Riparian buffers can influence water quality in downstream lakes or rivers by buffering non-point source pollution in upstream agricultural fields. With increasing nitrogen (N) pollution in small agricultural watersheds, a major function of riparian buffers is to retain N in the soil. A series of field experiments were conducted to monitor pollutant transport in riparian buffers of small watersheds, while numerical model-based analysis is scarce. In this study, we set up a field experiment to monitor the retention rates of total N in different widths of buffer strips and used a finite element model (HYDRUS 2D/3D) to simulate the total N transport in the riparian buffer of an agricultural non-point source polluted area in the Liaohe River basin. The field experiment retention rates for total N were 19.4%, 26.6%, 29.5%, and 42.9% in 1,3,4, and 6m-wide buffer strips, respectively. Throughout the simulation period, the concentration of total N of the 1mwide buffer strip reached a maximum of 1.27 mg/cm 3 at 30 min, decreasing before leveling off. The concentration of total N about the 3mwide buffer strip consistently increased, with a maximum of 1.05 mg/cm 3 observed at 60 min. Under rainfall infiltration, the buffer strips of different widths showed a retention effect on total N transport, and the optimum effect was simulated in the 6mwide buffer strip. A comparison between measured and simulated data revealed that finite element simulation could simulate N transport in the soil of riparian buffer strips.
The HYDRUS model is an efficient technical means to study the process of complex nitrogen (N) transport in farmland. Nonetheless, spatial variability in soil water and N parameters has been observed at the field scale, leading to uncertainties in the HYDRUS model. In this study, the HYDRUS‐1D model was corrected and verified by the measured data. Generalized likelihood uncertainty estimation (GLUE) was used to estimate the uncertainty and sensitivity of the model parameters. The differences in simulated and measured data were expressed in terms of root mean square error, normalized root mean square error and coefficient of determination, and the model calibration and verification indicated that the result was reliable. The soil saturated water content, dispersion and sorption coefficient were well identified, with values ranging from 0.42 to 0.45 cm3/cm3, 0.26 to 0.29 cm and 0.141 to 0.158 L/mg, respectively. The denitrification rate and saturated hydraulic conductivity greatly influenced the uncertainty in the model results. Considering the uncertainty of the parameters, the predicted leaching of N accumulation from 0 to 100 cm depth ranged from 9.5 to 12.7 kg/ha. The HYDRUS‐1D model can simulate water and N transport in paddy fields. The prediction interval can be narrowed by considering the uncertainty of the model parameters.
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