University of California Water Resources Center

Knowledge on the state of soil moisture is essential for improving predictability of the global energy and water balances on seasonal to inter-annual time scales. The exchanges of moisture and energy between soil, vegetation, and snowpack and the overlying atmospheric boundary layer impacts the near surface atmospheric moisture and temperature. Thus, reasonable estimates of soil moisture could significantly improve the accuracy of simulating precipitation and surface temperature globally and regionally. If the soil moisture estimation (or parameterization) is not reliable, a fully coupled climate and land surface model may simulate an erroneous climatic state that the forecasted precipitation and temperature deviate significantly from the observed values, especially in numerical forecasting of the extreme events. In this project, the impact of surface and groundwater interactions on soil moisture, evapotranspiration, runoff, and recharge are investigated. Through the two year project (extended for the third year without cost), a new parameterization to represent surface and groundwater interaction dynamics for land surface models is developed and implemented into a the VIC-3L (Three-layer Variable Infiltration Capacity) model, which is a hydrologically based land surface scheme. The new version of VIC (called VIC-ground) is applied to a watershed in Pennsylvania over multiple years. Results show that VIC can properly simulate the movement of the daily groundwater table over multiple years at the study site. Preliminary comparisons of VIC simulations with and without considering the dynamics of surface and groundwater interactions show important impact of such interactions on partitioning of water budget components. Results also show that it takes 3 to 4 years to have the effects of the initializations of groundwater tables disappear when the groundwater table is initialized to be deeper than the observed level, while it takes much less time (e.g., about 1.5 years) if the groundwater table is initialized to be shallower than the observed level. In addition, preliminary sensitivity studies at the site show that there is a more significant persistent signature of the impact of the precipitation (ppt) when its amount is halved (i.e., 0.5ppt) than that when its amount is doubled (i.e., 2ppt).