*2.1. Design of Rainfall Simulation Experiments*

Rainfall simulation experiments were conducted to study infiltration using spray-nozzle rainfall simulators and soil flumes in the rainfall simulation laboratory of Beijing Normal University, China, in 2015 (Figure 1). Four levels of rainfall intensities (including 30, 60, 90, and 120 mm/h), two soil moistures (8% and 30%, measured by TDR soil moisture sensor (Time domain reflectometry, CAMPBELL TDR200, Campbell Scientific, Inc., U.S.), representing the extremely dry and extremely wet runs), and 7% slope (defined as "intense erosion", according to the grade scale of soil erosion intensity standard in the experimental field) were designed for the laboratory experiments. Five residue coverages (15%, 35%, 55%, 75%, and bare slope as a control), determined by the digital photograph method, were designed to quantify the residue cover effect. Corn stalks were collected to use in the experiments. Each rainfall event was performed in both dry and wet runs and lasted for 1 h under various rainfall intensities. The interval was 24 h between runs. In total, 80 runs were conducted. All treatments were performed twice for reproducibility and precision.

The experimental black soil used was the top 20 cm of surface soil from farmland and classified by light erosion intensity, collected from the Jiusan Soil Conservation Station. The characteristics of the black soil included an organic matter content of 5.0% and a particle size distribution of 32%, 33%, and 35% for sand, silt, and clay, respectively. The soil was filled into the flume with a depth of 0.25 m (2 m length <sup>×</sup> 1 m width <sup>×</sup> 0.35 m depth) and layered with a soil density of 1.07 to 1.25 g/cm3 to model actual field conditions. The upper soil layer was 0.1 m deep and the sublayer was 0.15 m deep, and the contact surface of the two layers was roughened to reduce soil stratification which could affect infiltration [24]. A wooden board was used for the soil surface flatting to reduce the effect of soil roughness.

The precipitation of each rainfall event was controlled by simulators. The runoff was collected into bottles for measurement every 5 min during rainfall, and volumes were recorded after the rain. The infiltration amount was the difference between the rainfall amount and the runoff; evaporation

and residue retention during the rainfall events were not measured. The infiltration data obtained were used to determine the hydrological parameters in the models.

**Figure 1.** Laboratory experimental setup.
