Since the inauguration of the South Dam (Degania Dam) (1933) located 400 m from the Jordan River outlet, the maximal range of WL fluctuation varied between 214.87 and 208.30 mbsl (6.57 m). Prior to the dam’s construction, the fluctuations of the WL were controlled naturally by rainfall and river discharges. Geological stratigraphic research [11
] and sediment cores dating through fossil diatom fragment analysis [12
] have indicated WL fluctuation between 197 and 217 mbsl during the 9000 years, of which the last 90 years were affected by the dam’s operation. The bottom altitude of the Jordan outlet was 212.35 mbsl, as measured in 1932 by the National Electric Company Authority. The objective of the dam when constructed was to ensure a continuous sufficient water flow from Lake Kinneret for the operation of the Hydroelectric Station located approximately 10 km south of the lake. The electricity supply continued smoothly from 1933 until 1948, when the Naharaiim region was occupied by the Hashemite Kingdom of Jordan and the plant was never operated again. To ensure climate independence through a sufficient water supply, the short section of the Jordan River (app. 400 m) between its outlet and the Dam was deepened by 4 m. In the early 1950s, a national decision was accepted to utilize Lake Kinneret as a national storage of surface freshwater for domestic and agricultural supply, and the National Water Carrier (NWC) was constructed. Later on, a top priority of the Kinneret hydrological management was aimed at the storage capacity controlled by WL operation. The possibility to change the capacities of human water demands is quite limited, but the population size is enhanced. On the other hand, the quantity of available capacity is climate-dependent through rainfall and river discharges. Moreover, the storage capability is a multiannual option through surplus storage to be used later. Prior to the construction of the NWC, while the dam was operated, the WL was elevated gradually, aiming at water storage, whilst after the intensive operation (pumping withdrawal) of the NWC (the early 1970s) and the dam was almost totally closed, the WL was gradual—continuously declining with seasonal upward fluctuations. The focus of this study is limited to the range of annual changes of the WL. Lake evaporation is climatological and seasonal dependent, but not significantly affected by hydrological management. Besides the climate effect on water inflow and consequently on WL, the utilization of Kinneret water resources north of the lake has a partial impact on WL. Nevertheless, the range of northern annual utilization changes is minor (presently 50–70 mcm; 106
/y) and consequently has a negligible impact on WL fluctuations.
The validity of the formal legislation of the lower limit of WL in Lake Kinneret since 1934 was significantly modified. Before the operation of the NWC (1934–late 1960s), no uppermost and lowermost WLs were formally instructed. Therefore, the unlimited optional range of management decisions about the WL altitude was dependent during 1933–mid-1960s on two noncorrelated constraints: electricity production and environmental shoreline protection, whilst after the NWC operation, WL management achieved implementations aimed at both water supply (storage) and environmental shoreline protection. The determination of the uppermost WL altitude was fixed during the late 1960s after constructed devices surveyed within a close vicinity to the potential shoreline at various altitudes, whilst the lowermost permitted altitude was considered differently. Two major parameters are involved in the consideration of the lowermost altitude: the altitude of the NWC pumps intake and the potential ecological impact on nutrient dynamics. With regard to the NWC intake depth, the decision is of the engineering trait, which clearly indicates it is not lower than 215 mbsl. Nevertheless, the data recorded about the resulting effect of WL decline on nutrient dynamics were insufficient. Consequently, the experienced available data record was a useful tool for decision-makers. Nevertheless, when Lake Kinneret’s status was already dedicated to being a national reservoir of surface water for domestic supply, the water quality became a significant parameter of concern and the quantity and quality of Kinneret’s water are dependent on the implications of the WL measurement. The history of WL management (Figure 3
, Figure 4
and Figure 5
) indicates: prior to the early 1970s, the WL level altitude increased, but later declined. The legislated uppermost WL altitude was instructed to be 208.8 mbsl and was never changed, but included one exceptional case during January 1969 when the WL was recorded at 208.30 mbsl. Nevertheless, the lowermost limit was changed several times: 212.5, 214.0, and 214.87, and finally the legislated WL altitude level was fixed at 213.0. The essential cases of the lowermost level were due to seasonal dryness and the reduced availability of lake water.
4.1.1. Complex Interactions in WL Management
The consequences of the WL change evaluation since the dam operation, followed by the NWC construction and recent combating climate condition changes indicate partial ignorance of the involvement of water quality parameters among the management discretions. The intensive introduction of limnological features such as water quality into the management design considerations was enhanced during drought prior to the intensive supply of de-salinized sea water in 2010. Higher frequencies of drought seasons and lowering WLs strongly motivated national achievements towards the desalinization program. The rationale for it was an extreme decline of the WL, which initiated suspected enhancing nutrient flux from bottom sediments. Consequently, enhanced investments in desalinization plant constructions were promoted. Furthermore, from 2018, the climate conditions were changed into a wet type, a domestic water supply was almost fully achieved and the input of desalinized water into Lake Kinneret initiated a management complexity: the WL is high, availability of water is sufficient and the national domestic water supply comes almost totally from desalinization, and pumping is therefore diminished and the input of desalinized water already performed.
The achievement of desalinization of 650 mcm (106 m3/y) of sea water was initiated as the result of a long period of dryness. Nevertheless, it was predicted to prevent a WL decline below 214 mbsl, accompanied by water exchange enhancement in Lake Kinneret, and reinforcement of the irrigation supply, but increasing WL above 210 mbsl was not considered. The exceptional increase of the WL (>210.5 mbsl) is the result of a high level of rainfall and river discharge.
