Modeling Hydrologic Responses to Forest Management and Climate Change in Contrasting Watersheds in the Southeastern United States

Abstract

Hydrologic pathways and processes vary greatly from the coastal plain to the mountainous upland across the southeastern United States due to large physiographic and climatic gradients. The coastal plain is generally a groundwater dominated system with a shallow water table, while the mountainous upland is hillslope controlled system. It was hypothesized that these two different regions have different hydrologic responses to forest management and climate change due to different conditions: topography, climate, soil, and vegetation. The hydrologic impacts of climate change and forest management practices are complex and nonlinear, and a model is an advanced tool for addressing such tasks. The objectives of this study were: 1) to evaluate the applicability of a physically-based, distributed hydrologic modeling system - MIKE SHE/MIKE 11 - in the southeastern United States; and 2) to use the MIKE SHE/MIKE 11 modeling system to examine the hydrologic processes and responses to forest management practices and climate change on the coastal plain and the mountainous upland in the southeastern United States.

Four experimental watersheds, three wetlands on the coastal plain and one Appalachian mountainous upland, were selected. The model was first evaluated to determine if it could sufficiently describe the hydrological processes in these diverse watersheds in two contrasting regions. Next, the model was applied to simulate the hydrologic impacts of forest management and climate change at the four study sites, four simulation scenarios per site. These included the base line, clearcut, 2 °C temperature increase, and 10% precipitation decrease scenarios. Water table level and streamflow amount were two responses used to evaluate the forest management and climate change impacts.

This study indicated that forest management and climate change would have potential impacts on the wetland water table, especially during dry periods. The absolute magnitudes of streamflow reduction were larger in a wet year than in a dry year for the two watersheds under both climate change scenarios (2 °C temperature increase and 10% precipitation decrease). In terms of streamflow reduction percentages, the results seemed to suggest that climate change would have larger impacts on the coastal plain than the mountainous upland. However, more field data and research are needed to further test this hypothesis.

This study showed that MIKE SHE over-predicted soil evaporation from harvested lands. Thus, the model may have underestimated the streamflow impacts under the clearcut scenario. A process-based evapotranspiration model is needed to fully describe soil evaporation processes under forest harvest conditions.