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Soil-Water-Atmosphere-Plant (SWAP)

Jos van Dam, Wageningen University and Research centre, The Netherlands

Joop Kroes, Wageningen University and Research centre, The Netherlands





SWAP simulates transport of water, solutes and heat in the vadose zone in interaction with vegetation development. The model employs the Richards equation including root water extraction to simulate soil moisture movement in variably saturated soils. Root water extraction can be simulated with macroscopic and microscopic concepts. SWAP includes a detailed module on macroporous flow in clay and peat soils. For solute transport SWAP considers the basic processes convection, dispersion, adsorption and decomposition. Also SWAP generates soil water fluxes for detailed chemical transport models as PEARL for pesticides, ANIMO for nutrients and ORCHESTRA for heavy metals. SWAP simulates soil heat flow taking into account actual heat capacities and thermal conductivities. In addition to prescribed top boundary conditions, the generic crop growth module WOFOST is incorporated to simulate leaf photosynthesis and crop growth. The soil moisture, heat, solute and plant growth modules exchange status information to account for all kind of interactions. An extensive test protocol ensures the numerical code quality of SWAP. The source code is well structured, and available on the SWAP site.


SWAP model domain and transport processes.


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, vertical directed model. The flow below the groundwater level may include lateral drainage fluxes, provided that these fluxes can be prescribed with analytical drainage formulas. The model is very flexible with regard to input data at the top and bottom of the soil column. At the top in general daily weather conditions will suffice. For Nordic conditions a simple snow storage module has been implemented and the effect of frozen soil is incorporated. In addition to daily data, evapotranspiration and rainfall data can be specified on minute basis to simulate runoff or diurnal transpiration fluxes. At the bottom various forms of head and flux based conditions are used to accommodate interactions between soil water and surface water in delta areas.

In the horizontal direction, SWAP’s main focus is the field scale. At this scale most transport processes can be described in a deterministic way, as a field generally can be represented by one microclimate, one vegetation type, one soil type, and one drainage condition. Also many cultivation practices occur at field scale, which means that many management options apply to this scale. Upscaling of SWAP simulations from field to regional scale for broader policy studies is possible with geographical information systems.


Scientific articles

Van Dam, J.C., P. Groenendijk, R.F.A. Hendriks and J.G. Kroes, 2008. Advances of modeling water flow in variably saturated soils with SWAP. Vadose Zone Journal, 7, 640-653.

Kroes, J.G., and I Supit, 2011. Impact analysis of drought, water excess and salinity on grass production in The Netherlands using historical and future climate data. Agriculture, Ecosystems and Environment 144, 370-381.

De Jong van Lier, Q., J.C. van Dam, A. Durigon, M.A. dos Santos and K. Metselaar, 2013. Modeling water potentials and flows in the soil-plant system comparing hydraulic resistances and transpiration reduction functions. Vadose Zone Journal, doi:10.2136/vzj2013.02.0039

Extensive list of references on


Technical information

Operating system(s): Windows, Lunix

Licence: Open source

Output(s): Detailed data on water flow, solute transport, heat flow and crop growth

Export format(s): ASCII

User Manual:

Kroes, J.G., J.C. van Dam, P. Groenendijk, R.F.A. Hendriks and C.M.J. Jacobs, 2008.  SWAP version 3.2. Theory description and user manual. Alterra-report 1649, 262 pp, Alterra, Research Institute, Wageningen, The Netherlands

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