KNOWING ISRAEL

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Dead Sea

Located near the center of the region, the Dead Sea lies at the terminus of the Jordan River, flowing in from the north, and the Wadi Araba to the south. The shallow southern basin has been separated from the main water body by declining water levels and now contains manmade evaporation ponds.

The Dead Sea is the terminal lake of the Jordan Rift Valley. It is the lowest point on the surface of the earth, and the waters have the highest density and salinity of any sea in the world. The east and west shores of the Dead Sea are bounded by towering fault escarpments that form part of the African-Syrian rift system. The valley slopes gently upward to the north along the Jordan River, and to the south along the Wadi Araba. Historically, the Dead Sea is composed of two basins: the principal northern one that is about 320 m deep (in 1997), and the shallow southern one from which the Dead Sea has retreated since 1978. The two basins are divided by the Lisan (or Lashon) Peninsula and the Lynch Straits, which has a sill elevation of about 400 m below sea level.

Water from the Dead Sea currently is pumped into large evaporation ponds constructed in the shallow southern basin for the extraction of potash, magnesium, bromine, and industrial and table salts.

The closed watershed of the Dead Sea is 40,650 km2 . Most of the water flowing to the Dead Sea comes from the relatively high rainfall areas of the Jordan River watershed to the north, and the rift valley escarpments to the east and west of the Dead Sea. To the south, the Wadi Araba watershed covers the arid regions of the Negev and South Jordan Desert. The climate in the watershed varies from snow capped Mount Hermon (Jabel El Sheik), with annual precipitation in excess of 1,200 mm, to the arid regions of the southern Negev, where annual rainfall averages less than 50 mm. Over the Dead Sea itself, average annual rainfall is about 90 mm and the annual potential evapotranspiration is about 2,000 mm. Actual evaporation ranges from about 1,300 to 1,600 mm and varies with the salinity at the surface of the Dead Sea, which is affected by the annual volume of freshwater inflow. The average temperature is about 40 °C in summer and about 15 °C in winter.
The water level of the Dead Sea has a seasonal cycle. Prior to development of water resources in the watershed, the peak water level occurred in May and the low occurred in December, as shown for the period 1935-55 in the bottom graph on the next page. Within-year variation ranges from 0.3 to 1.2 m for most of the period of record. Intensive development of freshwater in the Dead Sea watershed has altered the seasonal variation in water level, typically increasing the decline and decreasing the rise.

LONG-TERM FLUCTUATION OF DEAD SEA WATER LEVEL AND REGIONAL CLIMATE

Comparisons of the timing of seasonal rainfall, runoff, and Dead Sea water-level changes illustrate the hydrology of this region. Typically peak rainfall precedes peak runoff by one month, and peak Dead Sea water level for undeveloped conditions by four months. These time lags represent the time to satisfy the extreme moisture deficit from the dry season, storage deficits in manmade and natural impoundments, and the travel time of stream-flow and shallow groundwater from areas of excess rainfall to the Dead Sea. This seasonality and time lag also is evident in the Auja Spring that discharges from the Eastern Mountain groundwater basin.

The water level of the Dead Sea has been monitored continuously since 1930, and has declined over 21 m from 1930 to 1997. Such a large decline raises questions of whether there are precedents for this water-level change and whether they can be explained by normal vari-ances in climate. Fortunately, evidence of histori-cal Dead Sea water-level changes may be found from several independent sources. 

METHOD OF RECONSTRUCTING HISTORICAL DEAD SEA WATER LEVEL Over 1000 years of historical water-level records were reconstructed using evidence from rainfall and tree ring widths, sedimentology, history, archeology, botany, and morphology.Tree ring and rainfall evidence: Periods of wider or narrower average width of the tree rings of a Juniperus phoenica (cut and measured in 1968) were found to correlate well with periods of rising or falling average rainfall in the watershed for the period 1846-1968, when concurrent rainfall records were available. Based on this correlation, changing average ring widths for a period extending back to A.D. 1115 were evaluated and found to agree with other indicators of Dead Sea water level. Sediment evidence: Aragonite, a calcium carbonate mineral, precipitates directly from Dead Sea waters at its surface, and leaves a crust that becomes a thick stripe where the level is steady for several years. Aragonite stripes form a definite record of historical Dead Sea water levels, and may be date-associated where they occur on archeological ruins, such as Qumran at about 330 m below sea level. At several lower elevations, the aragonite stripes are thick and composed of several layers, indicating recurring steady Dead Sea water levels. Intervals between these stripes are evidence of the steep rise or fall in water level during rainy or drought periods. History and archeology: Periods of habitation and abandonment of many archeological sites along the western shore of the Dead Sea were dated according to concurrent history, coins, pottery, and ruins. Plotting these sites according to their chronology and elevation, various points on the historical hydrograph were confirmed. History records periods when the Dead Sea could be forded on the sill (400 m below sea level) of the Lynch Straights in the early 19th century, and in the 18th, 17th, and 14th centuries. History also records periods of extreme drought or abundant harvests.Reference: Klein, C., 1985, Fluctuations of the level of the Dead Sea and climatic fluctuations in the country during historical times: International Asso-ciation of Hydrological Sciences, Symposium, Scientific basis for water resources management, September, 1985, Jerusalem, Israel, p. 197-224.

