Mass Balances of Nitrogen and Phosphorus in the Paddy Field District Implementing Cyclic Irrigation

Abstract

Mass balances of nitrogen and phosphorus in the conventional irrigation (Gyaku-sui irrigation; GI) system and the cyclic irrigation (CI) system were investigated in paddy field district close to Lake Biwa, where the CI was mainly run during the period from the puddling season to the start of the mid-summer drainage and the GI was run after the mid-summer drainage. As a result, it was confirmed that the CI system reduced outflow of surplus irrigation water with nitrogen and phosphorus. In the CI period, from the end of April to late June, there was less effluent water from the district even though about 80% of the irrigation water was the surplus water. From the comparison between mass balances in the CI system and GI system, it was shown that the CI system could reduce effluents of TN and TP by 120g ha^-1 d^-1 and 15g ha^-1 d^-1 respectively.