25-YEAR AVERAGE ANNUAL RECHARGE RATES FOR MAINE

New estimates of average annual recharge across the state of Maine are being developed by the U.S. Geological Survey (USGS), using a water-balance modeling approach and the USGS Soil Water Balance Code (SWB) software. SWB is a grid-based program that calculates recharge, runoff, and evapotranspiration on a daily time step. The program uses DayMet (version 3) gridded daily precipitation and temperature data as the primary input and apportions precipitation into rainfall or snow storage, snowmelt, direct runoff, soil moisture storage, delayed runoff, evapotranspiration from the rooting zone and from soil evaporation, and net infiltration to groundwater past the rooting zone (recharge). Gridded soil and land use data are used with adjustable model parameters to vary how the inputs are routed to each component of the water budget across the model domain. Calibration datasets were developed using twelve years of annual direct runoff and baseflow data from 32 unregulated watersheds across the state and 12 years of annual evapotranspiration estimates based on satellite data. The model has been calibrated by adjusting over 200 model input parameters to achieve a best-fit solution for all three components of the water budget—recharge, runoff, and evapotranspiration. Final grids of the 25-year average recharge rate for 1990-2015 and the 25-year minimum and maximum recharge rates are aggregated from daily output values. A data release and report, including a map of model uncertainty, are planned for late 2019.