Modeling Hydrological Processes with a Fully-Coupled Atmospheric-Hydrological Modeling System for the Poyang Lake Basin, China

Abstract:

For a rational quantification of past, current and future water availability, the feedback mechanisms among atmosphere, land surface and subsurface play a crucial role. Investigations of these feedback mechanisms require coupled atmospheric-hydrological modeling systems. Investigations of climate and land use changes on the regional water balance require model systems, which describe the feedback mechanisms between groundwater-, soil moisture dynamics and precipitation and which allow long-term simulations for climate-relevant scales. We have developed such a fully-coupled, meso-scale modeling system extending the atmospheric model WRF-ARW with the hydrological model HMS, which includes lateral water fluxes at the land surface and subsurface. In addition, two-way interaction between the saturated and the unsaturated zone is implemented by replacing the free drainage bottom boundary of the Noah Land Surface Model (LSM) with two approaches, a Fixed-head boundary condition assuming an equilibrium soil moisture distribution or a Darcy-flux at the boundary assuming a quasi-steady-state moisture profile below the LSM. The computational demand of this coupled model system allows long-term simulations.

The first application of the fully-coupled modeling system is for the Poyang Lake basin (160,000 km²) in South China for the years 1979-1986. The performance of fully-coupled simulations requires first rational setups of WRF and HMS. For WRF, a double-nesting approach is applied covering East Asia at 30 km resolution and the Poyang Lake basin at 10 km using ERA Interim data as global forcing. HMS simulations are performed on the 10 km grid. Simulation results are validated against CRU, GPCC, APHRODITE, CPC, GLEAM and streamflow observations. The performance of stand-alone WRF, HMS and the fully coupled simulations are shown. Furthermore, the impact of groundwater coupling on soil moisture, evapotranspiration, temperature and precipitation is investigated. The potential of the fully coupled model system, which closes the regional water balance, to investigate hydro-meteorological flux responses at basin scale is presented.