Makoto Tani

Runoff generation processes estimated from hydrological observations on a steep forested hillslope with a thin soil layer

Abstract In order to understand runoff generation processes on a forested hillslope involving large heterogeneities, this study monitored runoff from a steep hillslope with a thin soil layer as well as matric potential within it and analyzed their responses to storm rainfall. A comparison of storm runoff responses from the study slope with those from two adjacent catchments, one of which includes it, showed that physical properties of the slope reflected the runoff characteristics: although no responses occurred in very dry conditions because of the absence of wet zones near the stream, the area contributing to storm runoff more rapidly extended to the whole slope due to its topographic properties. They also caused its steep hydrographs produced in the wettest condition where almost all the rainfall contributed to storm runoff. In this wettest condition, tensiometric responses near bedrock showed the vertical quick propagation of the rainfall pulse, and a good agreement of storm hydrograph simulated through a kinematic wave runoff model suggested that runoff from the slope was produced by a lateral flow on the bedrock receiving the quick propagation. In a transition process from dry to the wettest conditions, the development of the lateral flow producing smaller responses at the downslope end was estimated from decreasing of matric potential near bedrock from high negative to low values with increasing cumulative rainfall.

The mechanism of general flowering in Dipterocarpaceae in the Malay Peninsula

The mechanism of general flowering in Dipterocarpaceae in the Malay Peninsula is revealed through field survey and meteorological data analyses. The regions of general flowering coincide with those which experienced a low night-time temperature (LNT) c. 2 mo before flowering. This supports the hypo- thesis that low air temperature induces the development of floral buds of diptero- carps. LNT was found to be caused by radiative cooling during dry spells in winter when the northern subtropical ridge (STR) occasionally migrates southwards with a dry air mass into the equatorial region. LNT events usually occur in La Nifia episodes, not in El Nifio episodes as believed previously. This is because the south- ward migration of the STR is associated with the intensification of local meridional Hadley Circulation in the western Pacific, which is strengthened in a La Nifia episode. Results suggest that El Nifio-like climate change in increased atmospheric carbon dioxide concentrations may be critical for the tropical rain forest biome in south-east Asia.

Soil physical properties and preferential flow pathways in tropical rain forest, Bukit Tarek, Peninsular Malaysia

Soil physical properties and water movement within soil were investigated using dyes in a tropical rain forest, the Bukit Tarek Experimental Watershed of Peninsular Malaysia. The saturated hydraulic conductivity (Ks) decreased with increasing soil depth. TheKs values were higher than those reported for other tropical soils. The geometric means of theKs values ranged from 4.69×10−3 (80 cm) to 4.07×10−2 cm s−1 (10cm). This suggests saturation overland flow may not be dominant but that subsurface flow must play an important role in stormflow generation. The shapes of the soil moisture characteristic curves resembled those of forest soils which have large changes in volumetric water content at pressure heads <30 cmH2O. The relatively high conductivities were due to the presence of a porous zone of decomposed root channels which existed continuously in vertical direction. Besides decayed roots, living roots also encourage preferential flow in vertical and lateral (downslope) directions. Termite activities may also form water flow pathways in tropical regions. These detailed results help us analyze water flow within the soil in tropical rain forests.

Environmental Effects on Oxygen Isotope Enrichment of Leaf Water in Cotton Leaves

The oxygen isotope enrichment of bulk leaf water (Δb) was measured in cotton (Gossypium hirsutum) leaves to test the Craig-Gordon and Farquhar-Gan models under different environmental conditions. Δb increased with increasing leaf-to-air vapor pressure difference (VPd) as an overall result of the responses to the ratio of ambient to intercellular vapor pressures (ea/ei) and to stomatal conductance (gs). The oxygen isotope enrichment of lamina water relative to source water \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(({\bar{{\Delta}}}_{1}),\) \end{document} which increased with increasing VPd, was estimated by mass balance between less enriched water in primary veins and enriched water in the leaf. The Craig-Gordon model overestimated Δb (and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\bar{{\Delta}}}_{1}),\) \end{document} as expected. Such discrepancies increased with increase in transpiration rate (E), supporting the Farquhar-Gan model, which gave reasonable predictions of Δb and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\bar{{\Delta}}}_{1}\) \end{document} with an L of 7.9 mm, much less than the total radial effective length Lr of 43 mm. The fitted values of L for \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\bar{{\Delta}}}_{1}\) \end{document} of individual leaves showed little dependence on VPd and temperature, supporting the assumption that the Farquhar-Gan formulation is relevant and useful in describing leaf water isotopic enrichment.

