Evaluation of nutrient loads from a mountain forest including storm runoff loads

Water quality and flow rates at a weir installed on the end of Aburahi-S Experimental Watershed (3.34 ha) were measured once a week from 2001 to 2003 and in appropriate intervals from 30 min to 6 h during five storm runoff events caused by each rainfall from 8 mm to 417 mm. The average annual loads of total nitrogen (TN) and total phosphorus (TP) were calculated to be 19.0 and 0.339 kg ha⁻¹ y⁻¹ from the periodical data by using the integration interval-loads method (ILM), which did not properly account for storm runoff loads. Three types of L(Q) equations (L=aQ^b) were derived from correlations between loading rates L and flow rates Q obtained from the periodic observation and from storm runoff observation. L(Q) equation method (LQM), which was derived from the storm runoff observation and allowed for the hysteresis of discharge of materials, gave 9.68 and 0.159 kg ha⁻¹ y⁻¹, respectively, by substitution of the sequential hourly data of flow rates. L(R) equation (L=c(R−r)^d) was derived from the correlations between the loads and the effective rainfall depth (R−r) measured during the storm runoff events, and L(R) equation method (LRM) calculated 9.83±1.68 and 0.175±0.0761 kg ha⁻¹ y⁻¹, respectively, by using the rainfall data for the past 16 years. The atmospheric input-fluxes of TN and TP were 16.5 and 0.791 kg ha⁻¹ y⁻¹.