Summary

SWAP (Soil-Water-Atmosphere-Plant) simulates transport of water, solutes and heat in the vadose zone in interaction with vegetation development. In the vertical direction the model domain reaches from a plane just above the canopy to a plane in the shallow groundwater. In this zone the transport processes are predominantly vertical, therefore SWAP is a one-dimensional, vertically directed model. In the horizontal direction, SWAP’s main focus is the field scale.

The SWAP model can be downloaded from site swap.wur.nl. The model input may consist of files for main input, meteorological data, crop growth and drainage. SWAP employs the TTUTIL library to read the ASCII input files in easy format. Output is generated in ASCII and binary files. The internet site contains a large number of SWAP applications in scientific literature (1  Model overview).

Soil water flow is calculated with the Richards equation. The Mualem-Van Genuchten relations, with a modification near saturation, describe the soil hydraulic functions. Scaling of main drying and wetting curves is used to describe hysteresis in the retention function. The bottom boundary is controlled by head, flux or the relation between flux and head. SWAP solves the Richards equation numerically with an implicit, backward, finite difference scheme. The Newton-Raphson iterative procedure ensures mass conservation and rapid convergence (2  Soil water flow).

For agricultural crops and grassland, SWAP computes the interception following Von Hoyningen-Hüne and Braden. The interception concept of Gash is available for forests. The Penman-Monteith equation can be used to calculate the potential evapotranspiration of uniform surfaces (wet and dry vegetation, bare soil). An alternative is providing input of reference evapotranspiration in combination with crop factors. Next the potential transpiration and evaporation fluxes of partly covered soils are derived, taking into account interception and soil cover. Actual transpiration depends on the moisture and salinity conditions in the root zone, weighted by the root density. Actual evaporation depends on the capacity of the soil to transport water to the soil surface. SWAP uses the soil hydraulic functions and semi-empirical equations to determine this transport capacity (3  Evapotranspiration and rainfall interception).

Surface runoff will be calculated when the height of water ponding on the soil surface exceeds a critical depth. The rate of surface runoff depends on a specified resistance. Interflow may occur when the groundwater level becomes higher than the interflow drainage level. Drainage can be calculated with the Hooghoudt or Ernst equations, with a table relating drainage flux and groundwater level, or with drainage resistances per drainage system. In order to calculate proper residence times of solutes, the drainage fluxes are vertically distributed according to so-called discharge layers (4  Surface runoff, interflow and drainage).

The water balance of the surface water system can be calculated to analyse water management options. Surface water levels can be imposed, or derived by setting soil moisture criteria (groundwater level, pressure head, minimum storage) in combination with a weir (5  Surface water management).

Macroporosity can be caused by shrinking and cracking of soil, by plant roots, by soil fauna, or by tillage operations. The macropore module in SWAP includes infiltration into macropores at the soil surface, rapid transport in macropores to deeper layers, lateral infiltration into and exfiltration out of the soil matrix, water storage in macropores, and rapid drainage to drainage systems. The macropores are divided in a main bypass domain (network of continuous, horizontal interconnected macropores) and an internal catchment domain (discontinuous macropores ending at different depths). The internal catchment domain causes infiltration of macropore water at different, relatively shallow depths. In addition, the macropores are divided in static and dynamic volumes. The dynamic volumes depend on shrinkage characteristics (6  Macropore flow).

The simple crop module prescribes crop development, independent of external stress factors. Its main function is to provide a proper upper boundary condition for soil water movement. In addition, SWAP includes the generic crop growth module WOFOST. In this module, the absorbed radiation is a function of solar radiation and crop leaf area. Next the produced carbohydrates (CH2O) are calculated, taking into account photosynthetic leaf characteristics and possible water and/or salinity stress. The carbohydrates provide energy for living biomass (maintenance respiration) and are converted into structural material during which weight is lost as growth respiration. The material produced is partitioned among roots, leaves, stems and storage organs, using partioning factors that depend on the crop development stage. The fraction partioned to the leaves, determines leaf area development and hence the dynamics of light interception. During crop development a part of the living biomass dies due to senescence (7  Crop growth).

Grass growth is special: it is perennial, very sensitive to nitrogen, and grass is either grazed or mowed. Therefore SWAP includes a separate WOFOST module for grass, which simulates these special grass features (7  Crop growth).

SWAP simulates transport of salts, pesticides and other solutes that can be described with basic physical relations: convection, diffusion, dispersion, root uptake, Freundlich adsorption and first order decomposition. In case of advanced pesticide transport, including volatilization and kinetic adsorption, SWAP can be used in combination with PEARL. In case of advanced transport of nitrogen and phosphorus, SWAP can be used in combination with ANIMO or Soil-N (8  Solute transport).

SWAP may simulate soil temperature analytically, using an input sine function at the soil surface and the soil thermal diffusivity. In the numerical approach, SWAP takes into account the influence of soil moisture on soil heat capacity and soil thermal conductivity. The top boundary condition may include air temperatures or soil surface temperatures (9  Soil temperature).

The snow module calculates the accumulation and melting of a snowpack when the air temperature is below a threshold value. The water balance of the snow pack includes storage, incoming snow and rain and outgoing melting and sublimation. Melting may occur due to air temperature rise or heat release from rainfall. When a snowpack is present, the soil temperature top boundary condition is adjusted in order to account for the insulating effect of the snowpack. In case of frost, reduction factors can be calculated for the hydraulic conductivity, root water uptake, drainage fluxes and bottom flux (10  Snow and frost).

Irrigations with fixed date, depth and quality can be specified as input. In addition, SWAP can be used to schedule and optimize irrigation. Timing criteria include allowable daily stress, allowable depletion amount and critical pressure head or water content. Depth criteria include back to field capacity and fixed depth (11  Irrigation).

The SWAP installation file includes cases which can be run directly after extraction. 12  Case studies describes the context and results of the following cases:

  1. HupselBrook (typical Dutch field with subsurface drains and crop rotation),
  2. GrassGrowth (growth and yield of a Dutch grassland field in Ruurlo),
  3. MacroporeFlow (field experiment where macro pore flow occurs, Andelst),
  4. SalinityStress (potato crop under saline conditions, Salt Farm, Texel),

Most cases include one or more R-procedures (R-Core-Team 2026) to graphically display some model results. Observations are sometimes included to have reference points during visual comparisons with simulation results.

SWAP is a numerical simulation model. As any other simulation model, SWAP is an approximation of the real world. Therefore, it is good practice that users perform verification, validation, sensitivity or calibration tests for their own studies. In this manual we provide some examples of verification and validation (13  Verification and validation), sensitivity analysis (14  Sensitivity analyses). SWAP does not come with an option to perform parameter calibration; however, this can be easily done by using external calibration programs (e.g., PEST by Doherty (2025)).