Unsaturated flow modeling with HYDRUS and UZF: calibration and intercomparison.

A groundwater flow model of the unconfined aquifer in a small interfluve area (~20 km²) of NE Belgium is presented. Two different packages are tested for the simulation of unsaturated flow: the HYDRUS package (Seo et al., 2007) for MODFLOW-2000 (Harbaugh et al., 2000) and the UZF package (Niswonger et al., 2006) for MODFLOW-2005. Transient boundary conditions (weekly potential evapotranspiration and precipitation for 14 years) are used to calculate the recharge. The discharge of groundwater flow to the ground surface (or seepage) is also simulated. In the HYDRUS-MODFLOW model, 8 homogeneous zones (based on land cover and steady-state groundwater depth) are defined, which keeps the computation time reasonable (3 min). In contrast, UZF (kinematic wave approach) makes calculations for every model cell of the top layer and the computation time of a single run is 11 min. A sensitivity analysis and local optimization is performed with UCODE (Poeter et al., 2005). The advantages and limitations of using HYDRUS or UZF for simulating unsaturated flow are discussed.