Effects of Forest Roads on Runoff Initiation in Low‐Order Ephemeral Streams

Abstract

Understanding hydrologic connectivity is essential for managing ephemeral headwater streams where upstream land use influences downstream aquatic habitats. This study relies on a field‐based approach to evaluate how precipitation and roads affect runoff generation in low‐order ephemeral streams of the U.S. Virgin Islands. Logistic regression analyses show that runoff delivery from unroaded catchments agrees with a water storage conceptual model typical for subsurface storm and saturation overland flow‐dominated settings. Without roads, runoff occurs only about 4 times per year in response to 10 and 78 mm of storm rainfall, depending on antecedent precipitation. In contrast, maximum 15‐min rainfall intensities are a better predictor of runoff generation on unpaved roads than are total rainfall and antecedent conditions. Intensities surpassing ~10 mm/hr lead to road runoff, and this occurs about 40 times per year. Road‐influenced streams represent an intermediate setting for which runoff generation depends on storm and antecedent rainfall, as well as the road surface area captured by drains and flow path distance. In our focus area, roads can provoke streams to deliver runoff to coral bearing waters 10 to 13 times every year as a response to 9.3‐ to 50‐mm storms, depending on antecedent rainfall and road drain characteristics. These results highlight the sensitivity of road connectivity to specific road drain characteristics and display the potential for connectivity as a guiding watershed restoration principle.

Non‐linear quickflow response as indicators of runoff generation mechanisms

“…Runoff processes also have high spatiotemporal variability ranging several orders of magnitude from seconds to years and from soil pores to hillslopes. This process complexity is partially explained by catchment properties like soil type (e.g., Wilson et al, 2017), geologic substrate (e.g., Fu, Chen, Jiang, & Dong, 2013), land use (e.g., Ramos-Scharrón & LaFevor, 2018), and climate; however, bottom-up hydrologic prediction using mechanistic models remains difficult without the calibration of several parameters to achieve flow fidelity (Hrachowitz et al, 2013). Alternatively, top-down estimates of runoff may be empirically derived from simpler relationships between hydrologic fluxes and catchment storage of water (Wittenberg & Sivapalan, 1999).…”

Assessing Effects of Sediment Delivery to Coral Reefs: A Caribbean Watershed Perspective

“…Unpaved roads stand out as sediment contributors due to the combination of: (a) erosion rates that are up to four orders of magnitude greater than natural rates (Ramos-Scharrón and MacDonald, 2005;McLaughlin, 2019); (b) the relatively minor contribution from other sediment sources due to a low degree of development and lack of widespread agricultural activity; and (c) the absence of widespread landsliding (MacDonald et al, 1997). Runoff and sediment delivery in these ephemeral-stream settings naturally occur only a handful of times per year (Larson et al, 2015), but unpaved roads can potentially induce a tenfold increase in the magnitude and frequency of sediment delivery to coastal waters (Ramos-Scharrón and LaFevor, 2018). Suspended sediment concentrations of runoff discharged from roaded watersheds are in the 100s to 1,000 mg L −1 range while those from undisturbed watersheds are only in the 10s to less than 100 mg L −1 (Ramos-Scharrón, 2004).…”

Effects of Hillslope Trenching on Surface Water Infiltration in Subalpine Forested Catchments

“…In the MNPA, unpaved roads have developed in part to facilitate access to trenches by workers tasked with cleaning trench litter and accumulated sediments. Unpaved roads in forests often lead to lower soil infiltration capacity, increased runoff, and enhanced surface connectivity between runoff source and depositional areas [67,68]. Additional research is needed to assess the potential for these and other environmental impacts (direct and indirect) to result from trenching in the MNPA.…”