Toward Understanding the Role of Ground Water in Hydroclimate Using a Single Column Climate Model

To better understand groundwater dynamics as coupled to atmosphere-land surface-subsurface exchange and feedbacks, we have performed a series of numerical simulations using the National Center for Atmospheric Research (NCAR) Single Column Climate Model (SCCM) at two locations, the Merced River Basin (MRB) in California and the Usadievskiy (Usad) watershed at the Valdai experimental station in Russia. The MRB is a semi-arid, mountainous region in southern California Sierra Nevada, and has needle-leaf pine trees at high elevation and grassland at low elevation. The Usad watershed is a flat, part wetland and part boreal grassland area southeast of St. Petersburg, Russia. This study provides an examination of the role of ground water on the local hydroclimate under different climate patterns and land use types at these two watersheds. The SCCM was simulated for MRB and the Usad watershed with the subsurface model extending 10 m in depth over the period 1961-1990, using the Reanalysis data from National Centers for Environmental Prediction (NCEP)/NCAR as initial and lateral boundary conditions. The simulated results show that SCCM reproduced the observed precipitation, 2 m height air temperature, and streamflow with fair to good skill. Our analysis based on the MRB simulations indicates that the winter table level is largely attributed to precipitation. The spring water table is positively correlated with the precipitation and snow water equivalent (SWE). In addition, spring surface temperature is found to play an important role in affecting the water table depth. The colder surface lifts up the water table by suppressing evaporation in this season, while the warmer surface lowers the water table by increasing evaporation. Meanwhile, a positive correlation between water table and runoff is very significant in spring. In summer, the higher (lower) water table produces stronger (weaker) evaporation, which significantly increases (decreases) precipitation, and such stronger (weaker) evaporation further results in colder (warmer) surface. In fall, the water table depth has a similar influence on evaporation to that in summer, but it is also positively correlated with runoff. The Usad simulations were evaluated with quality observations that include atmospheric fields, soil moisture, and ground water data. These simulations were also compared and contrasted with those for MRB to further understand the mechanisms of how groundwater level interacts with hydroclimate. In addition, we provide an evaluation of the impact on the water table due to different land use types at these two watersheds.