Hydrological processes in macrocatchment water harvesting in the arid region of Tunisia: the traditional system of tabias

In arid Tunisia, a tabia system is a traditional macrocatchment water harvesting system. It consists of a runoff area, which occupies two thirds of the slope arid is traditionally used for grazing; and one to five cropped plots within U-shaped soil banks arranged in a cascade in the third downstream area. These "run-on" areas accumulate and store the occasional runoff. Each soil bank is constructed with a discharge weir that allows modification of the flooded area and discharge of excess water towards downstream plots. Such a harvesting system, located in an area with 140 turn annual rainfall, was instrumented during four hydrological years (1995-1999) and 45 rainfall events were recorded. Eleven of these events gave a measurable inflow to at least one of the four plots. The observations showed that the traditional tabia system reduced total surface runoff from the catchment to essentially zero. The harvesting system significantly reduced peaks of surface runoff within the catchment, which also reduced erosion hazards. The cultivated area of about 5% of the total catchment could be supplied by a harvested water amount corresponding to about seven times the amount of each rainfall event larger than 20 mm.