The case of Arad

The ruins of the ancient city of Arad are located on the southern semi-arid flanks of the Hebron Mountains.

The average annual precipitation is about 200 mm, and there is no perennial water source in the region. The bedrock is composed of chalks of Eocene age (Avedat Group) overlain by loess of Late Pleistocene age (Issar & Bruins 1983). The chalks are mostly impervious, enabling the ancient and present inhabitants to dig cisterns for collecting rainwater flowing along the hillslopes. At the same time, dissolution processes have developed dissolution channels along fractured layers, causing the rock to become semi-pervious and thus waterbearing (Lerner et al. 1990). The chalks are underlain by a clay-marl impervious layer of Paleocene age (Takiya Formation).

The Early Bronze Age (2950-2650 BCE) walled city (Fig. 4), the area of which reached about 10 hectares, spread over the slope of a hill. At the lowest part of the hill within the walls of the city, a shaft was found and was excavated to a depth of about 20 m, but not reaching the bedrock. Its upper part is walled by masonry, while at the bottom the chalk bedrock is exposed (Amiran et al. 1987). Although the shaft is dry, its lower exposed part is moist. Since its excavation in the 1960s various opinions have been expressed about whether this shaft served as a cistern or a well. Rosenan (1978), Yair & Garti (1996) and Tsuk (2000) investigated the purpose of this shaft; they claimed that it functioned as a cistern collecting the runoff from the city draining from the streets, and sloped towards a sedimentation basin, and drained into the shaft. Yet, their calculations regarding the quantity of runoff, and taking into account the present value of precipitation, showed that the amount of runoff would not have been sufficient to supply the needs of the ancient city, which had a the population was about 3000 people. Amiran (1991) suggested that the climate was more humid and thus more water was available. Yair & Garti (1996) did not accept the climate change hypothesis, and suggested that in addition to the shaft, which was a communal project, each household collected its own water from its roof, which was made rain-proof and had an elevated rim and apertures from which the water flowed into big jars, many of which were found in the ruins.

The author's hydrogeological investigations in this region brought him to the conclusion that this shaft was an ancient well reaching a perched water table. This well was part of a complex system, which also served as an artificial recharge system. In other words the people of ancient Arad were innovative enough to develop a central water

Fig. 4. Aerial photo of the partly excavated Early Bronze Age city of Arad.

supply project, which utilized groundwater but at the same time recharged it by diverting the surface flow from the city into the shaft. The archaeologist who is presently carrying out excavations at this site, arrived at the same conclusion (Govrin, Y., pers. comm. 2005). At the same time, most probably each household collected its supply from its own premises.

The shaft, which is located at the lowest part of the city, touched the perched groundwater table on the clay-marl layer. This table was undoubtedly higher during the time in which this city flourished. As can be deducted from the palaeo-climate curve shown on Figure 3, the annual precipitation at Arad during the Early Bronze Period could have reached about 300-400 mm. Taking into account an order of magnitude of 3% recharge on this type of rock (Issar et al. 1984) and an area of about 1 km2 for local recharge, the annual amount of groundwater available to be pumped by this well would be in the order of 9000 m3. Based on the yield of shafts into the same rock in Israel, it can be assumed that such a well could have supplied about 15-20 m3 per day, i.e. 5200 to 7200 m3 per year.

Tsuk (2000), summarizing various estimations, came to the conclusion that in the pre-Byzantine periods the average demand for water per capita per year was about 5 m3, with an additional quantity of 25 m3 per year for livestock. Assuming that the latter were watered outside the city in the pasture areas from cisterns, and that the population, according to the archaeologists' estimates, reached about 3000 people, the total water demand could have reached 15 000 m3 per year. Thus the supply from the well could not cover the demand unless augmented artificially. Yet adding to this about 4500 m3 from the 15% runoff (Issar 1981) flowing from the built-up area extending over 10 hectares, and taking the higher value of the quantity supplied by the well, i.e. 7200 m3 per year, then the demand is nearly balanced by the supply, taking into account that all these figures represent order of magnitude estimates.

This delicate balance between demand and supply explains the fact that as this region became drier towards the middle of the third millennium before Christian era (BCE), the city was deserted, without any significant evidence of its destruction by war. It should be noted that an ash layer shows that at c. 2800 BCE the city was conquered and burnt, but was immediately rebuilt as the climate and thus living conditions were optimal.

As can be seen on Figure 3, optimal climate conditions reoccurred at c. 1000 BCE. The favourable climate enabled the kings of Judea to build a fortress atop the acropolis, with a temple. The water supply of the citadel came from the collected runoff, which was diverted into two cisterns with capacity amounting to 160 m3 (Govrin, Y., pers. comm. 2005). This is more or less the amount of runoff that could be collected from the area of 0.35 hectares of the citadel. A covered trench leading from the citadel outside may have served as a supply path for hauling water from the re-excavated well in the valley. Ash and ruin layers are evidence for destruction by war, which took place during the history of the Judean kingdom, and correlate quite well with events mentioned in the Bible. The citadel was totally destroyed c. 587 BCE when the king of Babylon conquered and destroyed Jerusalem and took many of its people into exile (Aharoni 1992).

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