Historical water-level records of the Dead Sea have been reconstructed for a period of over 1,000 years, including the very large rise and fall in water level around the first century B.C. (modified from C. Klein, 1985).

As shown above, there are many precedents in the historical record of larger, more rapid water-level changes than the 21 m decline over the last seven decades. Furthermore, the historical range of water-level fluctuations is about 83 m, nearly four times the 21 m decline in this century. Should further water-level declines reveal submerged trees, or should traces of historical submergence be found at elevations higher than 330 m below sea level, the historical range would increase.

Ruins of Essene community at Qumran on the northwestern shore of the Dead Sea

The largest change in water level shown on the estimated historical hydrograph occurred between about 100 B.C. and A.D. 40. Within this period, the water level of the Dead Sea rose some 70 m, from about 400 m to about 330 m below sea level (where Qumran was inundated) in about 67 years; and subsequently fell some 65 m in about 66 years. A second large rise, not shown on the graph, occurred between A.D. 900 and 1100 and crested at about 350 m below sea level. Could these extreme changes in stage be explained by climate fluctuations?

The surface area of the Dead Sea is known to have varied between about 1,440 km2 at its historical high of about 330 m below sea level, and about 670 km2 at 410 m below sea level, a greater than twofold difference. There is a corresponding difference in the volume of water lost to evaporation each year.

To address this question, investigators have made computer simulations of increased rainfall and runoff in the Dead Sea watershed, accounting for evaporation losses. These simulations indicate that rapid water-level changes on the order of 70 m over a 67-year period could occur if inflow increased by 33 to 48% over an average inflow condition. Likewise, persistent years of below average rainfall could cause rapid declines in the water level. Historical references lend weight to this conclusion. There are historical references to abundant harvests during the period of the rising Dead Sea water level prior to about 67 B.C., and there was severe drought during the period of the falling Dead Sea water level recorded by Josephus Flavius for 25-24 B.C. when Herod had to sell his treasures in order to buy corn from Egypt for the population.
The Dead Sea balances increased inflows not only by a rise in water level but also by increased evaporation losses. As the water level of the Dead Sea rises, its surface area increases causing a corresponding increase in the volume of evaporated water. The greater than twofold increase in surface area between the elevations of 410 and 330 m below sea level would increase the annual volume of evaporated water from 1,005 to 2,160 MCM, assuming a constant annual evaporation of 1,500 mm per year. Evaporation during periods of high water level is further accelerated by the dilution of saline waters near the surface, because in reality evaporation is not constant but increases as salinity decreases.

Water-level trends of the Dead Sea respond to measured rainfall trends in the watershed, except for the last three decades, when the effects of water use dominate the water-level trend.

Long-term fluctuations of the Dead Sea water level are caused by periodic fluctuations in rainfall over the watershed. The year-to-year water level is steady when the volume of water leaving the Dead Sea by evaporation is equal to the volume flowing in from perennial streams, flash floods in the wadis, and springs and seeps draining the groundwater. The water level rises following seasons of abundant rainfall and declines during drought years, as shown above in the graph of water level and rainfall from 1850 to 1997. In this graph, rainfall patterns in Jerusalem are assumed to be indicative of Mediterranean- based rainfall patterns over the Dead Sea watershed. When the annual rainfall is above average for several years, there is a cumulative effect (shown in the cumulative departure curve) leading to a rise in water level, such as occurred from about 1882 to 1895. The cumulative effect of below average rainfall periods leads to declining water levels as seen in 1930-36, and 1954-63.

EFFECTS OF DEVELOPMENT OF WATER RESOURCES ON DEAD SEA WATER LEVEL

During the last four decades, water resources in the Dead Sea watershed have been intensively developed to meet growing demands for this precious resource. Increasing amounts of water were diverted from surface and groundwater sources in the watershed to meet domestic, agricultural, and industrial needs. Since 1964, only a fraction of the flow from the water-rich areas of the upper Jordan River leave Lake Tiberias to move toward the Dead Sea. Most of this water and water from the Yarmouk and Zarqa Rivers is diverted for uses inside and outside the watershed. Under current conditions on an average annual basis, the combined inflow from all sources to the Dead Sea has been estimated as only one-half to one-fourth that of the inflow prior to development. Water also is pumped from the Dead Sea itself into evaporation ponds constructed in the shallow southern basin.The influence of rainfall and water-resources development on Dead Sea water levels is illustrated in the graph above. Until around 1970, Dead Sea water levels and rainfall showed a correlation. For example, a falling trend in Dead Sea water levels during 1954-63 corresponds to a period of below-normal rainfall. This downward trend was interrupted by above-normal rainfall that produced a rise in water levels during 1964-69. Since about 1970, however, the historical correlation between rainfall and Dead Sea water levels appears to deviate. Although rainfall generally increased during this period, water levels declined steeply, corresponding to decreased inflows from the Jordan River. Although the effects of rainy years in 1980, and especially 1992, are still evident, their influence on Dead Sea water levels is moderated. Development of water resources will result in a more pronounced impact of droughts on Dead Sea water levels. Thus, Dead Sea water levels continue to offer a record of the integrated effects of historical climate and water-resources development in this watershed.

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