Fens as Whole-Ecosystem Gauges of Groundwater Recharge Under Climate Change

Over the past century, mean air temperature has increased approximately 1 C in California. Since the 1950s, there has been an earlier onset of snowmelt and reduced snowpack (measured as snow water equivalent) in California as well as in much of the western United States. Because the snowpack is the main source of groundwater recharge in the mountainous west, reduced snowpack could result in decreased groundwater recharge through time. This could have important ramifications because groundwater recharge maintains groundwater springs, soil moisture, river baseflows and cool water temperatures. Reductions in groundwater recharge could not only impact water availability for human populations, but could also threaten long-term viability of ecosystems reliant on groundwater flows. Groundwater-fed peatlands called fens are potentially ideal ecosystems for studying changes in groundwater recharge, because they are groundwater discharge sites that rely upon corresponding recharge sites for their sustenance. When the amount of groundwater flow to a fen is reduced, the elevation of the water table decreases leading to desiccation, compaction and increased microbial oxidation of the organic soil. In cases where groundwater flow is reduced over an extended period of time, conversion of fen into wet or mesic meadow or even pine forest can occur resulting in contraction of fen boundaries. The purpose of this study is to evaluate the usefulness of fens as whole-ecosystem gauges of groundwater recharge. We studied 7 fens distributed throughout the Sierra Nevada by tracking their areas over 70+ years with aerial photography. All photos were geo-registered using the 2005 National Agricultural Inventory Program orthophotography as the base. Images were projected to UTM zone 10, NAD 83 using ArcGIS 10.0. Fen vegetation was identified in the aerial photos predominantly by (1) dark brownish-green coloring (or various shades of gray and black in black and white imagery) and (2) mottling of colors or shades and clustering of vegetation (which signifies a distinct moss canopy with overlying clumped sedge vegetation). Vegetation that was transitional between fen and wet meadow was classified as fen for the purposes of this project. Fen areas were delineated using ArcMap 10 for each site through the period of photographic record. Preliminary results indicate that five fens have decreased between approximately 10-17% in delineated area through time. The two other fen sites have had little change. Climate data were derived for each fen's watershed using a distributed-parameter water-balance model (the Basin Characterization Model (BCM)) with a horizontal resolution of 270 m. Decreases in delineated fen area occurred only at sites with steady increases through time in mean annual minimum temperatures (mean of monthly minimums) as estimated by the BCM. The results of this study suggest that fens are sensitive gauges for studying reductions in groundwater recharge due to climate change.