Quasi Steady‐State Simulation of the Unsaturated Zone in Groundwater Modeling of Lowland Regions

Well‐conceived and detailed simulation of soil‐moisture processes is a prerequisite for accurate watershed‐scale modeling of water quantity and quality processes. For this purpose, Richards' equation (and its extensions) is the conceptually preferable option. Applying the equation on the watershed scale, however, may overstretch available computer resources. At the other extreme, methods based on lumping are oversimplified. Approaches are therefore needed that are efficient and just accurate enough, and that provide the required detail in the vertical column. We have developed a quasi‐steady‐state model that uses a sequence of steady‐state water content profiles for performing dynamic simulations. The appropriate profiles are—for each time level—selected on the basis of water balances at the aggregate scale of control volumes. The groundwater coupling scheme involves an iteration cycle for the phreatic storage coefficient. In the postprocessing stage, the values of state variables obtained using the coupled model are disaggregated, thus delivering pressure heads, moisture contents, and fluxes at the detailed scale of compartments of a Richards‐type model. The plausibility of the simplified approach was tested by comparing its results to those of a Richards‐type model. The results appear promising for at least three‐quarters of the area of the Netherlands with a shallow groundwater elevation (within 2 m of the soil surface) and a thin root zone (<0.5 m thick). Customizing the modeling method used to the situation conserves computational resources, allowing more room for doing sensitivity analyses. This could be instrumental for quantification of model reliability.