) was applied to Takiya River (19.45 km²) to simulate daily runoff over the period 2000–2007. Snow accumulation and melt was simulated for each of three elevation zones using air temperature data at high and low elevations to estimate the lapse rate. Key model parameters were determined by analysis of the basin monthly water balance, in addition to model calibration using snowpack snow water equivalent and river discharge. Simulation of the IPCC Scenario A2 (high impact) for Niigata region showed that runoff would be 2–3 times greater in winter (Dec–Feb), and decrease by half in spring (Apr–May). Even with warming restricted to +2.0°C, major changes in monthly runoff volumes occur because of large shifts in the proportions of snow versus rain. In Niigata, and other regions that receive heavy snowfall at temperatures close to 0°C, a small rise in temperature causes large changes in the size of the seasonal snowpack and the seasonal distribution of runoff.

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This study examined regional-scale changes in stability conditions for the occurrence of summertime convective precipitation under global warming projected by global climate simulations. Precipitation events over the Kanto Plain on synoptically undisturbed days were specifically focused on. The outputs of the 20-km-resolution global model simulations for the present and a future warming climate were used for the analyses. It was shown that temperature and moisture content throughout the troposphere are projected to increase more in the rainy cases than in the August mean cases from the present climate to the future and that the moisture increase below the 700-hPa level is significantly enhanced. The effects of future warming result in the moisture increase favorably at the lower levels. Owing to the low-level moisture increase, some stability indices indicate a destabilizing tendency with a statistical significance. From the projected changes in the stability condition for precipitation occurrence, it is implied that the precipitation amount is considered to increase if a cumulonimbus cloud and its organized systems once develop. The degree of the destabilization of precipitation environments is projected to increase more significantly than that of non-precipitation environments, and therefore, the precipitation will be more intensified in a future climate.

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This paper performs a quantitative impact assessment of the climate change on typhoon wind risk, focusing on residential buildings in Japan. The risk is assessed based on (1) the typhoon event set extracted from the simulation by the super-high resolution atmospheric general circulation model developed within the KAKUSHIN program; (2) the probabilistic typhoon modeling scheme developed by our group; (3) a fragility model empirically estimated on the basis of the damage report of typhoon Songda in 2004 and the reproduced wind field by a mesoscale meteorological model; JMA-NHM. The main results are that in the future (2075–2099) at most locations of Japan: (1) extreme wind events (10-minute sustained wind speed exceeding 30 m/s) are more likely to occur; (2) the median of the annual maximum wind speed decreases; (3) the expected number of damaged residential buildings decreases, assuming that the profile of the building portfolio remains unchanged. Based on these results, the assumptions and inputs to the assessment are critically reviewed. Thereby, the needs of further research efforts toward more credible and comprehensive assessment are addressed.

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This study projects the hydrological cycle in the Tana River Basin, Kenya, under a changing climate due to anthropogenic greenhouse gas emissions in the late 21st century, using a 20-km mesh atmospheric global climate model (AGCM) and a 0.5°–mesh global river-routing model. In addition, 60-km mesh AGCM ensemble experiments forced with four different projected sea-surface temperatures were performed to quantify the uncertainty in the climate projections. All four climatological annual mean hydroclimate variables: precipitation, evaporation, total runoff, and soil water storage: are projected to increase in the future climate. With the exception of total runoff and river discharge in the headwater basin, these climatological annual mean increases are generally robust. The four climatological monthly mean hydroclimate variables at Garissa are projected to exhibit different seasonal changes. The long rains season is projected to have almost the same peak precipitation amount in April with an earlier onset, while the short rains season is projected to have significant precipitation increases. River discharges are projected to increase significantly in November to March with high consistent change in sign and in June. These results should guide the awareness programs of the changing hydrological cycles in the Tana River Basin and assist in formulating mitigation and adaptation strategies to meet these challenges.

