Groundwater recharge in northern New England: Meteorological drivers and relations with low streamflow

Meteorological drivers of groundwater recharge for spring (February-June), fall (October-January), and recharge-year (October-June) recharge seasons were evaluated for northern New England and upstate New York from 1989 to 2018. Monthly groundwater recharge was computed at 21 observation wells by subtracting the water levels at the end of each month from the level of the previous month; only positive monthly values were used to compute seasonal recharge. Precipitation, temperature, sea-level pressure, 500-mb geopotential heights, and various teleconnection indices were tested as explanatory variables for the interannual variability of recharge using random forest machine learning models. Precipitation within recharge seasons was positively correlated with groundwater recharge for most wells in all seasons. In general, whilst groundwater recharge in the study area was generally highest during the months of March and April, October precipitation was an important month for explaining the interannual groundwater recharge variability. This is likely because the variability in recharge in October may be high or low for given years. Sea-level pressure and 500-mb heights were typically inversely correlated with groundwater recharge during the recharge-year and fall recharge seasons, as higher sea-level pressure and heights are usually associated with clearer skies and less precipitation. The North Atlantic Oscillation, Pacific-North American pattern, and Pacific Decadal Oscillation teleconnections affected groundwater recharge differently by well and season. The influence of groundwater recharge on minimum daily streamflows during the subsequent summer/fall was also analysed. Summer precipitation was the most important explanatory variable for study streams whilst groundwater recharge and summer air temperature were significant variables for a few streams.