where P is the ‘climatically appropriate for existing conditions’ (Palmer, 1965) value of precipitation.
Evapotranspiration losses from the soil occur when PE > P. Losses from the surface layer (L,) are
assumed to occur at the potential rate. Losses from the underlying layer (L,) depend on the initial moisture
content, PE, and the combined soil water content (SWC) in both layers. The upper layer is assumed to
have a 25-mm water storage capacity. The available water capacity (AWC) of the underlying layer depends
on local soil type, and an appropriate value must be specified (see later). Moisture cannot be removed from,
or be replenished in, the underlying layer until the available moisture has been removed completely from,
or replenished in, the surface layer. Hence