We have estimated natural runoff independent of human activities using a watershed hydrological model for the Chao Phraya River basin, Kingdom of Thailand. In order to assess the flood control and water supply capacities of two large scale reservoirs, the observed daily river discharge have been compared to the estimated values. We observed that the reservoirs' operation affected not only the increase of low flow but also the stabilization of river discharge. We estimated that the peak discharge at Hydrological Station C.2 (Nakhon Sawan city), which is the most important station for water related policies, would decrease by a maximum of 57.8% of maximum discharge and an average of 42.4 % of maximum discharge due to reservoir operations. Moreover, small and middle scale flooding was decreased by about 40 % and large scale flooding was decreased by about 35 % due to the development of the two large scale reservoirs.

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In order to understand features of a meteorological field related to a heavy rainfall event, it is important to interpret detailed information of atmospheric stratification, especially, its horizontal variability. However, the horizontal features cannot be easily interpreted because the atmospheric stratification represents multi-dimensional data consisting of a vertical profile of water vapor and air temperature. Therefore, in this study, by considering multi-dimensional atmospheric stratification as a set of data, many varieties of patterns characterizing atmospheric stratifications detected in northern Kyushu in a rainy season were classified using the Self-Organizing Map (SOM) algorithm. The result shows that the SOM could recognize inherent patterns in the rainy season by investigating warm and humid air in lower layers, and dry air in middle layers. In the next analysis, all the atmospheric stratification characterized by the heavy rainfall event that occurred in northern Kyushu on June 29, 1999, were diagnosed on the basis of the classified results by SOM, and, subsequently, all the diagnosed patterns were displayed in the target area including northern Kyushu. As a result, the horizontal patterns of the atmospheric stratification in the heavy rainfall field could be easily interpreted by using some properties of SOM. From these analyses, it is expected that detailed information on the atmospheric environment causing heavy rainfall can be systematically and easily interpreted by combining the diagnosis of atmospheric stratification by using SOM with horizontal representation of its patterns instead of a conventional analysis of atmospheric stratification.

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Water management with appropriate gate operation is needed for irrigation areas where water shortages are serious. In this study, the irrigation water distributions of two irrigation projects in Northeast Thailand were analyzed and the efficiencies of water use were simulated from one day to 2 weeks. The efficiency of water distribution was calculated by using a hydraulic model under the operating schedule generated from human factors. Hydraulic structures such as a constant head orifice and spillway were included in the hydraulic model by using one-dimensional movement equations and continuity equations. The parameters of human factors were estimated from the measured data such as the gate operator density and water depth management. From the results, in the medium discharge, gate operation delays within two hours caused the error of water distribution within 5 %. However, if the gate operators operated the gates to the ideal opening height regularly every day, the error was within 5 % even in the actual density. In the high discharge, an overflow was caused regardless of gate operators density, the error of water distribution to the downstream was over 10 %

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This study evaluates performance of the neural network based radar rainfall estimation and examines the applicability of the radar rainfall input estimated from the neural network (R_NN) in streamflow modeling. The Uono River basin is selected as the study basin. A distributed hydrologic model is driven by using R_NN, the radar rainfall input obtained from the Z-R relationship (R_Z-R), and the gauge rainfall (R_G) respectively. The statistical results of radar rainfall estimation indicate that the radar rainfall product using the neural network is more accurate than that using the operational Z-R relationship. In addition, the streamflow simulation results show that the simulated hydrographs obtained from R_NN are more accurate than those obtained from R_Z-R. The study concluded that the neural network technique outperforms the existing operational Z-R relationship. The appropriately trained network at the rain gauge sites is accurate, stable, and robust for estimating radar rainfall over the whole basin. The study also suggested that the R_NN is an alternative input for hydrologic modeling when the gauge rainfall data is unavailable or insufficient.

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In order to evaluate an ability of a forested watershed for decreasing storm runoff and increasing baseflow discharge, this study analyzed the roles of soil layer and permeable bedrock in rainwater discharge processes by conducting hydrological observations and numerical simulations. Hydrological observations in a forested headwater catchment underlain by weathered granite indicated that saturated throughflow in a soil layer is extinguished just after a storm event while it contributes to a storm hydrograph. On the other hand, base flow discharge from the catchment was fed by the weathered bedrock. Based on these results, numerical simulations for saturated and unsaturated water flow were conducted for analyzing rainwater infiltration and discharge processes in a forested hillslope underlain by a permeable bedrock. Results showed that, while an exfiltration from the bedrock sustains base flow discharge, soil layer behaves as a buffer, moderating the infiltration intensity relative to the rainfall intensity and permitting large amount of rainwater to infiltrate into the bedrock. Thus, the result of this study indicates that measuring effective pore volume of soil layer is not enough for quantifying a water holding capacity of a forested watershed. Instead, evaluation of the buffer function of a soil layer is important.

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This paper proposes a hydrograph separation method based on spatiotemporal record of stream flow in a distributed rainfall-runoff model. It simulates spatiotemporal rainwater movement in a catchment, considering relationship between rainfall and runoff at each unit slope and at each flow pathway defined in the model. The proposed method is able to trace rainfall and runoff taking into account unsaturated, saturated subsurface, and surface rainfall-runoff processes, and to analyze the effects of the model structures and parameters on the simulated stream flow components.

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For delineation of current condition and characteristics of the nutrient concentrations in the Mekong River, continuous frequent observation was carried out in the mainstream at Vientiane. The water samples were taken twice a week from April 2002 to December 2005 and were analyzed for nitrogen and phosphorus species. Detailed seasonal variation characteristics which had not been detected by previous monthly observation were revealed by the frequent observation. Overall, the nutrient concentrations were low, possibly owing to lower population density and lower coverage of agricultural land in the upstream of the sampling point. The ammonium-nitrogen concentration, however, showed an increasing trend during the recent decades. The nitrogen concentrations abruptly increased in May, coinciding with the first small rise in the discharge after the low flow period. It is suggested that the first runoff after the dry season took the accumulated nitrogen in the basin and on the river bed to the river, bringing about the abrupt increase in the nitrogen concentrations. After June, the nitrogen concentrations steadily decreased, regardless of the discharge enhancement during the flood season. It is considered that the nitrogen supply after June was diluted by plentiful water during the flood season. On the other hand, the phosphorus concentrations gently increased from April to June and were greatly fluctuated during the flood season. Continuous dilution effects by the plenty of water and occasional larger scale soil erosions during the flood season may account for the variation characteristics of phosphorus concentrations during the flood season.

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