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Runoff processes govern the river hydrograph form, location of return-flow and biogeochemical water quality of tropical forest watersheds. This study reviews the literature on runoff processes from tropical rainforests and applies it to the situation in tropical Southeast Asia. The impact of clay mineralogy on permeability and thence water pathways within the soil, and the role of deep pathways with unconsolidated geological materials (regolith) or permeable rock (solid geology) are emphasised, and a new perceptual model, DELTAmodel, presented. Lastly, the implications of these findings for runoff processes within the Mekong Basin are discussed.

To analyze the relationships between land-use changes and water balance and flow regimes in the Chi River basin, Thailand, we used historical data of annual rainfall and seasonal and annual flow from 1951 to 2003, which corresponded to land-use changes derived from Landsat imagery acquired between 1973 and 2003. We found that during the past 52 years the forested area in the Chi River basin has declined by 20%, whereas agricultural areas, paddy fields, and urban areas have expanded rapidly. Upland agriculture (maize, cassava) fluctuated from more than 36% in 1973 to less than 20% in 2000, with a notable drop in cassava cultivation. In contrast, sugarcane cultivation increased during the past 5 years because of increased market demands, and rice fields were expanded from 20% to 42% in 2000. Although annual rainfall in the Chi River basin has tended to decrease, we found an insignificant relationship between land-use changes, in particular, the depletion of forested areas, and annual rainfall. We also found an insignificant relationship between the water budget component and land-use changes, with a rather small effect on seasonal and annual flows of the basin.

We assessed evapotranspiration in the Mekong River basin with a focus on the distribution of forested areas using geographic information system (GIS) datasets. We developed a new model to estimate evapotranspiration, a major component of the forest water budget. The model calculates transpiration (including forest floor evaporation) and interception loss separately. Transpiration was calculated based on the Priestley-Taylor equation. Interception loss assumed a constant interception ratio. After clarifying distributions of climatic conditions and forested area in the basin, we calculated the evapotranspiration rate distribution. We then identified significant factors to consider in accurate estimation of evapotranspiration by comparing evapotranspiration rates based on the model and those based on the original form of the Priestley-Taylor equation. Consequently, we concluded that the contribution of evergreen and deciduous broadleaf forests in the southern part of the basin is one of the dominant components of evapotranspiration from the whole basin, because those forests are distributed in an area with high evaporative potential and the forests cover a large area. Furthermore, it is essential to evaluate the transpiration control of evergreen broadleaf forests in the lower part of the basin because of decreases in soil moisture during the dry season.

Our previous study revealed that a hill evergreen forest in the Kog-Ma experimental watershed in northern Thailand, which is influenced by Asian monsoon cycles, transpired actively even in the late dry season. In this study, the impact of severe drought on the transpiration of this forest was investigated using data measured at the site over 8 years that showed seasonal and interannual variation in rainfall. To this aim, the impacts of soil drought on sap flow and water potential were examined during severe drought conditions. This site showed large interannual variation in the total amount of annual rainfall and in the length of the dry period. An unusually severe drought occurred in the late dry seasons of 1998 and 2004 as a result of the small amount of annual rainfall and a prolonged dry period coinciding with El Niño. Under the detected severe drought conditions in the late dry season of 2004, noticeable symptoms of water stress were apparent only in the smallest study tree. Decreases in sap flow velocity and water potential caused by soil drought were not apparent in larger trees. Deeper root systems of larger trees may explain the lower impact of severe drought on transpiration in larger trees. Transpiration in this forest could be maintained actively even under unusually severe drought conditions.

Tropical seasonal forests play an important role in global and regional carbon cycling and climates. Annual transpiration and primary productivity in tropical seasonal forests should be affected by the growing season length and physiological controls during the growing season. We investigated the year-to-year variations in the transpiration period as a measure of the growing season length in a teak ( Linn. f.) plantation in northern Thailand using sap flux measurements obtained over a 4-year period and examined the effect of soil drought on transpiration during the mid-growing season. The beginning and end of the transpiration period differed appreciably between years, corresponding to differences in the timing of soil moisture changes. These differences resulted in approximately 60 days interannual variation in the length of the transpiration period during the observation period, indicating that soil moisture changes are a major cause of large interannual variation in the transpiration period. Transpiration control caused by soil drought was sometimes observed during the transpiration period. The results suggest that soil moisture has two potential impacts on annual transpiration at this site; through modification of the length of the transpiration period, and through physiological control during the transpiration period. This regime contrasts with temperate deciduous forests and hill evergreen forests, another typical forest type in Thailand.

The scale dependence of the hydrological characteristics of a river basin was studied using three watersheds with different scales in northern Thailand. The discharge per unit area in the medium-scale watershed (Mae Chaem) had only small interannual changes even though large interannual changes occurred in the rainfall. The discharge per unit area in the small-scale watershed (Mae Tia) was about twice as large as in the medium-scale watershed and had larger interannual changes that were correlated with the interannual changes in rainfall. The long-term trend of discharge per unit area showed no distinct trend in either medium- or small-scale watersheds, whereas there was a distinct decreasing trend of low flow in the small-scale watershed. In the medium-scale watershed, however, this decreasing trend did not appear, suggesting that the land cover change in the uplands may have an influence on the discharge per unit area in the small-scale watershed, but only a minor influence on the discharge per unit area in the medium-scale watershed. The discharge per unit area in the microscale watershed (Huay Kog-Ma) was the largest and had the smallest seasonal change among the three watersheds. Even in the dry season, there was significant water flow in the microscale watershed.

