**2. Study Area**

The study area comprised two small watersheds, namely Harulin (45◦0'22"–45◦8'52"N, 80◦53'15"–80◦59'17"E) and Sailimu (44◦47'10"–44◦49'2"N, 80◦9'15"–81◦11'26"E), located in the Bortala River Basin of northwestern China (Figure 1). The two watersheds are in the upstream mountainous area of the Bortala River, separately covering an area of 47.9 km<sup>2</sup> (Harulin watershed) and 4.0 km<sup>2</sup> (Sailimu watershed). The climate in Bortala River Basin is a typical temperate continental climate with annual precipitation less than 100 mm and annual evaporation more than 1600 mm. With an arid climate, the primary source of the water system in the basin is seasonal ice and snow meltwater from the top of the mountain and the surface runo ff formed by heavy precipitation in summer [32]. Therefore, many streams in the basin are usually ephemeral, and the runo ff from April to August often accounts for more than 50% of the annual runo ff. Culverts with flood watermarks were only found in the main river channels of Harulin and Sailimu watersheds. Thus, the maximum flood peak flow of ephemeral rivers based on the water level can be calculated in the two watersheds. The main river channels in the two watersheds are separately referred to as river channel H and river channel S.

**Figure 1.** (**a**) Location of Bortala River Basin in China, (**b**) topography and water systems of Bortala River Basin, (**c**) Harulin watershed, and (**d**) Sailimu watershed.
