Drainage-Process Analyses for Agricultural Non-Point-Source Pollution from Irrigated Paddy Systems

Abstract

The aim of this study was to investigate the effects of drainage processes on non-point-source pollution in a complex irrigation and drainage system. Field experiments were conducted to measure quality and quantity of leakage and irrigation-return water in field drains, lateral drains, and the main drain during paddy-growing periods for 2 years. Water-flow and chemical-transport processes from paddy fields to field drains were simulated numerically. A modified Muskingum method was proposed to calculate the processes of water flow and chemical transport in lateral and main drains. Results showed that the Muskingum method was applied successfully to calculate the drainage and chemical-transport processes, as well as to quantify the effect of drain storage on the processes. Results from the field experiments and simulations indicated that the fate of ammonium (NH4+), nitrate (NO3−), and chemical oxygen demand (COD) in the system was primarily controlled by the drainage processes. Mass of NH4+ discharged from paddy fields to drains was mainly through subsurface flow (i.e., the leakage from paddy fields to field drains) and mass of NO3− mainly through deep leakage (i.e., the leakage from paddy fields to groundwater and finally to drains). The COD transportation was through irrigation-return water, subsurface flow, and deep leakage. Eighty-one percent of the discharged NH4+ mass by leakage and 64% of the discharged NO3− mass were through subsurface flow and deep leakage, respectively. Approximately 15% of the applied nitrogen in the paddy field was lost through the drainage processes. Within the total discharged nitrogen in the main drain, 38% and 62% were from subsurface flow and deep leakage, respectively. The methods and results from this study should be useful to characterize non-point-source pollution in paddy-irrigation district.