Evaluation of Nutrient Loads from a Citrus Orchard in Japan

We estimated four methods of calculating loads of nutrients from a citrus orchard in Japan. River water discharge and nutrient loads were examined every week for 2 years and more intensively at the time of one rainfall event. River water level was recorded automatically at 5-minute intervals. Nutrient concentrations and water discharge data obtained for the load varied with the evaluation methods, because the estimations made during rain events differed. Nutrient loads calculated by the four methods ranged from 2004 kg/year to 4194 kg/year for total nitrogen and from 30.3 kg/year to 44.6 kg/year for total phosphorus from the watershed area of 78.3 ha with citrus orchard and forest. Particulate form nutrients from farmland were loaded by runoff during rainfall events, and nutrient load increased quickly more so with phosphorus than with nitrogen during rainfall events. There is thus a need to quantify the degree of nutrient load from farmland in rainfall events. Continuous observation data will be needed to obtain appropriate values for load from diffuse pollution sources.