Drought, exurban expansion, and water yield in southern forest ecosystems

Background/Question/Methods

Relationships among hydrologic processes, climate variability, and land use are especially complex.  In the southern U.S., a rapidly expanding urban environment and increased frequency of severe drought has heightened concerns of threats to sustainable freshwater supplies throughout the region.  To address these concerns and plan and manage for the future, we need a predictive understanding of how hydrologic processes of various land covers respond to climate variability, especially with regards to key parameters such as streamflow and stormflow characteristics.  We use a combination of long-term gauged watershed data and landscape scale modeling to understand and predict interactions among climate variability, land cover, and water yield.  We analyzed long-term climate and streamflow data on treated and control watersheds at the Coweeta Hydrologic Laboratory, located in the southern Appalachians of western, NC.  Like many areas of the southern U.S., recent climatic patterns at Coweeta indicate an increased frequency of severe droughts, with at least three major events occurring in the past two decades. 

Results/Conclusions

Past droughts have substantially changed structural characteristics of southern Appalachian ecosystems, especially Pinus spp. due to southern pine beetle outbreaks, but also the proportion of large Quercus spp.  We hypothesized that changing structural characteristics would have an impact on streamflow characteristics and responses to future climate variability.  Our analyses indicated that land cover type interacts with climatic extremes to differentially affect streamflow, especially coniferous forest cover types.  Regional sensitivity analysis using a hydrologic model corroborated with the site-level observations at Coweeta and elsewhere that the impacts of climate change and/or vegetation cover change on water yield vary across climatic (wet vs dry) and disturbance (vegetated vs urban land) regimes.  Forest management strategies to mitigate the impacts of extreme climate change and urbanization must consider the differential hydrologic responses across a landscape/region and the interactions between climate and forest cover characteristics.