2.6.2.4.4 Evaporation

2.6.2.4.4.1 Analytic element model

Direct evaporation is not incorporated in the analytic element model. Direct evaporation occurs where the watertable is in proximity to the surface. This is not likely in the surface weathered and fractured rock layer where the regional watertable is generally several metres below the surface (Parsons Brinckerhoff, 2015).

Not incorporating evaporation is again in line with the precautionary principle. An evaporation top boundary de facto provides an upper limit to the simulated groundwater levels. By not incorporating this top boundary, the drawdown can only be over estimated.

2.6.2.4.4.2 Alluvial MODFLOW models

Within the alluvium watertables are shallower and direct evaporation is more likely. For past and future 30-year blocks, a value of half the measured pan evaporation was imposed as potential evaporation at the ground surface, with a linear extinction depth set to 2 m constant across the domain, that is, any groundwater at 2 m depth or deeper is not subject to evaporation.