SWAP 4 : technical addendum to the SWAP documentation

SWAP deals with soil water movement and soil water balance in the upper part of the soil consisting of the unsaturated (vadose) zone and the upper part of the saturated (groundwater) zone (see cover picture of the SWAP manual, and Figure 1.1 of the SWAP manual) (Kroes et al., 2017). Although the principal direction of soil water movement in the unsaturated zone is often vertical (upward, downward), due to interaction with groundwater and drains (drain tubes, ditches, canals) in practice soil water movement should be considered as a three-dimensional process. However, full 3D simulation of soil water movement would result in a complex simulation model with complex and detailed boundary conditions. Therefore, it was decided to set up SWAP as a 1D simulation model, in which interaction with the surroundings is modelled as additional sinks (drains) and sources (infiltration from ditches, drains). This makes then SWAP as a quasi-3D model.