タイ北部の丘陵性常緑林における4年間の樹冠遮断量とその年々変動

Rainfall, throughfall, and stemflow at Kog-Ma watershed, which is situated in a hill evergreen forest in northern Thailand, were examined for 4 years 1) to quantify annual throughfall and stemflow, 2) to quantify confident annual interception for four years, 3) to evaluate possible measurement errors of the presented interception, and 4) to understand inter-annual variation in interception. The annual rainfall was variable at the Kog-Ma watershed during the study period, ranging from 1641.9 to 2481.2 mm. The annual rainfall at the two neighboring sites differed by only 1-2%. The difference could be accounted by the altitudinal increase in annual rainfall, which was presented by a previous study at Kog-Ma watershed. On the other hand, the observationbased rainfall at two sites differed by more than 10% in half of all observations. These results indicated that spatial distribution of rainfall in the Kog-Ma watershed caused the difference in observation-based rainfall between the two sites, even though the two sites are only 200 m apart. Stemflow observations for 8 trees presented the relationship between tree size and stemflow coefficients, which were determined by regressing stemflow volume vs. rainfall. An attempt to estimate the total stemflow in the study site showed that 1.5% of annual rainfall, or 25-40 mm per year, was partitioned as stemflow at the Kog-Ma watershed. The estimation error suggests that the possible maximum and minimum stemflow was 3.0 and 0.5% of annual rainfall, respectively. Throughfall observation using 30 collectors showed that 1857.0, 1641.9, 1794.5, and 2481.2mm of rainfall was partitioned as throughfall at the Kog-Ma watershed in 1999, 2000, 2001 and 2002, respectively. These throghfall amounted to 86-91% of the annual rainfall. In addition, it was found that of observation-based throughfall occasionally exceeded rainfall every year. These extra throughfall amounts were caused by the occasional underestimations of observation-based rainfall just above the interception study site and by the additional precipitation, which was produced by fog occurrence, to the forest canopy in the Kog-Ma watershed. Annual interception in 1999, 2000, 2001, and 2002 at the Kog-Ma watershed, was 199.3，198.3，141.9, and 187.4 mm, or 10.7, 12.1, 7.9, and 7.6% of the annual rainfall, respectively. The relationship between annual rainfall and annual interception could not be expressed as a simple linear relationship at the Kog-Ma watershed. Moreover, a consideration of measurement error in annual rainfall just above the interception study site gave the possible range of annual interception at the Kog-Ma watershed. These ranges were 185.8 to 199.8, 141.9 to 184.4, and 187.4 to 210.1 mm for 2000, 2001, and 2002, respectively. These ranges of rainfall interception were confident enough to be applied to the parameterization of the model, which describes the evapotranspiration at the Kog-Ma forest. We assumed that only rainfall contributed to the total precipitation in the forest canopy, although fog precipitation also presumably occurred. That is, underestimations of annual interception could occur if fog precipitation in the Kog-Ma watershed was remarkably large. The significance of analyzing the rainfall interception with respect to fog occurrence was confirmed by this study. Comparison of interception rates between natural lowland tropical forests in Southeast Asia except Thailand and forests in Thailand showed remarkable contrasts. The interception rates at the natural lowland tropical forests in Southeast Asia normally ranged from 10 to 20%. However, the lowland forests in Thailand showed higher rainfall interception rate than 20%, while montane forests in Thailand indicated lower rate than 10%.タイの丘陵性熱帯季節林における森林フラックスサイト（コグマ試験地）における年間樹冠遮断量，その測定誤差，およびその年々変動を明らかにすることを目的として，４年間にわたって降水量，樹幹流下量，および樹冠通過雨量を観測し