Simulation of nitrogen dynamics in lowland polders using a new coupled modelling approach: Insights into management

A new modelling framework, the Polder Hydrology and Nitrogen modelling System (PHNS), was developed to
simulate the nitrogen dynamics and processes in polder systems. PHNS is a mass-balance model that simulates
water and nitrogen dynamics in soil and surface water systems through integrating the WALRUS-paddy, MUSLE,
and INCA models. The model explicitly considers the interactions among surface water, groundwater, and vadose
water, as well as irrigation, pumping, and fertilizer application, which are the key processes controlling the
nitrogen cycle in polders. The sensitivity analysis, calibration, and validation of the developed model were
conducted in a Chinese agricultural polder by using three years of measured hydro-meteorological data. The
calibrated and validated results proved that the model has a good performance with an R2 of 0.748 and a Nash-
Sutcliffe (NS) efficiency coefficient of 0.619 for total nitrogen (TN) concentration during the validation period.
The nitrogen budget results (net export of 40.4 kg/ha/yr) revealed that the polder is a nitrogen source for
downstream freshwater systems. Reducing the amount of fertilizers, retaining crop residues, and restoring
aquatic plants in surface water are effective countermeasures for reducing nitrogen export from polders. This
study provides an efficient modelling tool and useful insights into polder management.