The Indonesian Maritime Continent (IMC) is located in the tropics and consists of a number of islands and seas. This particular arrangement of land masses and seas produces unique weather and climate characteristics. Various hydrometeorology studies have been conducted exploring the variability of stable isotopes in precipitation over the IMC. Stable isotopes (δ¹⁸O and δD) in precipitation can be used to obtain information about atmospheric processes (e.g., precipitation, temperature and hydrological cycle). The Global Network of Isotopes in Precipitation, operated by the International Atomic Energy Agency, has been conducting worldwide monthly surveys of isotope levels in precipitation since 1961. To better understand the isotopic variability in the IMC, the Japan Agency for Marine-Earth Science and Technology began operating in the IMC region in 2001. There are three types of seasonal variation of stable isotopes in precipitation, depending on the level of precipitation amount and also stable isotopic value, namely, semiannual, anti-monsoonal and monsoonal. Negative correlations between the precipitation amount and stable isotope value (amount effect) were identified when using the monthly averages. The interannual variability of stable isotopes in precipitation was mainly because of El Niño Southern Oscillation activity, whereas intraseasonal variability was closely related to the Madden-Julian Oscillation. Stable isotope variability in short periods was found to be due to precipitation cloud types and post-condensation processes.

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In a coastal zone discharging submarine groundwater, we observed sea surface temperature using a drone equipping infrared thermography in order to verify the availability and defect on exploration of submarine groundwater using a drone. The difference of temperature between sea surface above the discharge point of submarine ground water and surrounding seawater was approximately 3 degree Celsius based on manual direct measurements using a thermometer. The difference of temperature was also observed by sea surface temperature image of a drone, showing that drone’s exploration method is successful for discovering submarine groundwater discharging at water depth of 2 meters. Furthermore, the submarine groundwater affected the temperature of surface seawater within a few square meters. On the other hand, submarine groundwater discharging at water depth of 4–5 meters was not identified by the image of drone, suggesting that the application of this method was restricted within very shallow parts of coastal area.

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In order to clarify loading processes of inorganic ions in a catchment accompanied by tephra and permeable bedrock, water sampling and electric conductivity monitoring were carried out in the forested (88.3% of catchment area) Oikamanai River catchment (62.47 km² in area) in the Tokachi coastal region of eastern Hokkaido. Surface geology of the catchment consists mostly of Neogene marine sedimentary rocks, siltstone, sandstone and conglomerate, accompanied by currently active faults. The forest soils on catchment slope include sandy tephra (Ta-b) at 30–40 cm depth, deposited by Tarumae Volcano eruption in 1667, and thicker tephra (Spfa-1) from Shikotsu volcanic eruption (ca. 40,000 years ago) and gravelly sediment at greater depths. The layers of tephra and gravelly sediment are relatively permeable, providing water pathway for subsurface flow. In this study, a linear relationship between ionic concentration (mg/L) and electric conductivity at 25℃ (EC25; mS/m)was found out for each of five major ions (Mg²⁺, Ca²⁺, Na⁺, SO₄^2-, HCO₃^-) in water sampled in the rainfall season of 2013. Hence, the EC25 monitoring allowed us to obtain the load time series of the five ions, using hydrographs from stage-discharge rating curves. The coupling of the tank model with power function reasonably simulated the discharge and ion load time series. As a result of runoff analysis, surface flow and intermediate flow occupied 74.2% of total discharge, and the groundwater leakage, 16.8%. Being common to the five ions, the ion load by intermediate flow prevailed, occupying 40.0–70.0% of total load. The intermediate flow could occur in the permeable layers of Shikotsu tephra Spfa-1 and gravelly sediment above the bedrock, then probably producing active dissolution of the five ions.

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