To elucidate the turbid filtration function of forest soil, we conducted field experiments in a deciduous broad-leaved stand and in a sugi (
Cryptomeria japonica) stand at Omyojin Experimental Forest, Iwate University. In one experiment, a 1055-kg suspension containing 4739mg/kgkaolin clay was discharged, divided into five, into a testing flume (2.0m long and 0.3m wide) at 10kg/min. Then, we investigated the surface flow rate of the suspension at the lower end of the flume. Two flumes were prepared in each stand and four experiments were conducted at each flume between September and July. Blocking efficiencies (removed kaolin/total kaolin) were 89%–96% in the first experiments but gradually declined as the experiment was repeated. Doubling leaf litter in a flume of the deciduous stand yielded similar results. After the experiments, a lot of kaolin clay was observed around the A
0–A layer boundary and infiltration capacities were lower than those observed in undisturbed forests. However, blocking efficiencies in June increased 1.6%–6.0%, potentially because the filtration function of new leaf litter was improved, and the permeability of forest floor recovered by melted snow. Filtration rate of suspension, which was expressed as an approximate curve of “superficial infiltration capacity” calculated from surface flow rate, slightly higher than the rate explained by percolation through the soil. This difference is due to the filtering effect of the A
0 layer, because the filtration function was composed of the filtration of surface flow by the A
0 layer and the water percolation.
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