Water management in Valencia
Quoted from T. Glick in Islamic and Christian Spain in the early Middle Ages, Princeton University Press, New Jersey, 1979. pp 71-3.
The distribution of water among the eight canals of the Valencian huerta is a particularly useful example of [water management] because the underlying principles of the distribution arrangements are well documented and quite easily associated with a specific Islamic model.
The river, now called by its Roman name the Turia, but in Islamic times known as the Wâd al-Abyad (Guadalaviar, "White River"), was considered to be divided into successive stages, each stage representing the point of derivation of one main canal which drew all the water at that stage, or of two canals, dividing the water among them. At each stage the river was considered to hold twenty-four units of water.
The twelve-base system… is standard in many areas of the Islamic world and is clearly related to the hours of the day. A paradigmatic system, so structured, would envision a river divided into 168 units (representing seven days and nights, or 144 if a day of rest was customary). The units were not, however, expressed in hours, but as simple proportions of a whole.
Thus, in times of abundance, each canal drew water from the river according to the capacity of the canal; in times of drought, the canals would take water in turn, for a commensurate number of hours or a proportional equivalent.
The same was true of individual irrigators (and herein lies the genius of the Valencia system): when the canal ran full, each irrigator could open his gate as he pleased, but when water was scarce, a turn was instituted; each irrigator, in turn, drew enough water to serve his needs (this style of irrigation was by submersion of the field, typically to a standard depth of an ankle). But he could not draw water again until every other irrigator in the system had his turn. Thus a relatively equal distribution was ensured, both in times of abundance and of scarcity, and no measurements of time or orifice of delivery were needed.
by: Quoted from T. Glick, Sun 21 July, 2002