**4. Discussion**

#### *4.1. E*ff*ect of Terraces and Vegetation on Runo*ff *Reduction*

To quantify the reduction effects of vegetation and terracing during runoff and sediment routing process, the absolute reduction in unit area (*AR*) and the reduction rate (*RR*) were calculated. In the case of runoff, for example, we first summed the hourly runoff discharge to obtain the total runoff of each rainfall, and counted the total simulated runoff of each event per case. We then calculated the *AR* of R1 and R2, and the *RR* of R1, R2, and R3, as shown in Table 6. The *AR* and *RR* were calculated as follows:

$$AR\_i = \frac{y - y\_i}{A\_i}, \ (i = 1, 2) \tag{15}$$

$$RR\_i = \frac{y - y\_i}{y} \times 100\% \text{ ( $i = 1, 2, 3$ )}\tag{16}$$

where *y* is total simulated runoff of O1 (m3); and *yi* is total simulated runoff of R1, R2 or R3 (m3), *Ai* is total area of R1 or R2 (km2).


**Table 6.** The efficiency of runoff reduction by vegetation and terraces in Pianguanhe Basin.

Annotation: *RR*<sup>1</sup> and *AR*<sup>1</sup> mean the *RR* and *AR* of vegetation, respectively; *RR*<sup>2</sup> and *AR*<sup>2</sup> mean the *RR* and *AR* of terrace, respectively; *RR*<sup>3</sup> means the *RR* of both vegetation and terrace; Difference denotes *RR*<sup>3</sup> minus the sum of *RR*<sup>1</sup> and *RR*2.

As shown in Table 6, the average *RR* of R1 on runoff was 33.58% in the 1980s. For the three events in the 2010s, the average *RR* of R1 was 51.02%, the average *RR* of R2 was 26.65%, and the average *RR* of R3 was 71.86%. The average *AR* of R1 was 3051.42 m3/km<sup>2</sup> per event, and it was 15809.18 m3/km2 for R2. The results show that the runoff reduction rate of vegetation was significantly higher than that of terraces, as the area of vegetation is seven times larger than that of terraces. However, terraces could reduce more runoff per unit area. Influenced by the revegetation and increase in vegetation coverage, the average *RR* of R1 in recent years increased by 17.44% over that in the 1980s. In general, the effectiveness of runoff reduction was highest under R3. Besides, the sum of *RR*<sup>1</sup> and *RR*<sup>2</sup> is inconsistent with *RR*3. The reason for the latter might be that when the terraces had played the role of water reduction, the water reducing capability of vegetation in the same time zone would be decreased. The assumption that all terraces in the Pianguanhe Basin are of good quality and have the same embankment height may be an oversimplification that overestimated the runoff reduction efficiency of the terraces.
