Eriko Ito

Leaf-Shedding Phenology in Lowland Tropical Seasonal Forests of Cambodia as Estimated From NOAA Satellite Images

Forest seasonality is an important element to ecosystem functions. Eco-hydro models describing the Indochina bioregion under the seasonal tropical climate need regional phenological information about leaf dynamics. We examined the utility of remote sensing technology to leaf phenological research in Cambodian lowland forests. For this purpose, we aimed to detect any difference in leaf-shedding phenology between evergreen forests and deciduous forests. We analyzed the NOAA/AVHRR normalized differential vegetation index (NDVI) obtained from May 2001 to April 2002. The local maximum fitting (LMF) processing combines the time series filtering and the functional fitting was used for creating cloud/noise-free 10- day composites 1.1 km pixel data. Firstly, we estimated seasonal changes in the NDVI dominated by lowland evergreen forests and by lowland deciduous forests. Secondary, we identified local minimum points and the antecedent local maximum points of the fitting trigonometric function curve to each pixel as indicators of leaf-shedding events. The heterogeneous seasonal changes in the NDVI were well detected. Deciduous forests demonstrated drastic and uniform leaf phenology; while evergreen forests did spatially and temporally heterogeneous one during the dry season. It indicated the difficulty in getting information of regional forest seasonality; thus it displayed the utility of remote sensing for phenological investigation. Leaf-flushing was detected during the dry season both in evergreen forests and in deciduous forests. It suggested that leaf phenology was not completely governed by drought stress. Leaf-shedding and leaf flushing in twice within a single evergreen pixel was the most striking findings in this study. In summary, remote sensing technology was of great service to getting phenological information that was considerably different between evergreen forests and deciduous forests distributed in the seasonal tropical zones.

Seasonal fluctuation of groundwater in an evergreen forest, central Cambodia : experiments and two-dimensional numerical analysis

This study of a water cycle was conducted in an evergreen forest located in the Mekong River Basin in central Cambodia. At the observation site, we measured the dynamics of the spatial distribution of groundwater levels. The groundwater movement was analyzed two-dimensionally using boundary conditions and parameters that had been observed in the field. The climate in the research area is dominated by two seasons, which occur annually: a rainy and a dry season. The groundwater levels are generally high during the rainy season and low during the dry season. Groundwater levels were measured along a stream, which flowed through the study site. The streambed was visible at the head of the stream in January. At the next downriver well point, the streambed appeared in March. Finally, it became visible at all well points in April, meaning that surface runoff had disappeared temporarily and instead flowed underground during the ensuing dry period. Groundwater levels of the studied lateral flow perpendicular to the stream that seeped and infiltrated into the stream were 1.2–2.5xa0m deep (in April), which was the lowest level recorded for the year. During that period, the depth of the groundwater of the studied lateral flow fell by as much as 56xa0mm per month. In addition, the lateral flow groundwater infiltrated into groundwater of the stream during that period. The groundwater level fluctuation was estimated based on a two-dimensional analysis of lateral flow perpendicular to the stream using a numerical simulation model with soil physical parameters and observed boundary conditions. The observations of ground water fluctuations were well reproduced. Deep seepage of groundwater was estimated using a uniform boundary condition that allowed efflux through the bottom, estimated as being approximately 30xa0mm per year. The simulated deep seepage rate was considered plausible considering other hydrological components such as soil water storage fluctuation.