Simulating Hydrologic Effects of Raised Roads within a Low-Relief Watershed

Abstract

Raised roads and their appurtenances (i.e., ditches and culverts) can considerably modify the natural hydrologic processes (e.g., flow direction and runoff spatial distribution) of low-relief watersheds. However, these effects are not well understood and are poorly documented in literature. Also, there is a serious lack of effective approach that can be used to describe these effects in common simulation models, including the Soil and Water Assessment Tool (SWAT). As a result, these effects have been rarely taken into account by the existing model applications. The results from those applications are likely to be misleading for watershed management decisions. The objectives of this study were to (1) develop an approach that enables the incorporation of the aforementioned effects into hydrologic models (e.g., SWAT), and (2) evaluate whether and how a model’s simulation performance can be improved by taking these effects into account. The evaluation was conducted for the low-relief Wild Rice River watershed located in northwestern Minnesota by comparing the SWAT-NoRoad model, which is based on a “conventional flow direction map” derived using the deterministic eight-neighbor (D8) algorithm, with the SWAT-Road model, which is based on a “corrected flow direction map” generated by using the approach developed in this study. The corrected map provided more realistic flow directions for the regions adjacent to the raised roads than the conventional map. Compared with the SWAT-NoRoad model, the SWAT-Road model had a compatible performance for a calibration period but was more robust and did a better job in reproducing the observed streamflows for a validation period at two U.S. Geological Survey gauging stations, as indicated by the larger values for Nash-Sutcliffe (