To investigate the impact of farm practices on suspended sediment production, soil physical properties (penetration resistance, basic intake rate and saturated hydraulic conductivity) were measured for sloping farmland and forest of the Nukibetsu River watershed, Hokkaido, Japan. Weirs were installed at upper and lower levels of the forest headwater stream to estimate the filtering and sediment trapping effects of the forest soil. On rainy days, suspended sediment concentrations were measured at the two weirs. Seasonal differences in penetration resistance profiles were observed between a wheat and a fall cropping radish farm. In both farms, hard soil layers occurred at depths of 10 cm and 40 cm. The penetration resistance profiles of the forest were lower than those of the farms, and there was no clear seasonal change in the profiles. The basic intake rate for the wheat farm was 255 mm h^<-1>, and for rows and furrows on the radish farm it was over 1000 mm h^<-1>, vs. 1 mm h^<-1>, respectively. The basic intake rate for the forest was over 1000 mm h^<-1>. Surface flows with suspended sediment often generated on the farms during storms. Field observations indicate that farm practices cause farm soils to have a lower basic intake rate and permeability, resulting in muddy surface flows. Suspended sediment load at the lower weir was generally lower than that at the upper weir. Muddy surface flows were absorbed into the forest soil. The structure of forest soil provides an effective means to decrease the suspended sediment load discharged from farms into streams.
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