Effects of the heights of farmland drainage outlets on nitrogen and phosphorus loss from surface runoff

The Erhai Lake is the second second-largest freshwater resource in the Yunnan Plateau (southwest China). The lake is also a national protected area and drinking water source for Dali residents. The water quality of the Erhai Lake is has gradually aggravated eutrophication, even above an acceptable level in the rainy season, due mainly to global natural conditions and intensified human activities in recent years. Specifically, nitrogen (N) and phosphorus (P) are the main factors causing eutrophication. Furthermore, the rice planting area accounts for about 10% of the total area of Erhai Lake Basin, a typical agricultural basin. The rice planting season is in the peak month of rainfall in the basin, where rainstorm events occur frequently. The loss of nutrients in farmland subjected to rainstorms has been a key environmental factor to determine the water quality of Erhai Lake in the rainy season. Much effort has been dedicated to the loss of nitrogen and phosphorus in the farmland of the Erhai Lake Basin, particularly on fertilization management and rotation mode. Farmland drainage outlets can serve as the channels for terrestrial pollutants to enter rivers, lakes, and other water bodies. However, only a few studies focused on the farmland drainage outlets for pollution prevention and flood control. Taking the height of the drainage outlet in the Erhai Lake Basin as a research object, this study aims to find an effective way to reduce the nitrogen and phosphorus loss from surface runoff in farmland. An artificial rainfall simulation was also adopted. Five drainage outlets were set with different heights, according to the height of the farmland drainage outlet (5-10 cm from the soil surface), and the height of the rice plant in the peak period of rainfall in the study area. The bottom distances of the drainage outlet were 5, 10, 15, 20, and 25 cm from the soil surface. An investigation was finally made to evaluate the control of drainage outlets at different heights on nitrogen and phosphorus losses in farmland runoff. The results showed that: 1) The low drainage outlet of farmland resulted in the increase of nitrate nitrogen and particulate nitrogen concentrations at the early stage of runoff generation. The drainage outlet height of more than 15 cm effectively reduced the concentrations of nitrogen and phosphorus in various forms, all of which be stabilized at a low level. 2) The losses of nitrogen and phosphorus were significantly reduced, with the increase in the height of farmland drainage outlets. A better control was achieved at the height of the farmland drainage outlet increasing from 5 to 15-25 cm. Specifically, the losses of total nitrogen, particulate nitrogen, ammonium nitrogen, and nitrate nitrogen reduced by 85.60%-93.13%, 88.39%-95.77%, 84.59%-91.72%, 63.05%-65.15%, respectively. The losses of total phosphorus and particulate phosphorus decreased by 86.75%-92.66%, 61.64%-94.61%, respectively. Moreover, there was an extremely high reduction of nitrogen and phosphorus losses, when the farmland drainage outlet was set at a height of 15 cm. But there was no significant change over 15 cm. 3) The main form of nutrient loss was the dissolved state in the paddy field. The dissolved nitrogen and phosphorus losses accounted for 52.37%-83.64% and 67.83%-92.29% of the total nitrogen and phosphorus losses, respectively. The inorganic nitrogen loss in runoff was mostly ammonium nitrogen, accounting for 47.85%-80.80% of the total nitrogen loss. Consequently, the drainage outlet at the height of 15-25 cm can be expected to achieve a superior performance for the runoff pollution control in farmland. Anyway, it is strongly recommended to be 15 cm high for the farmland drainage outlet in the Erhai Lake Basin. This finding can provide a significant support to control nutrient loss in farmland, thereby to improve improving the ecological environment.