*3.2. The Spatial Variation of Post-Fire Erosion Risk at SDWC*

There was great spatial variation in hillslope erosion rates over the SDWC area. Among the 204 drainage units within SDWC, the post-fire erosion rate (February 2020) ranged from 0.1 to 23.8 Mg ha−<sup>1</sup> month−<sup>1</sup> with a mean value of 4.9 and standard deviation of 4.2 (Mg ha−<sup>1</sup> month<sup>−</sup>1). Figure 6 shows the average monthly erosion risk in January and February 2020 that represents the effect of post-fire storm events on hillslope erosion across the SDWC area. Most of the highest post-fire erosion risk areas appear to be around the Warragamba Dam (Lake Burragorang) including Wild Dog (ID = 193), Lower Kowmung (ID = 99), and Cedar Ck (ID = 33) as shown in the Figure.

Figure 7 further presents the 10 drainage units with the highest erosion rates over the SDWC area after the 2019–2020 wildfires and storm events in February 2020. The erosion rates range from 13.2 Mg ha−<sup>1</sup> month−<sup>1</sup> in Brogers Ck (ID = 24) to 23.8 Mg ha−<sup>1</sup> month−<sup>1</sup> in Wild Dog (ID = 193), much higher than the average annual rate in NSW. Linking erosion risk information with these drainage units (the smallest water management units) helps the water management agency (i.e., WaterNSW) to prioritize the monitoring and mitigation actions.

**Figure 6.** The hillslope erosion in January and February 2020 showing the impacts of fire and storm events on erosion, and the ten high erosion risk sub-catchment (labelled with IDs) across the Sydney drinking water catchment: (**a**) erosion before storm events; (**b**) erosion after storm events.

**Figure 7.** The highest erosion risk drainage units within the Sydney drinking water catchment area after the 2019–2020 wildfires in February 2020.

#### *3.3. The Erosion Risk at Di*ff*erent Fire Severity and Erosivity Scenarios*

Erosion rates estimated at the high (75 percentile) and extreme (95 percentile) rainfall erosivity scenarios were compared with the average rainfall erosivity in 2000–2019 (the hillslope erosion modeling period). The vegetation cover levels before (July–August 2019) and after (January–February 2020) the

2019–2020 wildfires were estimated from satellite (Sentinel-2 and Landsat-8) derived FVC. It clearly shows the impact of fire severity on hillslope erosion, and the high fire severity class (canopy fully affected) has an erosion risk about two to four times higher compared to other classes. The erosion could increase up to 450% under extreme rainfall erosivity condition for severely burnt areas. Table 3 summarized the estimated erosion rates under different fire severity classes and rainfall scenarios. Unlike the relative change (%), the actual erosion rates are not always positively correlated to the fire severity classes because of the impacts of other factors such as rainfall, terrain, and soil (refer Equation (1)).

**Table 3.** Estimated mean erosion rates (Mg ha−<sup>1</sup> yr<sup>−</sup>1) at different fire severity classes and rainfall erosivity scenarios at Sydney drinking water catchment area, and the pre- and post-fire changes.

