In practice, the co-existence of eventual and definitive rights presents many difficulties, and more than one pretext for fraud. Sometimes the owners of eventual rights have access to the river higher up than the older intakes, which ought to be served first. A whole group of canals feeding land with eventual rights is in this way grafted upon the Tunuyan above La Paz, the rights of which are definitive and ancient. At Mendoza and San Juan the water-rights, codified in provincial laws which date, like the dams, from the end of the nineteenth century, are very different from the water-rights which hold in the Andean provinces of the north-west. The variety of the physical conditions is reflected in the institutions. Here water is not an object of private ownership independently of the soil. The concession of water is assigned to a definite estate, and it is formulated in superficial measurements. The law fixes the volume of water that goes with each unit of surface. If the output of the river is not large enough to provide the volume stated in the law to the whole of the irrigated district, all the lands with definitive rights receive at least an equal amount, and the available water is shared by the canals in proportion to the extent of the surface they irrigate. No law could secure for the farmers of Cuyo, even those with definitive rights, a constant supply of water, or save them from suffering in common from the variation in the volume of the torrents, and it was not even possible to guarantee them water in any permanent fashion. The _turno_ is used everywhere when the water is low. Lower down, where the drought lasts nearly the whole year, the _turno_ is the standing rule. At La Paz, on the fringe of the irrigated area, it has to be applied rigorously. The turn of each owner comes every eight, ten, or twelve days. In normal times he receives the _suerte de agua_; that is to say, the output of a sluice of a fixed size during a half-hour for each hectare (a little over two acres) of land. But if the river runs low, it becomes impossible to supply several neighbours simultaneously, and, in order to avoid making the interval between supplies too long, the duration of the _suerte de agua_ is reduced by half or three-quarters.