In order to better understand the hazard of secondary salinization in soils associated with inappropriate water management, the salt movements during the infiltration and evaporation after watering over the soil surface were investigated using soil columns with a shallow water table. Although the linkage of the infiltrated water and groundwater enhances the leaching of the salt contained in the soil surface layer, but the subsequent upward moisture movement due to the evaporation eventually increases the salt content near the soil surface more than the initial level before the watering.
Furthermore, a heat, moisture and solute transfer model in a saturated, an unsaturated and a dry soil layer was extended to calculate the condensation and crystallization of the salt content as well as the soil temperature, the volumetric water content and the vapor density profiles. The present model could well reproduce the experimental results. However, after the surface dry layer was appeared, the difference between calculated soil moisture content profiles and observed ones became conspicuous near the bottom of dry layer. It is pointed out that the reason may be attributed to the inaccuracy of the soil moisture characteristic curve and of the hydraulic conductivity in the low water content state.

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Land surface variables, especially the soil moisture, are central to the regulation of the land-atmosphere water, energy, and carbon interactions. The development of land hydrological parameterizations that emphasize soil moisture and temperature, together with innovative remote sensing and surface-based observations, promise a mechanism for the synthesis of regional fields of these vital hydrological variables. Data assimilation is the process of incorporating observations into a model of the land-atmosphere interaction system and filling the region of missing data with model predictions as well as providing a suite of data-constrained estimates of unobserved quantities.
In order to incorporate remote sensing and surface-based observations in a numerical forecasting model that estimates land-surface parameters, we have developed a Land Data Assimilation Scheme (LDAS) that can assess the model parameters and estimates vertical profiles of soil moisture and temperature by comparing passive microwave observations through a unique minimization technique called Very Fast Simulated Annealing (VFSA).
This manuscript mainly focuses on modifying LDAS and testing its capabilities in different bare soils and their surface conditions. So we conducted a field experiment and obtained well-controlled observations by employing a six channel Ground Based Microwave Radiometer (GBMR-6CH). The results of the LDAS testing were in good agreement with observational data compare to open loop simulations. Major model sensitivities to soil type, soil moisture, and surface roughness are explored in bellow section.

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To assess the spatial and temporal variation in net CH₄ fluxes in a forest watershed in Japan, we used the closed-chamber method to measure CH₄ fluxes from soils and investigated how they are affected by environmental factors. It was found that the water table and soil water content dictated redox conditions and affected CH₄ production and oxidation in several different parts of the watershed. In forest floor sites, CH₄ was consumed in dry parts throughout the year and was emitted in wet parts in summer. In wetlands, CH₄ emission was observed almost throughout the year, and increased markedly in summer, although emissions were constrained by the fall in water table in summer. While the CH₄ emission rate increased with increased soil temperature, CH₄ uptake rate was less affected by fluctuations in soil temperature. A net budget estimation suggested that CH₄ emissions exceeded uptake in this watershed during the high-temperature season, since the emission rate from wetlands was 3 orders of magnitude higher than the uptake rate at the forest floor. These results suggest the importance of measurements of CH₄ emissions from wetlands even if their geographical area is relatively small compared to the entire watershed area.

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This study presents an effort to develop a new conceptual rainfall-runoff model based on a proposal of storage channel concept. The model implicitly solves the flow equations and was successfully calibrated and validated by applying it to a lowland catchment in the central province of Vietnam for comparison with the observed data. Moreover, the applicability of the model was also tested by comparing its results to those computed by a well-known model in Japan, the ordinal Tank model developed by Sugawara. From both the performance statistics and the hydrographs of the model results, it has been found that the model integrating a proper discharge control device provides satisfactory estimation of runoff discharge. The full model structure has totally twelve parameters to be calibrated. Sensitivity analysis for these parameters shows that they contribute effects to both the magnitude and the shape of the runoff hydrographs. Discharge control devices installed at the channel's outlets have a high advantage when considering the effects of time-varying boundary conditions at the channel′s outlets, in contrast some traditional lumped rainfall-runoff models using fixed discharge coefficients may not. The use of water control devices has made the model possible to consider the effect of tidal regime in lowland catchments while also play an important part in the model toward the shape, peak and timing of the runoff hydrographs as well as express the nonlinearity of the rainfall-runoff process. In short, the newly developed model has several good advantages over lumped hydrological models. Furthermore, the new model is basically able to analyze both runoff and flow movement in river basins that include mountainous, hilly and lowland tidal areas.

