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Study on the role of Watershed classes in water flow and its characteristics of Ping-Wang river Basin, twelve representative subwatersheds were selected, they are Mae Rim, Mae Klang, Mae Jan, Mae Ngat, Jae Hom, Mae Kuang, Mae Khan, Mae Taeag, Mae Li, Mae Chaem, Mae Wang and Mae Ping. They were also classified into 4 area-size groups from small to large, and each group is consisted of three subwatersheds. Ten Year of historical data of runoff was recorded during 1973 to 1982 by Royal Irrigation Department (RIC). The percentage of existion forest area obtained from LANDSAT - images were analyzed by regression analysis, scatterplot and correlation and correlation matrix.
The results revealed that the influence of watershed classes was correlated with classes I and II. The more percentages of watershed classes I and II would cause the higher amount of discharge into the stream and also regulate flow all year round, especially forest areas more than 70 percents.
The result of research showed that the size of watershed area influenced the amount of runoff in the stream. The information obtained from statistical analysis found that the bigger size of watershed areas would increase the amount of runoff water in the stream.
Whenever the deforestation increased, the peak flow found high concentration in the early period and less releasing water into stream in the summer. This result indicated for all subwatersheds.
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