Integrated into buildings

The roof of the building is utilised as impluvium or catchment area before the rainwater is stored in the subterranean cisterns. Until the beginning of the last century the technique of collecting and storing rainwater in cisterns was essential for the supply of drinking water in urban areas, villages and isolated accommodations in several Tunisian regions (e.g. the islands of Jerba and Kerkenna and the mountainous regions or the arid plains in Central Tunisia). The city of Tunis had numerous cisterns, which were integrated into the accommodations or public buildings, with a total capacity of 12 million m³.

Isolated, with sealed impluvium

Remains of the Roman era on the island of Kerkennah show that this type of cistern also has a very long tradition. The subterranean cisterns are surrounded by a sealed impluvium aiming at the increase of the runoff coefficient. This method is expensive but it has a long life expectancy. The surface area of the impluvium varies according to the precipi­tation of the region. More than 200 public cisterns of this kind exist on the island of Jerba. Each of them has a storage capacity in the order of 50 m³ and an impluvium with a mean surface of about 250 m² (El Amami, 1984).

Natural impluvium

These cisterns are either open, if they are big, or covered, if they are smaller. The natural impluvium is protected and kept clean. It is sometimes cleared of vegetation and loose stones to reduce the interception of rain and the obstruction of overland flow. As figure 15 illustrates small ditches and stones collect and direct the rainwater to the cistern, which is equipped with a decantation basin for the removal of silt and an overflow.

Figure 19: Fsagui (left) and majel (right) with a natural impluvium (Photo: Meinzinger)