Simulation of nitrogen transport and transformation processes in a Tibetan Plateau irrigation district

The objective of this study was to investigate the effects of hydrology processes on the nitrogen transport and transformation in a Tibetan Plateau irrigation district. Field experiments were conducted to measure the quality and quantity of leakage within the soil and loose rock medium and the surface drainage water in the Danniang irrigation district, Nyingchi, Tibet during the hulless barley growing periods in 2014 and 2015. The lateral seepage flux and the NH₄⁺-N and NO₃^--N concentrations in the soil and loose rock medium and at the outlet of the drainage canal were monitored. A procedure using a stepwise method was proposed to describe the flow and transport processes in the loose rock, and a lump first-order kinetics method was used to describe the process of nitrogen transformation during the seepage and drainage processes. The deep seepage accounted for 26.2% of the total rainfall and 40.9% of the total irrigation. The total mass of NH₄⁺-N and NO₃^--N discharged into the drainage canal accounted for 20.2% and 25.1% of the total mass of NH₄⁺-N and the total mass of NO₃^--N leached from the soil into the loose rock, respectively. The mean values for system errors, Nash-Sutcliffe efficiency, the relative root mean square error, and the fractional gross error between the simulated and monitored flow rates and nitrogen concentrations at the outlet of the drainage canal were 0.043, 0.694, 0.081 and 0.242, respectively, indicating that the developed method was applied successfully to simulate nitrogen transport and transformation in the seepage and surface drainage processes.