An availability factor is necessary for the classifications above as
it is not possible to capture 100% of the precipitation that falls on
these areas. These availability factors will vary according to land use
type and to regional features. There are a number of ecological
factors that determine the availability of water in the catchment
area. One of the most important aspects is the amount of water
needed for environmental flows, and the variations in the flow concept, but also the routes of trade: for imported goods, the region
of the virtual water and its regional water balance would have to be
taken into account. Virtual water embedded in exported goods is
included in the WSF model, as it has to be part of the regional water
supply e.g. as industrial water. To make this share explicit in the
WSF matrix would be possible; we do not consider it to be of major
interest for regional water supply planning because this information
offers few strategic choices other than to cut back export oriented
industrial production which is unlikely to sit well with
regional economic policies.