Nitrogen (N) transfer to runoff contributed to nutrient loss and water pollution. Experiments were conducted to study the ammonia nitrogen (NH₄-N), nitrate nitrogen (NO₃-N) and total nitrogen (TN) transfer from loam soil to runoff in response to various rainfall intensities (RIs) (0.4±0.02, 1.0±0.04, and 1.8±0.11 mm min^-1) and slope gradients (SGs) (5°, 10°, 15° and 20°). A typical mathematical model based on effective mixing depth (h_m) and a refined model which replaced the time-average h_m in this typical model with a time-increasing h_m were both applied to predict N transfer to runoff. These models were verified with experimental data to evaluate the applications in simulations of surface N dynamics. NH₄-N and TN concentrations in overland flow presented large deviations but NO₃-N concentration highly declined from the initiation of runoff and then stabilized with slight deviations. The effective mixing depth deduced from fitted results coincided positively with RI but negatively with SG. The linear regressions between model prediction and experimental results revealed better agreements for NO₃-N (r² = 0.696; Slope = 1.1617) than NH₄-N (r² = 0.2538; slope = 0.7916) and TN (r² = 0.224; slope = 0.6658). The refined model showed improved performance compared with the original model for the NO₃-N (r² = 0.8267; slope = 0.9996; intercept = -0.2675 versus r² = 0.696; slope = 1.1617; intercept = -0.0438).