We conducted a year-round observation of meteorological elements using a meteorological observation tower 60 m in height to evaluate evapotranspiration in an evergreen broadleaf forest watershed in central Cambodia. The period of observation was from November 2003 to October 2004. Solar radiation was consistent throughout the year. The integrated values of net radiation and downward and upward shortwave radiation were 5.09, 6.79, and 0.76 GJ m year, respectively. The temperature observed above the forest canopy was lowest and highest in the first and latter half of the dry season, respectively. The mean air temperature was 26.4°C. The saturation deficit was high in the late dry season (>30 hPa) and low during the rainy season (<25 hPa). The evapotranspiration rate was estimated from these observed meteorological parameters using the heat-balance method incorporating the Bowen ratio. The evapotranspiration rate was higher in the dry season than in the rainy season. Seasonal variation in evapotranspiration corresponded to the variation in the saturation deficit above the forest canopy. The amount of year-round evapotranspiration was 1139.7 mm. The water budget calculations from observation data suggested a water loss of 1202.8 mm for the experimental watershed. Thus, the observed evapotranspiration and water loss amounts were similar.

The wind characteristics of speed, direction, and vertical profile were studied to determine the effects of undulations in the local topography and canopy surface on flux observations made from a tower in an evergreen forest in Kompong Thom Province, Cambodia. Three seasonal patterns of wind speeds and directions were identified. The first occurred in December and January and was characterized by northerly monsoons that persisted all day, as well as a diurnal variation in wind speed, with a maximum and minimum around noon and near sunset, respectively. A second pattern, in February, was characterized by southerly to westerly prevailing monsoon winds, along with an easterly mountain wind observed in the early morning. Wind speed was low throughout the day. The third pattern was similar to the second but included brief, strong winds associated with squalls. Thus, regional effects on circulation were limited, and monsoon winds were found to dominate the meteorological system above the evergreen forest of central Cambodia. In the forest, the estimated roughness length and zero plane displacement height averaged 18.3 m and 7.5 m, respectively, and the average canopy height was 27.2 m. The dependence of roughness length and zero plane displacement height on the wind direction was within the standard deviation. Thus, the undulating canopy surface had little effect on the tower flux observations.

We explored diurnal and seasonal variations in stomatal conductance in dry evergreen and dry deciduous forests in Cambodia and examined the stomatal response characteristics at two sites using a Jarvis-type model. Although stomatal conductance had maximum values at 9:00 (0900) or 10:00 (1000) in the morning and decreased continuously during the evening, transpiration showed peak values in the daytime and minimum values in the morning or evening at both sites in correspondence with the vapor pressure deficit. Stomatal conductance decreased in the rainy season to the late dry season; the pattern was clearer in the dry evergreen forest than in the dry deciduous forest. Stomatal conductance and volumetric soil water content had similar seasonal patterns, although these patterns differed between the dry evergreen and dry deciduous forests. The seasonal patterns of stomatal conductance and transpiration were different in the dry evergreen forest in the rainy season as a result of the moist air conditions. Clear differences were observed in maximum stomatal conductance and the function of the vapor pressure deficit between the two sites. In particular, compared to the results of other studies, the two sites showed large differences in their responses to the vapor pressure deficit. The functions of photosynthetically active radiation and the vapor pressure deficit showed wide daily change, suggesting that these factors may greatly impact the diurnal change of stomatal conductance. The vapor pressure deficit and volumetric soil water content also showed large seasonal variations and remarkable differences in function. The vapor pressure deficit had a large influence on stomatal conductance in the early dry season, whereas volumetric soil water content had a large effect in the late dry season.

Changes of soil water conditions in a soil profile were observed and estimated using a one-dimensional vertical soil water movement model for a dry evergreen forest area of Kampong Thom Province, Cambodia. The research site was in a dry evergreen forest where a meteorological observation tower had been established. Soil water matric potentials were measured at 20-, 50-, 100-, 150-, 200-, and 250-cm depths in an observation plot. Groundwater levels were observed at the site. Soil water matric potentials at each observation depth in a soil profile were simulated using a one-dimensional water movement model that was based on Richards’ equation. Results of observations and simulation revealed the following. (1) The site’s water-saturated zone was close to the ground surface during the rainy season. Water conditions in the unsaturated zone, which was above the groundwater level, were influenced strongly by groundwater. The groundwater level was 400 cm deep even in the dry season. The entire soil profile, from the surface to the bottom, never dried completely. (2) At the beginning of the rainy season, at the surface and subsurface depths of 20 cm and 50 cm, respectively, soil matric potentials were increased by rainfall events, which often supply water to the ground surface. Meanwhile, matric potentials at 200 and 250 cm depths were influenced directly by groundwater; they retained high potential values even during the dry season. In the middle zone, at depths of 100 and 150 cm, the soil was quite dried; the minimum matric potentials in the rainy season resembled those of the surface zone. (3) At the beginning of the dry season, matric potentials at 20- and 50-cm depths decreased because of soil water loss by transpiration of trees and evaporation from the ground surface.