Simplified spatially distributed, coupled soil moisture–groundwater model, evaluated for Flanders (Belgium)

Study region

Flanders region of Belgium.

Study focus

In hydrological studies, there is a need for computationally fast, spatially distributed models involving the unsaturated zone soil moisture, groundwater and their interactions, while maintaining good accuracy. This study builds on an approach based on the AquaCrop model for the unsaturated zone and a 2D explicit groundwater flow model. The model’s speed stems from its simplicity, while AquaCrop’s wide use provides a basic database for parameterization.

Capillary rise, often not (explicitly) assessed in distributed models, is considered. For coupling the unsaturated and saturated zones better, the capillary rise algorithm by AquaCrop was modified. Groundwater flow to the rivers is represented combining Darcy’s law and the linear reservoir concept. Surface runoff modelling is modified to better incorporate the effects of urban environments. The model was parameterized using publicly available data (involving minimal calibration) and tested for Flanders.

New hydrological insights for the region

Despite its simplicity, the model often performed acceptably. Its accuracy was generally higher for the degree of saturation in the upper 70 cm of the soil (R=0.90, RMSE=0.14) than the first 10 cm (R=0.68, RMSE=0.17). The groundwater levels for wells with observed depths less than 5 m were simulated better (R=0.82, RMSE=0.72 m) than the deeper ones (R=0.63, RMSE=4.59 m). Capillary rise was found to be a potentially substantial source of water to the crops.