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The study on influences of headwater zones and deforestation on streamflow of the
peneplain river basins of Mae Wang and Mae Yom are classified into three different sizes
of watershed area. (1) The small watershed area (less than 600 sq.km) consists of the Mae
Ngao, Nam Ngim and Nam Pi subriver basins. (2) The medium watershed area (600-1,500 sq.km)
consists of Upper Mae Wang, Mae Chang and Mae Tui subriver basins. (3) The large watershed
area (more than 1,500 sq.km) consists of Mae Wang, Upper Yom subriver basins. The
eighteen-year data during 1968 to 1985 are utilized to this study. The data comprise of
streamflow (annual, wet period and dry period) and rainfall recorded by Royal Irrigation
Department and Meteorological Department, existing forest area on selected watershed by
Royal forest Department and area of watershed classification zones (WSC.) obtained by
Department of conservation, Faculty of Forestry, Kasetsart University. Those data are
taken into account for the analysis of this study using multiple regression by defining
the discharge as the independent variable. The results of this study as following :
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The influence of headwater zones and deforestation on streamflow reveals that prevalence
of the headwater zones of WSC 1-WSC 2 on discharge controlling and flow period is clearly
observed. In every size of watershed area through the forest area of WSC 1 - WSC 2 is
decreased by deforestation, nevertheless both of them influence on streamflow, but
influential degree becomes less. In addition, the WSC 3 indicates the influence with
respect to WSC 1 - WSC 2 but the influence of the WSC 1 - WSC 2 is more obvious than the
influence of WSC 1 - WSC 2 is more obvious than the influence of WSC 3. Except during dry
period, they show corresponding influence. The WSC 4 indicate the influence contrastly on
discharge, that is, the forest area reversely to the discharge. As for the WSC 5, the
influence show unobviously.
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The influence of headwater zone and deforestation on streamflow in the wet period and
dry period reveals the following results. In the wet period, the WSC 1 - WSC 2 with forest
covering influence obviously on the discharge controlling while the other watershed
classes indicate lower influence, but the difference is very close. In the dry period, the
influence of the WSC 1 - WSC 2, the influence trends to decrease while trend of the other
watershed classification zone increased. In opposite direction, especially in the WSC 4,
during the dry period the waterflow flow down the upper headwater zone to the lower area
of river basin.
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Moreover, the results of study shows that the size of watershed area affects the
streamflow quantity. The more area of watershed, the more quantity of streamflow are
found. If the size of watershed becomes smaller, the specific yield of streamflow will be
increased. Simultaneously, the rising and falling limb of hydrograph change more sharply.
From those relationships, the development on the peneplain of Mae Wang and Mae Yom river
basin can be formulated to sustain water yield throughout the year. To approach this
objective, the conservation of headwater zone WSC 1 - WSC 2, and the appropriate
controlling of the forested area on the other watershed classification zone for suitable
condition.
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