Rainfall-runoff responses and roles of soil moisture variations to the response in tropical rain forest, bukit tarek, peninsular Malaysia

Field observations were conducted at Bukit Tarek Experimental Watershed in Peninsular Malaysia to investigate the relationship between rainfall-runoff responses and variation in soil moisture in a tropical rain forest. Stormflow depended strongly on the antecedent wetness as represented by the initial runoff rate. Though heavy rains fell in almost every month, the soil moisture decreased when fair weather was sustained. The soil moisture depleted and became dry at 160 cm depth during occasional dry spells. During dry conditions, streamflow responded quickly to rain events but declined rapidly after the rain stopped, and the soil moisture of surface soil (≤20 cm) increased but remained dry at lower depths (≽80 cm). This suggests that the rain water was mostly retained in the soil and only small proportions appeared as stormflow. As soil moisture conditions became wetter, the recession limb of the storm hydrograph was more gradual. Stormflow volume increased with increasing soil moisture. During wet conditions, the soil profile was moist at all parts of the slope. The hydraulic gradient was around 1.0 and there was downward soil water flux, which followed the pressure gradient. This suggests that subsurface flow from the upper part of the slope might also be important for stream-flow production. Positive pressures were observed at 10 cm and 160 cm depths during large storms. The behavior of the subsurface flow might be an important determinant of stormflow.

CO 2 exchange in a temperate Japanese cypress forest compared with that in a cool-temperate deciduous broad-leaved forest

To examine the characteristics of carbon exchange in coniferous forests, we analysed the seasonal and diurnal patterns of CO2 exchange, as measured using the eddy covariance method, in a Japanese cypress forest in the Kiryu Experimental Watershed (KEW) in central Japan. The net CO2 exchange data during periods of low-friction velocity conditions and during periods of missing data were interpolated. The daily CO2 uptake was observed throughout the year, with maximum values occurring in early summer. Periods of low carbon uptake were seen in late summer owing to high respiratory CO2 efflux. The diurnal and seasonal patterns of daytime CO2 exchange at KEW were compared with those in a cool-temperate deciduous forest of the Tomakomai Experimental Forest (TOEF) in Japan. The environmental differences between evergreen and deciduous forests affected the seasonal patterns of carbon uptake. Although there were great differences in the mean monthly air temperatures between the sites, the mean monthly daytime carbon uptake was almost equal at both sites during the peak growing period. The carbon-uptake values at the same PAR level were greater before noon than after noon, especially at TOEF, suggesting the stomatal regulation of carbon uptake.

Rainfall Characteristics of Tropical Rainforest at Pasoh Forest Reserve, Negeri Sembilan, Peninsular Malaysia

We investigated the rainfall at the Pasoh Forest Reserve (Pasoh FR), Peninsular Malaysia. Pasoh FR is located in the Southwest rainfall regime, in which the average annual rainfall (1,500–2,000 mm) is less than that in other regions of Peninsular Malaysia. Monthly rainfall in 1996 and 1997 ranged from 2.2 to 206.7 mm with a mean of 115.6 mm. The rainfall in 1997 was much smaller due to the El Nino Southern-Oscillation (ENSO) event. The longest period of dry days was 49 days. Dry periods as well as fluctuation in rainfall are major factors affecting the growth of vegetation. A distinct diurnal cycle in rainfall, in which 52% of the rainfall occurred between 13:00 and 19:00 h, was apparent. The frequency of the amount of rainfall in each event was an inverse J-shaped type distribution. The amount of rainfall in one event ranged from 1.2 and 93.1 mm with a mean of 11.4 mm and a median of 5.6 mm. The rainfall was characterized by a short duration (range:1.0-22.0 h, mean: 3.8 h) and high intensity. The maximum hourly rainfall intensity during a rain event ranged from 0.6 to 63.8 mm h-1 with a mean of 7.8 mm h-1 and a median of 3.8 mm h-1.

