[1] The Madden-Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in the tropical atmosphere. This study examines the evolution of the hydrologic regime from before the onset of the MJO (pre-onset period) to the MJO onset period, using deuterated water vapor (HDO) measurements from the Tropospheric Emission Spectrometer (TES) and from ground-based stations. Ground-based observations reveal a clear transition between high HDO/H 2 O isotope ratios during the pre-onset period to a period of repeated abrupt decreases in the HDO/H 2 O isotope ratio associated with intense convection. Each observed minimum in the HDO/H 2 O ratio corresponded to a maximum in stratiform rainfall fraction, which was derived independently from radar precipitation coverage area. The ground-based observations are consistent with the satellite observations of the HDO/H 2 O ratio. In order to attribute the mechanisms that bring about the isotopic changes within the MJO convection, an isotope-enabled general circulation model (GCM) constrained by observed meteorological fields was used to simulate this MJO period. The GCM reproduced many of the observed isotopic features that accompanied the onset of an MJO. After the development of deep convection, large-scale stratiform cloud cover appears, and isotope ratios respond, as a consequence of diffusive exchange between stratiform raindrops and the surrounding vapor. In this diffusive exchange process, heavy isotopes tend to become enriched in precipitation and depleted in the surrounding vapor, and thus successive stratiform rainfall results in decreasing isotope values in the middle and lower troposphere. On the basis of these characteristics, isotope tracers can be used to partition stratiform and convective rainfall from observed isotope data and to validate the simulated proportions of convective/stratiform rainfall.

Both meteorological data and stable isotope data were used to investigate the role that local moisture recycling plays in maintaining moist land surface conditions over the middle of the Tibetan Plateau during the summer monsoon season. Past studies have shown that precipitation events of the summer monsoon season can be categorized according to synoptic conditions as east-migrating trough types, heat low types, and regional circulation types. Precipitation events during an intensive observation period from 13 to 27 August 2004 were therefore classified into these three types. The contributions of locally recycled moisture in each precipitation type were investigated using isotopic features. The isotope data include precipitation, near-surface atmospheric moisture, and evapotranspiration. First, using a simple Rayleigh distillation model, the isotopic content of the moisture source of the rainfall was estimated from observed precipitation isotope data. The contribution of lower-atmospheric moisture to the precipitation was then evaluated by comparing isotopic values. Next, when rainfall was mainly fed by lower-atmosperic moisture, the contribution of evapotranspirated water in the lower atmosphere was assessed by considering the factors controlling the isotopic variability of lower-atmospheric moisture. The results show that, in the case of trough-type rainfall, moisture flux convergence occurred in this area and a remarkable increase in precipitable water was observed when a trough approached the site. Thus, observed large isotopic variation associated with the passage of a trough reflects the isotopic content of moisture advected from the surrounding areas. With the exception of trough-type rainfall, the simulated isotopic values agreed well with the isotopic value of lower-atmospheric moisture. This finding indicates that lower-atmospheric moisture is the dominant source of such rainfall. In these periods, temporal isotopic variation in lower-atmospheric moisture showed gradual increases accompanied by an increased contribution of evapotranspirated water that had relatively heavy isotopic values. In particular, when the regional circulation type of rainfall was observed, the local recycling ratio, which is the contribution of locally evapotranspirated water in the boundary layer, increased from 30% to 80%. Locally recycled moisture thus plays an important role in precipitation associated with regional circulation. Active moisture recycling contributes to a high frequency of precipitation events so that the moist land surface is maintained during the summer monsoon period in this region.

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Tropical peatlands store soil carbon constituting up to 15-19％ of global peat carbon. That huge carbon pool is presently being disturbed on a large scale by land development and management, and has consequently become vulnerable. Peat degradation occurs most rapidly and massively in Indonesia's peatlands because of fires, drainage and deforestation of swamp forests. Peat burning releases carbon dioxide (CO 2 ) intensively but occasionally, whereas drainage increases CO 2 emissions steadily through accelerated aerobic peat decomposition. Under such circumstances, tropical peatlands might become a huge source of carbon emissions to the atmosphere. Nevertheless, the effects of drainage on the carbon balance of tropical peatland ecosystems are not well understood; more field data must be accumulated. Therefore, we measured soil respiration (RS), which is a major source of CO 2 efflux, continuously for more than one year using automated chamber systems, with consideration of microtopography, at two sites in Central Kalimantan, Indonesia: undrained and drained peat swamp forests. The RS was determined mainly by local hydrology. In the undrained forest, RS decreased sharply under flooded conditions because of anoxia. In contrast, in the drained forest, with its lower groundwater level (GWL), RS showed a quadratic relationship with GWL and gradually increased as GWL decreased when GWL was lower than about -0.8 m, which was caused chiefly by the enhancement of peat decomposition. These relationships indicate that lowering GWL by drainage increased RS, whereas annual RS was larger in the undrained forest (1347 gC m -2 y -1 ) than in the drained forest (1225 gC m -2 y -1 ) in 2005. The difference in annual RS was probably attributable to higher forest productivity in the undrained forest.

Abstract:Permeability of a streambed is an important factor regulating nutrient and oxygen availability for aquatic biota. In order to investigate the relationship, an accurate permeability should be measured. However, it is difficult to measure permeability in a coarse gravel bed using a conventional permeability test. Moreover, turbulent flow may occur in coarse bed material, and then deviations from Darcy's law do occur. Thus, permeability calculated following Darcy's law may be overestimated under turbulent flow conditions and should be corrected. The packer test can be used in highly permeable gravel beds. We developed a field method applicable to a gravel bed using the packer test and derived an equation adopting a law of turbulent flow to study the problems under any type of flow condition. The accuracy of the equation was examined using a laboratory flume with a gravel bed. The results suggested that permeability calculated from Hvorslev's equation is overestimated for turbulent flow. In contrast, our equation, developed here, could evaluate permeability accurately under any type of flow condition.

The Makomanai River in northern Japan has suffered considerable fine sediment deposition, especially in a reach where channel works have been constructed. Four contiguous reaches were examined for deposition of fine sediment and the effects of such on periphyton biomass; two of the reaches had channel works and bank protection, respectively, the other two being unmodified. The influence of fine sedimentation on epilithic periphyton biomass (chlorophyll a weight and organic matter weight) and the autotrophic index (AI), and the relationship between these and hydraulic variables was emphasized. Fine sediment increased, chlorophyll a decreased and AI increased in the reach with channel works and in the unmodified reach immediately upstream. In addition, the current velocity and Froude number tended to decrease in the reach with channel works. Correlation analysis showed that with an increase in fine sediment, chlorophyll a decreased and non-living periphyton, indicated by AI, increased. It was also confirmed that fine sediment increased with an increase in weight of periphyton organic matter and decreased with an increase in current velocity or Froude number. These results suggest that accumulation of fine sediment, which substantially reduces light penetration for photosynthesis under low current velocity conditions, results in lowered periphyton levels. The channel works have lowered the current velocity, thus promoting deposition of fine sediment and an increase in non-living periphyton.

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