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The study on relationships between streamflow characteristics and proportion of
watershed classes in northern Thailand is divided into 3 different sizes of watershed area
with a total of 28 areas. Among these 28, 11 of are medium (500-1,000 sq.km.) and large
(bigger than 1,000 sq.km.) subwatershed respectively. This study is base on the
fifteen-year data of streamflow and rainfall gathered during 1971 to 1985 by Royal
Irrigation Department and National Energy Authority of Thailand. The percentage of
existing forest area recorded by Royal Forest Department ,and percentage of watershed
classes 1-5 (WSC 1-5) obtained by Department of conservation, Faculty of Forestry,
Kasetsart University are also taken into the analysis study using multiple regression. The
study reveals the following results:
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The relationship between annual yield of streamflow and proportion of watershed classes
of all sizes is directly related to WSC1-WSC2 with little effect from other independent
factors.
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The percentage of WSC1-WSCI2 tends to increase annual yield of small and large
watershed, prevalence of WSC1 on large one was clearly observed.
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The relationship between wet flow and the proportion of watershed classes showed that
wet flow is directly attributed to WSC1-WSC2 in small and large watersheds. In addition,
highly significant relationship of the large watershed and wet flow, was found. Besides,
the Percentage of WSC1-WSC2 gave increase of the wet flow in every-size of watershed, and
increase was clearly related with the amount of rainfall.
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The relationship between dry flow and the proportion of watershed classes indicates that
Dry flow is little direct related to WSC1-WSC2 in small and large watershed ,the
Percentage of WSC 1-WSC2 tended to increase dry flow in every size of watershed.
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In opposition way, setting proportion of WSC1-WSC5 as dependent variables then applying
regression equation; both the annual specific yield and wet flow trends were increase with
respect to WSC1-WSC2 in small and large watershed. Besides, the results indicate that
there is the rapid change of the rising limb of small watershed. And also the peak flow is
higher in the small watershed other than the middle and the large. This is due to the
highest average percentage of WSC1 is contributed within the small watershed. And also the
peak flow is higher in the small watershed other than the middle and the large. This is
due to the highest average percentage of WSC1 is contributed within the small watershed.
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