Figure 3: Simulated Maize relative transpirations as a function of Total Water Supply in dry years, considering a shallower water-table at 2-m depth (after Utset et al., 2006).

3.2.2 SWAP Model. Generalities

Feddes et al. (1978) developed the agrohydrological model SWATR (Soil Water Actual Transpiration Rate) to describe transient water flow in cultivated soils with various soil layers and under the influence of groundwater. The model was further developed to accommodate more boundary conditions (Belmans et al., 1983), crop growth (Kabat et al., 1992), shrinkage and swelling of clay soils (Oostindie and Bronswijk, 1992), and salt transport (Van den Broek et al., 1994). More recently, the model SWAP (Van Dam et al., 1997) was released as a result of a combination of SWATR with WOFOST (Van Keulen and Wolf, 1986). Several improved versions of SWAP were released; the most updated includes also the solute-leaching simulation model PEARL (Kroes, 2001).

SWAP is a computer model that simulates transport of water, solutes and heat in variably saturated top soils. The program is designed for integrated modelling of the Soil-Atmosphere-Plant System. Transport processes at field scale level and during whole growing seasons are considered. System boundary conditions at the top are defined by the soil surface with or without a crop and the atmospheric conditions. The lateral boundary simulates the interaction with surface water systems. The bottom boundary is located in the unsaturated zone or in the upper part of the groundwater and describes the interaction with local or regional groundwater.

Van Dam (2000) provides a detail description of the SWAP theoretical background. SWAP solves Richards's equation numerically, subject to specified initial and boundary conditions and the soil hydraulic functions.

The maximum root water extraction rate, integrated over the rooting depth, is equal to the potential transpiration rate, which is governed by atmospheric conditions. The potential root water extraction rate at a certain depth may be determined by the root length density, at this depth as fraction of the total root length density.

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