EXTREME PRECIPITATION EVENTS AND SUBSURFACE WATER STORAGE DYNAMICS OF GLACIATED LANDSCAPES

Glaciated environments host a wide variety of subsurface aquifer materials that have strong spatial heterogeneity in hydraulic properties. These aquifers fill and drain water to/from storage that are essential elements in maintaining baseflow to streams. The long-term response of these systems to hydrologic change is critical for appropriately understanding impacts from climatic change. To evaluate the response time of stream-aquifer systems to extreme precipitation events we use a long-term isotope dataset from Western Massachusetts with drainage areas ranging from 0.1 to > 800 km². The year of 2011 was the wettest calendar year on record and the months of August and September of 2011 were the wettest consecutive two-month period in the 123-year record. Stable isotopic composition of surface waters of catchments ranging from 1 – 1000 km² show an enrichment due to summertime and Tropical Storm precipitation. Enrichment in potential recharge water is shown to have a significant long-term impact (> 3 hydrologic years) on the isotopic composition of both surface and groundwater. The length of isotopic recession of stream water are also a strong function of watershed area. It is concluded that the stream water isotopes are consistent with a large pulse of water being stored and released from enriched groundwater emplaced during this period of above-average precipitation. This paper is the first to document stream water enrichment at multi-annual time scales due to a single large precipitation event. This highlights the importance of groundwater sources of baseflow to streams and the transient storage and release mechanisms of shallow groundwater storage. Ultimately the results point to the importance of considering hydrological processes of streamflow generation and their role in hydrologic processes beyond traditional unit hydrograph response analysis.