Law of nitrate transfer and loss in purple sloping farmland and its numerical simulation

Abstract: The nitrate transfer and loss plays a critical role in groundwater contamination. Specially, the purple soil sloping field accounts for large areas of farmlands in China and the nitrate transport and loss in purple soils causes serious pollution towards waterbodies. To research the transfer and loss of nitrate in sloping field of purple soil, we conducted laboratory experiments using soil tanks and artificial rainfall device to study nitrate loss features by surface flow (SF) and subsurface flow (SSF) subjected to various precipitation intensities and slope gradients. In this study, 3 precipitation intensities (0.4, 1.0, 1.8 mm/min) coupled with 3 slope gradients (5°, 15°, 20°) were used, and totally 9 treatments were conducted with 2 repetition. Besides, numerical modelling approach was also applied to investigate the nitrate transfer and loss characteristics of purple soil in sloping field. The modified effective mixing depth model and convective-dispersion equation were applied in simulations of nitrate loss via SF and SSF, respectively, with the effective mixing model was modified by a time-increasing effective mixing depth and convective-dispersion equation was solved by HYDRUS-2D software. The results showed: 1) exponential decrease between nitrate concentration and time through SF and linear increment through SSF. 2) The loss concentration of nitrate in SSF was 19-72 folds more than that in SF, and the nitrate loss cumulative loss loads through SF and SSF both presented linear increments with time. Additionally, the variabilities of measured nitrate concentration in SSF were much larger than that in SF. 3) The nitrate loss load presented linear increment over time subjected to all treatments and the proportion of nitrate loss load in SF increased with increasing precipitation intensities. As a result, the loss load of nitrate was mainly through SF in response to large precipitation intensity, but nitrate loss was mainly through SSF subjected to low precipitation intensity. In detail, the proportion of nitrate loss through SF increased from 17.3% to 66.0% as response to increasing rainfall intensity from 0.4 to 1.8 mm/min. 4) The precipitation intensity was a very influential factor for nitrate nitrogen loss, while the impact of slope gradient on nitrate loss showed no consistent pattern. 5) The linear regressions between model prediction results and experimental data and evaluation index of accuracy for simulation results both revealed good agreements for nitrate transfer and loss through SF and SSF, respectively. The mean absolute error (MAE), root mean square error (RMSE), coefficient of Nash-Suttcliffe ENS and R2 reached 0.872 mg/L, 1.009 mg/L, 0.590 and 0.826 7, respectively, for nitrate loss via SF. Similarly, the MAE, RMSE, ENS and R2 of prediction for subsurface nitrate loss reached 9.889 mg/L, 13.084 mg/L, 0.792 and 0.842 6, respectively. This study provided better understanding for nitrate transfer and loss mechanism of purple soil in sloping farmland.