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We evaluated the effects of forest thinning on peak flow and recession characteristics of storm runoff in headwater catchments at Mie Prefecture, Japan. In catchment M5, 58.3% of stems were removed, whereas catchment M4 remained untreated as a control catchment. Storm precipitation and runoff was monitored from June 2004 to January 2007 for the pre-thinning period (113 events) and from March 2007 to June 2009 for post-thinning (103 events). Based on paired-catchment analysis between M5 and M4, volumes of peak flow did not increase significantly after thinning. Recession constant K increased, while recession time of storm runoff did not differ between pre- and post-thinning periods. Storm hydrograph recession tended to be more gradual (i.e., higher K values) after thinning due to increases in available soil water associated with higher net precipitation and decreased evapotranspiration. The lack of changes in peak flow can be attributed to the minimal soil disturbance during thinning. Our hydrograph analysis in paired catchments indicates that thinning may alter specific internal hydrological pathways, such as subsurface flow and groundwater flow.

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A massive flood, the maximum ever recorded in Thailand, struck the Chao Phraya River in 2011. The total rainfall during the 2011 rainy season was 1,439 mm, which was 143% of the average rainy season rainfall during the period 1982–2002. Although the gigantic Bhumipol and Sirikit dams stored approximately 10 billion m³ by early October, the total flood volume was estimated to be 15 billion m³. This flood caused tremendous damage, including 813 dead nationwide, seven industrial estates, and 804 companies with inundation damage, and total losses estimated at 1.36 trillion baht (approximately 3.5 trillion yen).
The Chao Phraya River watershed has experienced many floods in the past, and floods on the same scale as the 2011 flood are expected to occur in the future. Therefore, to prepare of the next flood disaster, it is essential to understand the characteristics of the 2011 Chao Phraya River Flood. This paper proposes countermeasures for preventing major flood damage in the future.

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Regions Of Freshwater Influence (ROFI) existing between oceans and estuaries often yield significant fishery resources. To inform fishermen of the real-time ocean state, an operational prediction system focusing on a specific coastal region would normally require accurate and timely runoff data for all rivers. In this paper, hydrometeorological procedures providing the required runoff data on a daily basis were coupled with an Ocean General Circulation Model (OGCM) to evaluate the impacts of runoff processes on ocean simulations within a ROFI. The procedures we adopted employ a distributed tank model based on water mass and heat budgets derived from predicted meteorological datasets. An exponential relationship between runoff rates and watershed areas was used to determine model parameters in order to estimate the runoff from many other small rivers. The coupled model reproduced a surface salinity field in the bay that was in good agreement with observations, and simulated the expected clockwise circulation generated by the high net total discharge associated with snowmelt. Our results underline the fact that implementation of hydrological processes into ocean simulations is essential for a better understanding of water circulation driven by runoff into semi-enclosed bays over interannual timescales.

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The warming of the Earth’s atmosphere system will change temperature and precipitation distributions across the globe. This will affect the hydrological cycle and, therefore, the hydrology of river basins worldwide. In this study, we model the stream flow of the Chao Phraya River Basin (CPRB), Thailand, in response to two climate change projection data sets under scenario A1B of the Special Report on Emissions Scenarios. We used Japan Meteorological Research Institute (MRI) atmospheric general circulation model 3.1 and 3.2 output data as input to a watershed hydrologic model to assess the impact of climate change for the basin. We found that, in the future, the mean annual river discharge is likely to increase in the CPRB due to increased rainfall. Furthermore, increases in annual maximum daily flows will occur toward the end of the 21st century.

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Soil hydraulic properties in field scale porous media are distributed with large spatial heterogeneity. Thus, detailed understanding of micro-physical properties leading to macro-scale heterogeneity is important for generalizing purposes in modeling. Hysteretic water retention properties greatly affect solute transport under unsaturated condition. In this study, laboratory infiltration experiments were carried out to investigate the importance of hysteresis in numerical modeling of water flow under heterogeneous distribution of hydraulic conductivity. Soil water content was observed by time domain reflectometry using small printed circuit board probes (PCBP). The PCBP observations were compared to results from numerical simulations involving saturated-unsaturated seepage analysis and two-phase flow with/without hysteresis effects. Observations were compared to both uniform and heterogeneous parameter modeling. Differences between unsaturated-saturated and two-phase flow were quite small in the numerical results for soil water content. The uniform parameter model, however, could not properly reproduce experimental water content change. The numerical results of water content change including hysteresis matched better observations for the unsaturated-saturated and two-phase flow cases. Results displayed that it is important to take heterogeneity and hysteresis into consideration in the numerical modeling in both two-phase and saturated-unsaturated flow analysis.

