Ir. J.G. Wesseling
DLO Winand Staring Centre (SC-DLO)
Dept. of Agrohydrology
6700 AC Wageningen
Fax : +31.317.424812
SWAP (Soil, Water, Atmosphere and Plant) simulates vertical transport of water, solutes and heat in unsaturated/saturated soils. The program is designed to simulate the transport processes at field scale level and during entire growing seasons.
Basic, daily meteorological data are used to calculate daily, potential evaporation according to Penman-Monteith. If basic meteorological data are not available, potential evaporation or reference evaporation can be input. Precipitation may be provided either at a daily basis or at actual intensities. Short-term rainfall data allow the calculation of runoff and preferential flow.
Crop growth is simulated by the code WOFOST. The processes considered include rate of phenological development, interception of global radiation, CO2 assimilation, biomass accumulation of leaves, stems, storage organs and roots, leaf decay and root extension. The assimilation rate is affected by water and/or salinity stress in the root zone. If simulation of crop growth is not required, the user should prescribe leaf area index, crop height and rooting depth as function of development stage.
Irrigation gifts can be prescribed or are calculated. The user may choose various timing and depth criteria in order to optimise irrigation application.
Soil water movement SWAP employs the Richards' equation for soil water movement in the soil matrix. The Darcy equation is used to calculate infiltration and evaporation fluxes at the soil surface. A physical description rather has been chosen beyond a parametric description of water flow, as it allows the use of soil physical data bases and the simulation of all kind of management scenario's.
Root water uptake
Root water extraction at various depths in the root zone is calculated from potential transpiration, root length density and possible reductions due to wet, dry, or saline conditions.
In cracked clay soils the shrinkage characteristic is used to determine crack volume, area and depth. Water collected in the cracks will infiltrate according to Darcy at the crack bottom. In the clay matrix the Richards' equation applies. The program will calculate solute leaching by fast moving crack water from top-soil to sub-soil.
Flow and transport in water-repellent soil is based on the concept of a mobile and an immobile soil volume. The actual mobile volume is derived from the soil layer and the soil water pressure head. In the mobile volume the Richards' equation and the solute transport equation apply. Solute difusion between mobile and immobile volume is taken into account.
Drainage is calculated with the Hooghoudt or Ernst drainage equations, which allow evaluation of drainage design. Also the user may specify a drainage resistance or tabular values of the drainage flux as function of groundwater height.
Surface water systems The program offers the possibility to simulate the management of a surface water system.
Options offered include flux, groundwater height, flux as function of groundwater height, free drainage and lysimeter with free drainage.
SWAP simulates convection, diffusion and dispersion, non-linear adsorption, first order decomposition and root uptake of solutes. This permits the simulation of ordinary pesticide and salt transport, including the effect of salinity on crop growth. In case of detailed pesticide transport or nitrate leaching, daily water fluxes can be generated as input for the models PESTLA and ANIMO.
The program calculates the residence time of the solutes in the saturated zone analogous to mixed reservoirs. In this way solute breakthrough to surface water is derived.
The program calculates heat conductance and temperatures in the soil. Soil temperatures have a large affect on solute decomposition. Either an analytical or a numerical solution can be used. The numerical solution uses the ambient air temperature as upper boundary condition.
Hoogland, J.C., R.A. Feddes and C. Belmans. 1981"Root water uptake model depending on soil water pressure head and maximum extraction rate." Acta Horticultura 119 (1981), 121 136. Also: Technical Bulletins I.C.W. nr. 6 (new series).
Belmans, C., J.G. Wesseling and R.A. Feddes. 1983. "Simulation model of the water balance of a cropped soil: SWATRE." J. Hydrol., 63(1983), 271-286.
Feddes, R.A. 1984. "Crop water use and dry matter production: state of the art." Les besoins en eau des cultures. Conference internationale. Paris, 11-14 sept. 1984.
Feddes, R.A., P. Kabat, P.J.T. van Bakel, J.J.B. Bronswijk and J. Halbertsma. 1988. "Modelling soil water dynamics in the unsaturated zone - state of the art." J. Hydrol. 100 (1988), 69-111. Also: Technical Bulletins I.C.W. nr. 78 (new series).
Feddes, R.A., M. de Graaf, J. Bouma and C.D. van Loon. 1988. "Simulation of water use and production of potatoes as affected by soil compaction." Potato Research 31 (1988), 225-239.
De Jong, R. and P. Kabat. 1990. "Modeling water balance and grass production." Soil Sci. Soc. Am. J. 54 (1990), 1725 - 1732.
Ragab, R., F. Beese and W. Ehlers. 1990. "A soil water balance and dry matter production model: I. Soil water balance of oat." Agron. J. 82 (1990) 1, 152-156.
Ragab, R., F. Beese and W. Ehlers. 1990. "A soil water balance and dry matter production model: I.I Dry matter production of oat." Agron. J. 82 (1990) 1, 157-161.
Kabat, P., B.J. van den Broek and R.A. Feddes. 1992. "SWACROP: A water management and crop production simulation model." I.C.I.D. Bull. 41 (1992) 2, 61-83.
Zepp, H. and A. Belz. 1992. "Sensitivity and problems in modelling soil moisture conditions." J. Hydr. 131 (1992), 227-238.
Faria, R.T., C.A. Madramootoo, J. Boisvert and S.O. Prasher. 1992. "A comparison of the versatile soil moisture budget and swacrop models in Brazil." A.S.A.E. meeting presentation. Paper no. 922114.
Wesseling, J.G., 1993. "Balance: a package to show the components of the water balance of a one-dimensional soil profile in time." Environmental Software 8 (1993), 247-253.
Clemente, R.S., R. de Jong, H.N. Hayhoe, W.D. Reynolds and M. Hares. 1994. "Testing and comparison of three unsaturated soil water flow models." Agric. Water Manag. 25 (1994), 135-152.
Van Dam, J.C., J. Huygen, J.G. Wesseling, R.A. Feddes, P. Kabat, P.E.V. van Walsum, P. Groenendijk, C.A. van Diepen, 1997. SWAP version 2.0, Theory. Simulation of water flow, solute transport and plant growth in the Soil-Water-Air-Plant environment. Technical Document 45, DLO Winand Staring Centre, Wageningen. Report 71, Department Water Resources, Wageningen Agricultural University.