As all the water is utilized, and the rights of all are equally entitled to respect, the division of the water into _marcos_ (_demarcacion_) is in practice merely a proportional distribution of it amongst those who have rights to it. If the sum total of rights expressed in _marcos_ represents something like the total flow of a stream during an average season, in the time of low water it is disproportionate, and the water no longer flows to the tops of the _marcos_. In other words, the quantity of water granted to each rises or falls with the rise or fall of the stream itself. Theoretically, when the water-right is defined in _marcos_ it is permanent. Often, however, it is impossible to grant each proprietor a permanent title to the water. Even in oases where the water is "demarked," the _turno_--that is to say, the turn of the proprietors to have water--which is the absolute rule in the poorest oases, reappears during the months of scarcity, in winter, when there is no rain, and at the beginning of summer. It reappears also when the right of ownership has been broken up into fractions that are too small, and it is better to grant a larger volume of water for several hours instead of a constant stream of water which would be too scanty for profitable use. At Andalgala the "turn" is sometimes obligatory, and regulated by custom, in channels where the irrigating proprietors are too numerous; at other times optional, and settled by convention amongst the owners themselves, when water is scanty. At Valle Viejo (Catamarca), when the water runs low, they set up the _mita_; that is to say, the sluices remain closed in each channel during four days out of eight, each proprietor in turn giving up his right to a permanent supply in order to have a double allowance when his turn comes. The _turno_ is, therefore, a general practice. Everywhere we can see the farmers on the watch along the _acequias_, waiting for the moment to close their neighbour's trench with a pellet of clay and to let the stream into their own trenches with a blow of the spade. The most minute precautions are taken in order that no one shall suffer injury. As the irrigation is always slower and less thorough during the night, they take it in turns to have the day and the night alternately. When the community receives the water from another community higher up the stream, the succession of "turns" amongst its members differs every time. The water comes down charged with sediment, pushing in front of it a mass of liquid mud, as the flush of a torrent does. It takes some time for the stream to become regular and clear. The first irrigator therefore exercises his right under unfavourable conditions. In the local phraseology the _volcada de agua_ is not as good as the _corte de agua_, which means the irrigation that begins when the _acequia_ is full.