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Although the depths of water uptake by plants can be estimated by comparing oxygen 18 or deuterium in stem water with that of possible water sources in semi-arid regions, it is difficult to apply this technique to investigations of water use in relatively humid areas because the vertical profile of the isotopic ratio in soil water in such areas is more complicated than that in arid and semi-arid regions. However, the d-excess (deuterium excess) of rainfall in Japan shows clear seasonal variations. Therefore, we attempted to utilize seasonal variations of d-excess to estimate water sources of 20-year-old Japanese cypress (Chamaecyparis obtusa) and Japanese cedar (Chriptomeria japonica). Source water could be distinguished based on differences in their seasonal pattern of d-excess. The seasonal variations of d-excess in the stem water of both species were similar to those of shallow soil water (<1.0 m), but different from those of other water sources, indicating that the main water source was shallow soil water.

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Climate change is anticipated to escalate flood impacts, and thus it is important to assess flood risk closely in terms of extent and location. This study aimed to assess present and future flood risks, particularly flood risk change, over the Asia-Pacific region with consideration of climate change impacts by using a topography-based analysis method. By analyzing the output of the super-high-resolution global atmospheric general circulation model, it was found that future flood risk will increase in response to extreme rainfall under climate change. Results of this study also indicated that flood risk will further increase in the far future (2075–2099) than in the near future (2015–2039). Analyses of inundation area and flood inundation depth (FID) also showed upward trends; most of flood plains in the Asia-Pacific region may experience a 0–50cm increase in FID.

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This paper aims to determine the accuracy of composite polarimetric variables, horizontal polarization (Z_H), differential reflectivity (Z_DR), and specific differential phase shift (K_DP), and rainfall rates derived from a network of four X-band polarimetric radar stations during localized convective precipitation over Tokyo on 28 September 2010 by comparison with data from a Joss–Waldvogel disdrometer and a surface rain gauge network. The four X-band polarimetric radars were complementary with respect to signal extinction, and they yielded composite maps of the polarimetric radar parameters and rainfall rates. The composite maps were validated by cross-comparison of data from the four individual radar stations, and by ground-truthing with surface observations. The raindrop size distribution (median volume diameter and normalized number concentration) was estimated from the composite maps of polarimetric parameters, and validated by the disdrometer data. We found that composite polarimetric radar parameters can provide useful information, not only for hydrological applications, but also for microphysical analysis.

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The Global Satellite Mapping of Precipitation (GSMaP) project provides semi-global and hourly rain rate estimates based on multi-satellite measurements. However, the quality of GSMaP estimates degrades when no microwave radiometers are available or when a cloud moving vector is applied. This study proposes a new rain detection method for the interval between microwave measurements based on variations in surface soil moisture. The new method is tested with Tropical Rainfall Measuring Mission Microwave Imager data and evaluated with rain gauge measurements from the Oklahoma Mesonet. Generally, 10 GHz with horizontal polarization (10H) is the optimal channel to detect temporal increases in surface soil moisture induced by rainfall. Compared with GSMaP, the performance of hourly rain detection in the new method is not high. However, the new method is expected to perform better if measurement intervals are shortened by use of multiple microwave radiometers. The new method is good for less vegetated areas where GSMaP performance is reduced. Currently, the new method is mostly independent of GSMaP as 10H is not used for rain retrieval over land. Incorporation of the new method into GSMaP is expected to improve the latter.

