The technique of the jessour(or jesr) is known since antiquity as a way of exploiting the surface runoff water for agriculture in arid regions. It is a typical system of the highlands in the southeastern regions of Tunisia and constitutes the foundation of the agri­cultural activities in this zone. It is based on a retention dam made of earth or stone perpendicular to the runoff, behind which the crops, mainly fruit trees, are cultivated. The dam stops and stores the runoff and supplies in this way water to the crops.

Jessour (singular: jesr) are generally used in mountainous areas, where they are often built into wadis, but they are also constructed on plains. The dams encourage the infiltration of the rainwater, which not only intensifies the agricul­tural production but also recharges the ground water. During extreme rainfalls a part of the disastrous runoff can be retained behind the dams which helps to reduce the damage that may be caused by floods.

Components of the jessour system

A jesr is a small hydraulic unit which comprises several parts: the dam, the terrace and the catchment area or impluvium (see figure 15 and 16). It can have a dimension from a few hundred to a few thousand square metres.

Figure 15: Jessour system (adapted from Belgacem, 1999)

Normally, the dam is constructed across the wadis or thalwegs. On gently sloping hills it is perpendicular to the slope. It aims at the reten­tion of sediments, which contribute arable soil, as well as the accumulation of runoff water, which is essential for the cultivation in arid zones. The dam is either made of stones or of consolidated soil. Its length can reach up to 200 meters in wide valleys. The height of the dam can vary between 50 cm and 5 m. As figure 16 illustrates the cross section of the dam shows a more or less trapezoidal form. The longer base is formed by the surface of the slope.

Figure 16: Cross section of a jesr (adapted from Belgacem, 1999)

Generally the dams are equipped with one or two spillways, that discharge the excess water to the downstream side of the jessour. The runoff water is collected up to a height of about 20 cm or more before it is discharged into a downstream jesr system via the spillway. According to the size of the impluvium several jessour connected with each other can be superposed. Some considerations have to be taken into account for the construction of a spillway. If the threshold of the spillway is too high, the jesr on the downstream side will not receive enough surplus water and its production will decrease. If the spillway is not wide enough, the dam is threatened by undermining and the appearance of holes. Besides, the breaking of one tabia results nearly unavoidably in flooding and the breaking of downstream dams as well. If on the contrary the threshold is too low or the spillway too large, the water and soil retention capacity is not sufficient and the yield of the jesr will be unsatisfactory.

Figure 17: Dam with spillway (Photo: Meinzinger)

The terrace stretches behind the dam on the upstream side and provides the surface for farming activities. Runoff water and eroded soil accumulates on this plain surface. The soil depth varies in relation to the dam and can reach as high as 2 m. The surface of the terrace is often less than 2 ha. The CCR varies normally from 4 to 6, but it can also reach values as high as 100 (Achouri, 1994).