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Stable isotopes, such as deuterium (D) and oxygen-18 (O), are widely used in hydrology as environmental tracers because they move with the water itself. Information obtained from stable isotope data can improve our understanding of the processes associated with the source of water and system dynamics and also provide quantitative estimates related to flow dynamics and transport parameters. In this chapter, stable isotope ratios (δD, δO) of rainfall and stream water were studied from 2003 in four forest watersheds in the Kampong Thom Province of Cambodia. The stable isotope ratios of rainfall during the dry season from November to April lined up below the local meteoric water line (LMWL), implying that rainfall during the dry season may be affected by secondary evaporation during its descent. When these data were discarded, the slope and the intercept of the LMWL were 7.95 and 9.11, respectively, and close to those of the global meteoric water line (GMWL). The volumetric weighted means of δD and δ18O values in rainfall were −6.7‰ and −43.9‰, respectively. During the period from January to March, when little rainfall occurred, the δD values of stream water were near the volumetric weighted mean of δD in the rainfall. During the period from April to December, when a considerable amount of rain fell, the temporal variation in δD in rainfall was less evident in stream water. The range of variation in the δD value of stream water differed among the watersheds, which may indicate that the residence time of stream water differs from watershed to watershed.

We measured precipitation, runoff, and several meteorological factors associated with evapotranspiration in evergreen broadleaf forest watersheds in Kampong Thom Province, central Cambodia. All the studied watersheds have flat topography, with and as the primary plant species. The mean tree height in the upper crown layer was 27 m and the maximum tree height was 45 m. Meteorological factors were observed from a 60-m-high meteorological observation tower. The heat budget method, which incorporates the Bowen ratio, was used to calculate the energy balance above the forest canopy. To estimate evapotranspiration, meteorological data were collected during two sampling periods: October 2003, near the end of the rainy season, and March 2004, in the middle of the dry season. Average daily evapotranspiration levels calculated for the late rainy season and middle of the dry season were 4.4 mm/day and 4.9 mm/day, respectively. A continuous simulation model (modified HYCY model) was then applied with the obtained streamflow data for the watersheds. Evapotranspiration calculated using the tower observations was included as a model parameter. The estimated runoff matched observed runoff comparatively well for small watersheds. The model parameters varied in correspondence with the watershed size.

Some regions of the Mekong River basin still have considerable forest resources, but the pressure for exploiting these resources is very high. Changes of forest cover may strongly affect the water circulation of this region. Therefore, it is important to monitor changes of land cover of this region. In this study, we tested an objectoriented classification method to create a land cover classification map in Cambodia. A commercial object-oriented image analysis software package (eCognition) was used in this analysis. In an object-oriented classification method, the success of classification depends largely on the result of image segmentation. In this study, we overcame the difficulty in image segmentation by combining temporal images acquired in the early and late dry season. The overall accuracy was 0.70, and the Khat statistics value was 0.60. Although the accuracy was moderate, the discrimination between evergreen and deciduous forest types was good. However the mixed or the degraded land cover types were still hard to distinguish from each other. Using images taken in different phenological stages made it possible to both segment the images accurately and classify objects appropriately in an object-oriented classification process.

The objective of this study was to produce land cover maps for the period between 1982 and 2000 using the Normalized Differential Vegetation Index (NDVI) data from the National Oceanic and Atmospheric Administration (NOAA) Pathfinder Advanced Very High Resolution Radiometer (AVHRR) 8-km land dataset for monitoring forest cover changes in the Mekong River basin. Time-series analysis, named Local Maximum Fitting with Kalman Filter (LMF-KF), was applied to the NDVI data to remove noise such as clouds and produce cloudfree images at 10-day intervals. Multitemporal metrics such as annual mean, maximum, minimum, standard deviation, and range were calculated using LMF-KF-processed NDVI data. Classification was performed to produce land cover maps based on signatures from the multitemporal metrics of the NDVI time-series data. The GLC2000 land cover database produced by the Joint Research Center of the European Commission was used as training data for the first classification, which is for the year 2000. Then, the results of the first classification were used as training data for the next classification, which is the previous year. Consequently, classification results for the period between 1982 and 2000 were obtained. It was found that the total forested area was stable in the classification images, whereas the proportion of deciduous forest area had increased.

To predict the lifespan of Lam Phra Phloeng Reservoir, in which the capacity has been reduced by sedimentation, both sediment inflow volumes generated from upstream areas of the catchment and deposition in the reservoir were calculated. The rating curve of the relationship between discharge and sediment at station M.145 from 1996 to 2000 obtained using water level discharge and a sediment estimation (LQS) showed that the annual sediment volume resulting from inflow to the Lam Phra Phloeng catchment (820 km) was 0.36553 Mm (million cubic meters). The siltation rate and annual sediment volume in the reservoir were obtained for a series of periods. During the first period, from 1970 to 1983, the annual sediment volume was 2.23 Mm and the erosion rate was 2.72 mm/year/km. This period coincided with a 73.57% decrease in forest area. The second period, from 1983 to 1991, had a lower annual sediment volume of 1.625 Mm and a lower erosion rate of 1.98 mm/year/km. The forest area increased 1.05% during this period. In the recent period, from 1991 to 2000, the annual sediment volume was 0.36553 Mm and the erosion rate was 0.445 mm/year/km. These low rates were associated with a 4.95% increase in forest area. Since the 1960s, the agriculture of Thailand has shifted from subsistence farming to a cash crop culture to develop the social economy. This shift has resulted in the conversion of forests to cultivated lands. As a result, rapid deforestation has occurred, and soil erosion in crop fields has become a serious problem with regard to resource degradation. Soil erosion from crop fields has generally been recognized since the old days.

