15  Calibration

The main core of SWAP, as stated before, consists of the solution of the governing flow equation for water movement in soils. The solution is determined by initial and boundary conditions, as well as by soil properties. All the conditions and properties need to be supplied by the user; those are not hard-coded in SWAP. Only universal constants or scientifically accepted parameter values have been coded hard in the source code. Technical calibration of internal parameters is not operational (this will only occur internally when new routines will be implemented). In summary, technical calibration is not implemented, because:

Most model variables are model input variables, which allows the user to calibrate, validate, verify or check for plausibility whether the outcome matches observations.

Several papers have been published in the scientific literature where SWAP was compared against other models, against analytical solutions and/or against available data: for example, Eitzinger et al. (2004), Vanderborght et al. (2005) (see also comment by Groenendijk et al. (2006)), Bonfante et al. (2010), Hack-ten Broeke et al. (2013), Groenendijk et al (2014), and Kroes et al. (2015).

SWAP can be combined with optimization algorithms as PEST (Doherty 2025) to perform sensitivity analysis or automatic calibration. For instance, the SWAP-PEST combination has been used by Van Dam (2000), De Wit et al. (2024) and De Melo et al. (2025).