2.2.2. Extraction of the Terrace Units and Vegetation Units Watershed

The impact of terrace and vegetation units on water and sediment flow depends upon their size and position in the catchment. The flow direction of the natural hillslope is shown in Figure 4a. When there is a terrace in the hillslope, the connectivity of its original flow pathways is broken, and the whole slope could be divided into three segments: upstream section, terrace section, and downstream section (Figure 4b). Here, we define the upstream section as the watershed of terrace (or runoff contributing area). The area impacted by the terrace units in terms of water and sediment reduction is the total of the terrace and its watershed.

The terrace watershed was extracted by searching upward from the terrace cell based on deterministic-8 (D8) flow direction method [69]. For each cell, once there was a route that water could follow to reach the terrace cells, the cell was considered to belong to the terrace watershed. The extraction of vegetation watershed was the same as that for the terrace watershed.

**Figure 4.** Schematic diagram of flow direction of (**a**) natural hillslope, and (**b**) the hillslope with vegetation and terrace practice. Numbers in the grid are elevations. Orange represents natural hillslope unit. Green represents vegetation unit. Yellow represents terrace unit. Arrows represent flow direction.
