Total phosphorus in water from an adjustment reservoir in Isahaya reclaimed land has regularly exceeded environmental standards. However, sludge from purification plants is known to have the capacity to absorb phosphate. In this study, to promote recycling of sludge and to develop a method for total phosphorus reduction, evaluation of phosphate removal by sludge from a purification plant was performed, in addition to simulation of concentration changes in an adjustment reservoir to which sludge was added. Absorption experiments were conducted using a mixture of phosphate solution and sludge. The coefficient of absorption isotherm and the mass transfer capacity coefficient were measured in relation to the absorption rate. During mixing, water in the reservoir is pumped into the water treatment plant. Phosphate is removed from it by complete mixing with sludge of about1,400-2,000 t/y of sludge. When using the immersion method, sludge is added to the reservoir. Approximately 3,000t/y of sludge is required. However, both methods have shortcomings: a huge absorption tank (e.g. the order of 10⁵m³) must be used for mixing; in the immersion method, sludge remains in the adjustment reservoir and phosphate is not removed from the reservoir. Because the required amount of sludge and scale of treatment plant are large, further investigation is necessary.

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 A method to evaluate and compare nonlinear property of flood mitigation capacity of watersheds is proposed in this report. First, the observed rainfall-outflow data in the watershed are expressed using a runoff model. Then a rain pulse of R (mm) is applied to the model to ascertain the peak discharge Q_max(mm/h). Ratio τ_R (= R/Q_max), which is designated as the peak discharge relaxation time, is the index of flood mitigation capacity. Values of τ₅₀ and τ₁₀₀ for typical watersheds of Japanese rivers are shown. For analyses of them, the literature describes some constructed tank models.

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 To ascertain the relation between the infiltration rate, cover material and hydraulic conductivity of forest soils in Japanese cedar (Cryptomeria japonica) and Hiba arborvitae (Thujopsis dolabrata) plantations, we conducted artificial rainfall experiments using an oscillating nozzle rainfall simulator. The maximum infiltration rate (FIR_max) was higher than that found in previous studies of cypress plantations. Especially in conditions where surface cover materials are less than 1000 g/m², FIR_max tends to become higher than the value of the cypress forest in several magnitudes. FIR_max showed over 100 mm/h, irrespective of the amount of surface cover materials. The result confirmed little or no correlation between FIR_max and surface cover materials in either of the studied tree species. FIR_max were lower than hydraulic conductivity. FIR_max showed no correlation with hydraulic conductivity and fine fraction content. Therefore, the differences of FIR_max in cedar and hiba plantation were not explainable by surface cover materials, hydraulic conductivity, or the fine soil fraction content, which contrasts with results of previous studies of cypress plantations.

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