A DISTRIBUTED INTEGRATED HYDROLOGIC MODEL OF THE CHESAPEAKE BAY WATERSHED

We applied ParFlow.CLM to the 400,000 sq km area encompassing the Chesapeake Bay Watershed to quantify groundwater stores, fluxes and flowpath lengths. We evaluated the effects of model spatial resolution using 1-, 2-, and 4- km grid sizes and found that integration of both groundwater and land-surface processes led to a transient sensitivity of model outputs to grid resolution, with energy fluxes and evapotranspiration dominating grid-size effects during dry conditions and pressure gradients dominating those effects during wet conditions. Hydrogeologic properties from literature and other sources were assembled and processed to derive a conceptual hydrogeologic model consisting of 29 hydrofacies and a three-dimensional hydraulic conductivity field. The model was run at an hourly time step for the years 2004 and 2005. The modeled terrestrial water storage changes compared well with GRACE satellite data. Model results showed the large contribution of subsurface storage changes (90%) to terrestrial water storage changes in the region. The relationship between storage and streamflow was investigated and both threshold-based and hysteretic relationships were found at the scale of the Chesapeake Bay Watershed and within each of the five modeled physiographic provinces. Evaluation of groundwater fluxes within main rivers in the watershed showed a marked spatial variability with a strong effect of topography and hydrogeology on the magnitude of groundwater fluxes. Model outputs were used to determine groundwater flow paths within selected basins in the watershed and demonstrated the high variability of flow path lengths across physiographic provinces.