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Inter-annual variations of snowmelt runoff timing in 15 basins across central Japan were analyzed across 30 years, from 1980–2009, to determine if mountain hydrology has been affected by global warming. Observed daily river discharge was utilized to calculate center time (CT) of mass of flow. CT was found to be occurring significantly earlier in the year at two northern basins, with a rate of change of around five days per decade. While decreasing trends in CT in the other basins were not significant, negative correlations between CT and winter temperature was significant except for the central to northeastern basins. The effect of winter warming on snowmelt runoff was more significant in northern basins on the Sea of Japan side, where CT also correlated with the flowering date of cherry trees. Positive correlations between precipitation and discharge were stronger in southern basins, disturbing winter warming effect on spring discharge. These findings support the notion that winter warming accelerates snowmelt runoff, although year-to-year fluctuations were more pronounced than progressive warming over the three decades. Our results highlight inter-basin differences in hydrological response to climatic change, serving to validate down-scaling of climate simulations over the Japanese Alps region.

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The results of coupled high resolution global models (CGCMs) over South America are discussed. HiGEM1.2 and HadGEM1.2 simulations, with horizontal resolution of ~90 and 135 km, respectively, are compared. Precipitation estimations from CMAP (Climate Prediction Center—Merged Analysis of Precipitation), CPC (Climate Prediction Center) and GPCP (Global Precipitation Climatology Project) are used for validation. HiGEM1.2 and HadGEM1.2 simulated seasonal mean precipitation spatial patterns similar to the CMAP. The positioning and migration of the Intertropical Convergence Zone and of the Pacific and Atlantic subtropical highs are correctly simulated by the models. In HiGEM1.2 and HadGEM1.2, the intensity and locations of the South Atlantic Convergence Zone are in agreement with the observed dataset. The simulated annual cycles are in phase with estimations of rainfall for most of the six regions considered. An important result is that HiGEM1.2 and HadGEM1.2 eliminate a common problem of coarse resolution CGCMs, which is the simulation of a semiannual cycle of precipitation due to the semiannual solar forcing. Comparatively, the use of high resolution in HiGEM1.2 reduces the dry biases in the central part of Brazil during austral winter and spring and in most part of the year over an oceanic box in eastern Uruguay.

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Previous unpublished research led to the establishment of a relationship between effective porosity and specific capacity, based on well construction data in a series of laboratory experiments. In this paper the relationship was tested on a regional aquifer system using data from 609 wells which met certain criteria. The relationship was applied to each well, with results showing that the relationship needed revision. The relationship was thus reevaluated and revised to produce results that reflected the conditions in the aquifer system which consisted of 9 layers of aquifers and aquitards of various lithologic descriptions, ranging from unconsolidated sediments to volcanic rocks. Average values of effective porosity could be calculated and the distribution of effective porosity was determined for each unit and compared with the original estimates. The result was a new relationship to determine effective porosity directly from specific capacity, which can be applied without detailed information on well construction or lithology. The new relationship is useful for distributing effective porosity within 2 or 3 dimensional groundwater and particle tracking models on a cell-by-cell basis. More importantly, the new relationship can be used to determine effective porosity for contaminant transport models.

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In this study, we conducted sap flow measurements in Japanese cedar and cypress trees growing on a steep slope to examine circumferential variation. Sap flow measurements were conducted for upper and lower slope aspects and in four directions (north, east, south, and west). We also measured the width of the tree crown to examine the effect of sunlight. Japanese cedar and cypress growing at this site extended their crowns toward the lower slope. Individual trees displayed circumferential variation in sap flux density (F_d). For Japanese cedar and cypress, the maximum daily F_d were 1.92 and 3.80 times as large as the minimum, respectively. However, the circumferential variation in F_d did not appear to be dependent on direction or slope aspect. These results suggest that large errors are produced when circumferential variation in F_d is ignored during the estimation of whole tree transpiration. Therefore, it is necessary to use sensors to capture circumferential variation in F_d, but sensors can be inserted randomly without the need to consider the shape of the tree crown or the direction of the tree trunk.

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