Effects of closed roads, traffic, and road decommissioning on infiltration and sediment production: A comparative study using rainfall simulations

- Water repellency limited forest infiltration rates, but sediment production was low.
- Closed roads had equal infiltration but lower sediment production than trafficked roads.
- 80 passes of an ATV increased available fine sediment and tripled sediment production.
- Ripping is not a very effective decommissioning treatment at the plot scale.
- Mulching after ripping greatly increased infiltration and reduced sediment production.

Road closures and road decommissioning are increasingly being used to reduce runoff and sediment production from unpaved roads, but few studies have quantitatively assessed the effectiveness of these treatments. This study used rainfall simulations to: 1) quantify the differences in infiltration and sediment production among five treatments: undisturbed forest, closed roads, closed roads exposed to all-terrain vehicle (ATV) traffic, and two decommissioning treatments (ripping only, and ripping plus wood-strand mulch); and 2) quantify the effects of key site variables on infiltration and sediment production. Four replicate rainfall simulations were conducted for each treatment in northcentral Colorado, with 44 mm h−1 of rainfall being applied to 1 m2 bounded plots for 45 min. The mean infiltration rate for the last 5 min (“infiltration capacity”) for the forest was 28 mm h−1 and highly variable, while the closed roads with and without traffic had nearly identical mean values of only 5 and 4 mm h−1, respectively. Ripping only increased the mean infiltration capacity to 9 mm h−1, while adding mulch more than doubled this to 20 mm h−1. Mean sediment production from the forested plots was only 3 g m−2 as compared to 43 g m−2 from the closed roads with no traffic. Eighty passes of an ATV tripled the mean sediment production compared to the closed roads with no traffic. The mean sediment production for the ripping treatment was 72 g m−2 or 67% higher than the mean value from the closed roads, while adding mulch decreased the mean sediment production to just 16 g m−2. These results first show the importance of roads and even small amounts of traffic for increasing plot-scale runoff and sediment production, and second that ripping plus mulching is a more effective road decommissioning treatment than just ripping. The results provide important guidance for future road decommissioning efforts.