Characteristics of Energy Exchange and Surface Conductance of a Tropical Rain Forest in Peninsular Malaysia

Energy exchange above tropical rain forest was studied using micro-meteorological monitoring from a 52 m tower established in the Pasoh Forest Reserve (Pasoh FR) in Peninsular Malaysia. The meteorological conditions were comparatively drier during the first half of the year and wetter toward the end of the year due to the seasonal variation of rainfall. The five-year observational period from 1995 to 1999 included a low rainfall duration due to the El Nino from 1997 to 1998. The latent heat flux estimated by the Bowen ratio method occupied a dominant portion of the energy exchange even in the driest condition in early 1998. Although evapotranspiration from the dry canopy tended to be smaller in this period than in a wet period during the end of 1998, the surface conductance estimated using the Penman Monteith Equation was consistently controlled by the same function of solar radiation and specific humidity deficit. This suggests the evaporation did not suffer from severe stress of soil water even in the driest condition.

An approach to annual water balance for small mountainous catchments with wide spatial distributions of rainfall and snow water equivalent

Abstract One of the major problems in understanding the hydrological cycle in high mountainous regions with much snow is evaluating the spatial and temporal distribution of precipitation. This study evaluates the water balance in two small neighbouring catchments, namely Honryu (HN) and Shozawa (SH) in the Takaragawa Forest Watershed Experiment Station, Japan, by analysing records of precipitation in rainfall seasons and snow water equivalent (SWE) in snowfall seasons. Because both records were satisfactorily correlated with the filtered elevation, the distributed values of precipitation and SWE for each grid point of the digital elevation map were evaluated based on this correlation in each of the seasons. Total precipitation during the snowfall seasons was estimated from the SWE records using a relation of the mean catchment SWE to the cumulative precipitation monitored continuously at the observation station. An annual water balance in the two catchments averaged over several years was calculated from the total mean catchment precipitation in rainfall and snowfall seasons. The catchment evapotranspiration derived from the water balance was compared with Hamons potential evapotranspiration. Although a large difference between the two catchments in evapotranspiration was calculated from the water balance, several probable causes could be suggested for this difference, and it is concluded that the annual water balance could be estimated with an acceptable accuracy.

Vertical profiles and storage fluxes of CO2, heat and water in a tropical rainforest at Pasoh, Peninsular Malaysia

Ambient CO2 concentration, air temperature and relative humidity were measured intermittently for a 3-year period from the floor to the canopy top of a tropical rainforest in Pasoh, Peninsular Malaysia. Mean diurnal CO2 storage flux (Sc; μmol m-2 s-1) and sensible and latent heat storage fluxes (Qa and Qw; W m-2) ranged from -12.7 to 3.2 μmol m-2 s-1, -15 to 27 W m-2 and -10 to 20 W m-2, respectively. Small differences in diurnal changes were observed in Sc and Qa between the driest and wettest periods. Compared with the ranges of mean diurnal CO2 eddy flux (-14.7 to 4.9 μmol m-2 s-1), sensible eddy flux (-12 to 169 W m-2) and latent eddy flux (0 to 250 W m-2), the contribution of CO2 storage flux was especially large. Comparison with summertime data from a temperate Japanese cypress forest suggested a higher contribution of Sc in the tropical rainforest, probably mainly due to the difference in nighttime friction velocity at the sites. On the other hand, differences in Qa and Qw were smaller than the difference in Sc, probably because of the smaller nighttime sinks/sources of heat and water vapour.