Effect of Soil Conservation System as Hillside Ditch with Mixed Cropping among Banana and Arabica sp. upon Hydrologic Behavior at Ban Sado Phong Watershed, Khao Ko Sub-District   Somchai Inthasothi , Sompong Limsakul , Chukit Aiemsaart , Pinit Hutawatana and Jurai Thongmark   ABSTRACT         Study the effect of conservation practices in agricultural land on upland watershed was conducted during the year 1987 to 1993 at  Khao-Ko sub district, Phetchabun province. The area has elevation ranging between 600 to 700 m. of mean sea level. Soil properties was in group soil no. 29 and 62. Three mini watersheds were selected for different conservation practices which are buffer strip, contouring and hillside ditch. The results were concluded that annual streamflow and subsurface flow were correlated with annual rainfall distribution. The efficiency of these three systems of soil conservation practices are not significant in controling soil erosion which not only the magnitude of soil loss are under the tolerable limit but also the streamflow quantity are equal in all watersheds. The intensive exploitation of agricultural land has caused decline not only annual streamflow but also subsurface flow. The rate of subsurface flow has 53 % of total flow in the fifth year. But contouring conservation system can improve subsurface flow more to than 90 % of total flow because of higher infiltration rate. Its amount has however declined after construction of contouring in 3.0 % per year, and has become to 82 % of total flow in 5 years. Its has also found that soil erosion was increased with time after constructing contour which caused the rate of about 4 times of begining year in 5 years. The changes of land use from field crop to coffee orchards under soil conservation system as hillside ditch caused higher annual streamflow and increase subsurface flow. Each conservation method without soil improvement program can not prevent the decreasing of soil fertility.         Keywords : Soil and water conservation, Water yield, Subsurface flow