As the problem of climate change increases in importance, evaluations of its impact are urgently required.Through a series of studies, we estimated the impact of climate change on Japanese water resources using a waterresources model comprising five sub-modules: land surface hydrology, irrigation, rice growth, dam operation, andstream flow. The model has the benefits of simulating not only the natural water cycle on land but also rice yields and theeffects of human activities such as dam operations, water withdrawal, and irrigation. This paper presents the water resourcesmodel and verifies its accuracy for Japanese river basins. Using observed meteorological data during 1994-2003, we analyzedriver discharge, rice yields, and water stresses. The main results are as follows: （1) Simulated rice heading date,harvesting date, and yield agreed with the prefectural statistical data. （2) Estimated river discharges showed good agreementwith observed stream flow records in 20 first-grade rivers, with no parameter calibrations. The simulated data for riverdischarges were improved using a dam operation module and by correction for snowfall data. （3) Water stresses in Japaneseriver basins were computed by calculating the cumulative withdrawal to demand ratio （CWD). Calculated values showedgood agreement with a statistical drought map, suggesting that the CWD reflects actual water shortages in Japanese basins.

View full abstract

 This study estimated the impacts of climate change on Japanese water resources using a hydrological model proposed in an earlier study. Using forcing outputs from a super high-resolution global climate model (MRIAGCM3.1S), we analyzed river discharge, rice yields, and water stresses under present (1979-2003), and anticipated nearfuture (2015-2039) and late 21st century (2075-2099) climates. The main findings were the following: (1) Climate change will engender large changes in seasonal discharge in basins facing the Sea of Japan, which experience heavy snowfall. Snowfall depletion will increase winter discharge and decrease spring discharge. (2) Changes in water stress were evaluated by calculating the cumulative withdrawal to demand ratio (CWD). Water stress will increase despite the increase in water resources in many basins, especially in the Tohoku District. Results show that many Japanese basins will face severe challenges in water resource management under future climate scenarios. (3) Warm temperature trends will cause rice yields to increase in northern, eastern, and central Japan. Under climatic conditions predicted for the end of the century, late plantings will be a necessary adaptation to avoid high-temperature and low-temperature damage in northern and western Japan. However, late plantings will also negatively affect yields by increasing water stress in basins in the Tohoku District that experience heavy snowfalls.

View full abstract

 To evaluate the applicability of the Soil Water Assessment Tool model (SWAT) for estima-tion of phosphorus load from a suburban catchment, we examined the reproducibility of river discharge, suspended solids (SS), particulate phosphorus (PP) and dissolved inorganic phos-phorus (DIP) concentrations. Results can be summarized as follows. First,the reproducibility of daily river discharge was good throughout the year, although it tended to be overestimated or underestimated in high discharge periods. This result implies that the reproducibility of discharge is a.ected by spatial and temporal resolution of weather data. Secondly, the repro-ducibility of SS concentration shows acceptable results despite empirical equations developed in the United States of America. Reproducibility of PP concentration was also acceptable as well as SS. Therefore, the model was capable of estimating PP caused by soil erosion. Never-theless,results showed that estimation methods used for DIP in urban areas are not fit for use in Japan. The possibility exists of making an error of estimation of DIP in a watershed that is highly in.uenced by domestic wastewater. In this study, the estimation of DIP was improved by considering domestic wastewater as a point source. Consequently, results con.rmed that SWAT is capable of estimating phosphorus discharge. However, results suggest that the model should be used carefully for DIP estimation in urban areas. Furthermore, results underscore the necessity of improving DIP estimation methods.

View full abstract