Abstract: Mixing zone depth is an important parameter in chemical transport from soil into surface runoff, which implies chemical transport extent and loss. It provides the basic data for calibrating the model parameter. An indoor experiment was conducted to study the mixing zone depth during chemical transport from soil to runoff. Different vertical hydraulic gradients were imposed by setting the flow cell to free drainage condition (-5 cm), saturation condition and artesian seepage conditions (5 cm) under simulated rainfall at 3, 6 and 9 cm/h and simulated flow rate to rainfall at 2, 4 and 10 times of simulated rainfall. Results showed that mixing zone depth increased linearly with rainfall intensity. The mixing zone depth increased inversely with runoff flow rate under free drainage condition, but increased linearly under artesian seepage conditions. Thus the mixed zone depth is a multi-directional, complex dynamic chemical transfer quantity.