For irrigation purposes the volume of water made available by these concessions depends on per-hectare quotas and available water resources and so are variable over time. The initial water allocation is hence based on the basis of historically determined water rights and irrigated surface. Economic criteria do not play an important role, although there is some limited scope for changing the initial assignment of water rights through trading (see below). Effects which are external to the trading partners occur, for example, as a result of alterations in river flows, and the relatively high cost and lack of transport infrastructure limits the scope for trade.