The Songkhram River is a 420-km-long tributary of the Mekong River and is the last remaining, free-flowing, undammed Mekong tributary in northeast Thailand. This chapter seeks to clarify the framework of competition and harmony in land use of the seasonally flooded community forest on the banks of the Songkhram River. In a study of Thai forest policy, we identified two kinds of policy: a strong policy for excluding illegal farmers from the national forest, and a realistic response to the farmers involving a partial release of national forestland and community forestry. The participation of local people in forest management should be a key factor for solving the land problem in the national forest. The seasonally flooded forest in the Songkhram River Basin grows at the periphery of agricultural land and lies between water resources and agricultural land geographically. While flooded, the land is unsuitable for agriculture, but this prevents deforestation and provides rich natural resources for the local inhabitants.

Accounting for 57% of its entire national area, the forests and forestland of Vietnam cover about 19 million hectares (ha), of which 12.6 million ha is covered by forests (). Because of its diverse climate conditions and complicated topography, Vietnam has a wide range of vegetation and soil types. The forest vegetation includes six major types: (i) evergreen and semideciduous broad-leaved forests, (ii) deciduous forests, (iii) bamboo and palms, (iv) coniferous forests, (v) open broad-leaved forests, and (vi) scrub. Forest soils comprise 14 major soil groups and 31 soil units. The major soil groups include Arenosols, Salic Fluvisols, Thionic Greysols, Acrisols, Ferralsols, and Leptosols. Of those soil groups, Acrisols cover the largest area, followed by Ferralsols and Thionic Fluvisols.

We enumerated all trees 10 cm or more in DBH with respect to DBH, height, and species identity in 29 circular plots of 20-m radius from Kampong Thom, Kratie, and Mondolkiri Provinces, Cambodia. The composition data were analyzed using cluster analysis with group-averaging protocol, and Sorensen’s similarity index based on basal area data and the resulting clusters were also described with respect to height structure and indicator species. We found four main clusters corresponding to traditional qualitative forest types known as evergreen forest, deciduous forest, hill evergreen forest, and swamp forest. The evergreen cluster was further divided into two stand types of dry evergreen forest and two stand types of secondary evergreen forest. The deciduous forest cluster was divided into three stand types of deciduous dipterocarp forest and a mixed deciduous forest. We describe the correspondence between the forest stand types of our study and the many regional names previously used for the different forest types in varying classification systems. Some of the stand types, for example, an evergreen forest overtopped by deciduous dipterocarp () or by a pine (), and a stand on seasonally waterlogged habitat, seemed to be unique in Cambodia. The application of this method and the needs of regional forest mapping are discussed.

Leaf area index (LAI) is a key biophysical variable in most process-based models of forest ecosystems and water cycles. We compared the LAI of two types of tropical seasonal forest in Kampong Thom Province, Cambodia. The two forest types are extremes of crown-cover density, i.e., closed dry evergreen forest (DEF) and open dry deciduous forest (DDF), suggesting marked spatial variation in forest site conditions such as soil moisture. Monthly changes in LAI were estimated indirectly using a plant canopy analyzer and hemispherical photographs. Both methods of LAI estimation showed instrument errors, i.e., low reproducibility in the plant canopy analyzer data and LAI-saturation in hemispherical photograph data; nevertheless, LAI values differed between DEF and DDF. The average LAI from three years of measurements was about 4.6 times higher in DEF than in DDF. DDF exhibited much greater seasonality than DEF. The annual minimum LAI averaged 76% and 84% of the annual maximum LAI for DDF and DEF, respectively. LAI showed high peaks in the rainy season and decreased in the dry season. However, in DEF, LAI decreased twice annually, at the beginning of the dry season and the beginning of the rainy season. Seasonal changes in LAI could be approximated using a third-degree Fourier-series equation.

Open woodland patches scattered in dense evergreen forest are a landscape peculiar to Kampong Thom Province. One of these open woodland patches was studied by setting a belt transect, and floral composition, stand structure, and habitat conditions were examined. Based on a census for trees 10 cm or greater in diameter at breast height (DBH), the forest along the transect was divided into three types that varied with topography. Most of the area was covered by gentle slopes and was dominated by , which displayed 50% canopy openness and poor species richness. The stands were located on rectilinear to convex sloping sites with low clay content that were waterlogged in the rainy season. stands occurred in a small swamp, whereas on the slope was mixed with . The stands were geographically isolated from the coastal location more characteristic of the species. Along the stream or beside the swamp, where no waterlogging occurred even in the rainy season, we found stands. Physical habitat conditions associated with the topography, such as clay content and soil water conditions, enable the three forest types with different physiognomies to coexist at this small spatial scale and may also explain the outpost patches of .