Biogeochemical response to extreme events at the five USGS WEBB watersheds

Abstract:

The USGS Water, Energy, and Biogeochemical Budgets (WEBB) program has been making intense hydrologic and biogeochemical measurements at five pristine headwater watersheds across the U.S. for 25 years. Here we analyze the biogeochemical response at these sites to extreme events that occurred during this period. We quantify the magnitude and duration of the response, and assess whether extreme events cause lasting change in watershed function. Extreme events across the WEBB climate gradient include hurricanes and drought at wet tropical Luquillo, Puerto Rico; opposite extremes of snowmelt and a rare summer flood at alpine Loch Vale, Colorado; the most extreme drought in the past 75 years at the northern temperate lakes landscape of Trout Lake, Wisconsin; large rain and snowmelt events and deep summer droughts at Sleepers River, Vermont; and drought, extratropical storms, and intense thunderstorms at Panola Mountain, Georgia. Extreme drought at Sleepers River (2001) caused historic low groundwater levels, exposing sulfide minerals to oxidation. As groundwater recovered, stream sulfate concentrations tripled and remained elevated for nearly a year. At Panola Mountain, sulfate inputs are stored in the watershed during drought and flushed out on rewetting. At Luquillo, hurricanes and other large rain storms limit the biogeochemical influence of the watershed and the stream channel becomes a conduit for new rainfall. Even dissolved organic carbon (DOC), after an initial increase, dilutes at the highest flows. At the groundwater-dominated Trout Lake, stream chemistry is fairly resilient to extremes of precipitation and drought. At Loch Vale, drought (2000-2005) caused large increases in solute concentrations. A summer flood in 2013 caused a persistent increase in DOC concentration, but at a much lower level than during the typical spring snowmelt period. The size of the annual snowmelt event at Loch Vale is the major determinant in the amount of acid anion export. In addition to these analyses of individual events, we examine the biogeochemical response to unusually wet and unusually dry years in order to extrapolate to future extremes.