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In Okinawa, red soil runoff has been a social problem for long time, and many measures have been adopted. However, effects of those measures are estimated to be not enough and some models for the prediction of red soil runoff are required. With this background, we have been developing the suspended sediment (SS) runoff analysis model. In this study, the SS runoff analysis model, that is a lumped storage-type runoff model and includes a sediment runoff process, has been improved by adopting a concept of the slope erosion model shown by Foster in a soil runoff process. The model has been applied to two watersheds that have different land use conditions from each other. As the result, though there were differences in some cases of suspended sediment concentration (SSC) peaks, the SS runoff analysis model gave good agreement between the observed data and the calculated data. It was recognized that the optimized parameters represented differences of the characteristics of the watersheds. Moreover, it was confirmed that the model could reproduce a first flash that was the typical characteristics of red soil runoff from farmlands.

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This paper is reported on reservoirs operation in Chao Phraya River basin, Kingdom of Thailand. The Chao Phraya River basin has two large-scale multi purpose reservoirs. These reservoirs did not have any operation rule until 1995. After 1995, the EGAT that is the operator made the reservoir operation rule in 1996. The EGAT had made the upper rule curve and the lower rule curve, then the storage volume has been operated between the upper rule curve and the lower rule curve. In the dry season (from January to June), the EGAT had the special rule for release water. The committee on water resources of Thai government had made the weekly demand and then order to the EGAT.
Spectrum of flow was analyzed by using FFT for daily discharges in Nakhon Sawan and immediate downstream of the Bhumibol dam (Royal Irrigation Department′s code P.12) after the Bhumibol dam construction. Flow in Nakhon Sawan has periodic characteristics of 3.5 days and 7 days, and the released water from the Bhumibol dam at P.12 has periodic characteristics of 2.3 days, 3.5 days and 7 days. Reservoir operation affected the hydrological cycle considerably.
The effect of reservoirs operation on hydrological regime was estimated in Chao Phraya River basin. As the result, the minimum discharge had been increasing year by year. And the discharge had been maintained. The reservoir operation had huge impacted on the hydrological regime in lower basin.

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A method with multi-layer and grid-based runoff model using Hydro-BEAM (Hydrological River Basin Environment Assessment Model) is proposed to analyze the integrated hydrological processes. The spatiotemporal water flow is calculated with the kinematic wave model for surface runoff, Richards' equation for unsaturated subsurface flow and the unconfined flow model for groundwater. The initial loss of rainfall due to interception by depression storage process is considered here. Moreover the basin division and land use dynamics are introduced to analyze reservoir operation and land utilization with human activities. The proposed model is calibrated for different initial conditions and parameters, and applied to the Yasu River to verify the dynamic linkage between surface and groundwater.

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Atmospheric water vapor collected by cryogenic trapping method is in danger of changing its isotopic composition due to isotopic fractionation during condensation. However, there is no research to assess in detail the isotopic modification associated with the process of water vapor sampling. In the present study, we designed a new experimental protocol for examining the isotopic fractionation effects and attempted to check reliability of the water vapor collection method. Experimental results under different conditions suggest that isotopic composition of collected water vapor can be measured with sufficient accuracy in practice (±1‰ for δD and ±0.5‰ for δ¹⁸O) if air flow rate through a trap is no more than a critical value (e.g., 10 l/sec).

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This study demonstrated the validation of the terrestrial water storage simulated by Japan Meteorological Agency-Simple Biosphere model (JMA-SiB) forced with the European Center for Medium-Range Weather Forecasts 15-year Reanalysis against the terrestrial water storage estimated by the land-atmosphere combined water balance method.
Taylor diagram was used to provide a concise statistic summary of the validation. The JMA-SiB did a good job of reproducibility when difference of the definition on the terrestrial water storage between the model and the water balance method was taken into account. The maximum of the lagged correlation coefficients (LCC) in most basins exceeds the value of 0.9. The present study clarified that the diagram can summarize the merits of a collection of different basins or trace changes in the LCC.

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