Reduction of nitrogen, phosphorous and runoff by coordination controlled drainage with basin and ditch in paddy field

Abstract: Heavy rains occur frequently in South China during rice growth stages. The large amount of discharge from paddy field combined with high fertilization has caused serious environmental problems. The present controlled drainage methods pay much attention to holding more water in basins and reducing pollutant from basins, however, how to control runoff from field ditches and non-cultivated areas are often ignored. Coordinated Controlled Drainage (CCD) was proposed in the paper to solve the problem above by making most use of the wetland effects of both paddy field and field ditches that were widely distributed in the farmland in South China. The CCD technique tries to store rainwater in basins as much as possible by using rain-catching and controlled irrigation (RCCI) technique that maintains lower irrigation limit and higher water depth after rain depending on the waterlogging-tolerant and drought-tolerant ability of rice so as to reduce runoff from basins. Furthermore, it also impouned runoff from basins and non-cultivation lands such as roads, ditches and canals by a construction at the outlet of the field ditch. Field experiments were conducted in 2013 to verify the feasibility of the model. Controlled and uncontrolled drainages treatments were designed in rice field. Each controlled field of 4.5 hm2. Three replicates were designed. During the experiment, the drainage from field and trench were collected for determination of total nitrogen and phosphorus (TN and TP). Rice yield was measured. The results showed that at field scale, CCD could reduce evaportanspiration and water consumption by 18.8% and 15.3%, compared with frequent and shallow irrigation technique (FSI) respectively. Irrigation quote, drainage quote and irrigation frequency declined 28%, 60.6% and 4 times while TN and TP loss reduced 58.6% and 58.8%. At field control scale, surface drainage volume, the TN and TP burden from controlled ditch decreased by 55.9%, 59.7% and 66.7%, respectively under CCD without reduction of grain yield compared with uncontrolled ditch combined with FSI. The investigation also found that TP and TN concentrations were high both in basin water and ditch water at early stage after rainfall and decreased rapidly with time, indicting that holding water in basins and ditches at that time could remove TN and TN more efficiently. Runoff from non-cultivated area accounted for 31.3%-38.7% of the total runoff from ditches, indicating it was an important source of pollutant. The results above suggested CCD was an effective method to reduce water application, cost of labor, and pollutant burden in paddy field in South China.