Hydrologic Processes Of Reference Watersheds In US Forest Service Experimental Forests
Wednesday, 16 December 2015
Poster Hall (Moscone South)
Long-term research at small, gauged, paired forested watersheds in the USDA Forest Service, Experimental Forest and Range network (EFR) has contributed substantially to our current understanding of relationships between forests and hydrologic processes. This cross-site comparative assessment provides an overview of factors that influence streamflow dynamics at ten relatively undisturbed reference watersheds in the USFS EFR network across the continental United States, including low-gradient watersheds with wetlands that generally have lower water yields, lower runoff ratios, and higher evapotranspiration than upland-dominated watersheds. We used daily flow duration curves (FDCs) to illustrate and compare streamflow magnitude and distributions from these watersheds located in vastly different eco-hydrologic regions. Monthly average flows were also calculated to characterize variability within and among sites and a dryness index (a ratio of mean annual potential evapotranspiration to precipitation) was used as an indicator of energy limited versus moisture limited watersheds. Results generally indicated that snowmelt-dominated watersheds have lower relative variance in discharge, that larger watersheds have dampened high flows with large baseflows, and that none of the site characteristic parameters considered, except rainfall, had a significant (α = 0.05) relationship with annual streamflow. Despite the differences among site characteristics, widely different sites produced very similar FDCs. While some seasonal flow patterns were observed among sites along the eastern and the western near-coastal areas, rain and snowmelt water flow paths were shown to vary greatly across and within six reference watersheds, potentially affecting the timing and peak of storm runoff. However, using the dryness index, a threshold of 0.50-0.70 was found to separate sites with high or low runoff coefficients (the ratio of streamflow to precipitation). The long term data from USFS EFR reference watersheds have been and will continue to be valuable in developing, calibrating, and validating hydrologic models to address contemporary issues, such as the impacts of climate and land use change on hydrology, biogeochemistry, and water supply throughout the country.