Recharge from irrigated agricultural lands in arid and semi-arid areas is a dynamic process that depends on various interactive factors. Rainfall, irrigation, evapotranspiration, land use, soil type and drainage conditions all have to be considered. The temporal and spatial variability of these factors as well as their interactive nature prevents the use of a simple water balance method for recharge evaluation. Instead a more dynamic approach is needed such as application of the transient finite-difference model, SWAP, developed by Feddes, et al. (1978). This model is one-dimensional and considers saturated and unsaturated vertical flow (see Section 2.5.3). To estimate the recharge for larger areas, the model must be applied to the range of conditions (e.g. soil types and crop rotations) occurring in the area. This can be achieved by subdivision of the area into homogeneous calculation units.