The question is, therefore: When and how much of the dam should be open? If the WL is high, whilst desalinized water is available and pumping is therefore diminished, management turns toward procedures aimed at the prevention of damage to beach facilities by the appropriately enabled control of salt accumulation. An efficient tool for optimization of WL control is the dam operation and withdrawal enhancement for agricultural irrigation.
The benefit of gradual dam opening is also the shortening of the hydraulic residence time and water exchange enhancement for water quality improvement.
The schematic formulation of the WL management and hydraulic parameters involved is as follows: Evaporation is a natural parameter dependent on climate conditions and, in comparison with the other parameters, its amplitude of changes is negligible:
A = Total Water Input,
Where: A1 natural River Flow; A2 = Desalinized Sea-Water Insert;
B = Total Water Output,
Where: B1 = Evaporation; B2 = Pumping; B3 = Open Dam Spilling;
C = Lake Volume, WL measure (the consequence of C changes is WL fluctuations);
D = Hydraulic Residence Time;
D = C/A = (A1 + A2) minus (B1 + B2 + B3)/(A1 + A2)
Therefore: Increasing A with a decline of C results in an improvement of the water quality by shortening D and the water exchange enhancement;
The decline of C is enhanced by increasing B2 and B3;
The optimization of WL management relies on natural water availability, which is dependent on the climate conditions and pumping regime.
Conclusively, WL management design is comprised of a comprehensive evaluation where priority grading is included.
Since the dam construction at the Kinneret outlet (1933), four long-term periods of prolonged WL decline were recorded: 1988–1991, 1992–2001, 2004–2008 and 2013–2018: The maximum and minimum of the periodical WL amplitude varied between 208.90 and 214.87, respectively; the decline ranged between 4.1–6.0 m, which is 9–13% of the maximum lake depth; the exposed bottom size resulting from the respective WL decline ranged between 7.7–11 km2, which comprised 4.6–6.5% of maximal lake bottom surface area; and the time duration of the WL decline period varied between 3.4–7.4 years. Though focusing on the limitation to time duration and bathymetric features whilst eliminating hydrological and ecological parameters, the incomparability of Lake Kinneret to the other three SAT lakes is prominent.
Conclusively, during 1933–1948, Lake Kinneret was proposed to supply water for electricity production. From the early 1970s, the lake designation became a water supply. This designated purpose completely dictated WL management. During 1964–2010, three principal factors controlled WL management in Lake Kinneret: water availability (rainfall, river discharge), and water supply demands. Later on, prior to the implementation of the desalinization program, the control parameters became climate condition changes (drought) and salt loads in the lake. Nevertheless, throughout the entire period, the management tools for objective achievements were the aam and NWC constructions.
4.1.3. Lake Sevan
The principal limnological features of Lake Sevan before the anthropogenic involvement are [13
]: volume—58.5 km3
; water surface—1416 km2
; maximum depth—100 m; and mean depth—41 m. Due to severe regional distress in agricultural land, and available water for irrigation and electricity, a decision was taken to transport water through a tunnel, lower the outlet river from Lake Sevan into the adjacent Ararat valley (1933–1949) and construct Hydroelectric plants. The lake WL decline was intensively implemented between 1933 and 2002, the WL declined by 19.9 m and its volume constricted by 44%, the surface reduced by 13%, and maximum and mean depths were reduced by 20% and 34%, respectively. Similar parameters for Lake Kinneret, but during much shorter periods of 3–4 years (69 years in Lake Sivan), were significantly smaller: 15–23% and 9–13% for the maximum and mean depths, respectively. The hydrological balance was extremely modified: an inflow reduction of 12% with an outflow enhancement by 94% resulted in an annual deficiency of 1258 m3
, in spite of a reduction of evaporation and bottom infiltration. Fishery management became uncontrollable, resulting in overfishing and the introduction of exotic species. Additional deteriorating factors were inputs of an intensive supply of pollutant and waste substance loads from the watershed. The trophic status of the Lake Sevan ecosystem severely deteriorated: the devastation of biodiversity, severe enhancement of primary production, the devastation of natural habitats, and water quality deterioration were the results. In spite of the morphometrical similarity between Lake Kinneret and Lake Sevan, the major differences that deny comparability between the two lake ecosystems are as follows: the amplitude of the WL decline 19.22 m in Sevan; 4–6 m in Kinneret; and the time duration of the change, short (3–4 years) in Kinneret and much longer (69 years) in Sevan. Moreover, fishery (harvest, stocking) in Kinneret is regulated, anthropogenic improvements in the Kinneret drainage basin are thorough, and sewage and agricultural pollutants are removed. The consequent conclusion of the comparative evaluation between lakes Kinneret and Sivan is that mild WL decline during a short time duration is implementable. The incomparability of the WL decline achievement in Lake Kinneret and Lake Sevan is indicated by their morphometric features resulting from a maximal WL decrease of 6.07 m (during 1–3 years) and a 19.22 m (during 1930–2001; 71 years) decline in LK and LS, respectively, given in Table 3
as a range of change in %:
A significant issue that emphasizes the incomparability of the WL decline in LK and LS is fishery: a historical record of 11 exotic fish species stocking in LK was documented as a failure, whilst in LS a similar introduction accompanied by a WL decline crucially damaged the fishery of endemic species. Moreover, the successful introduction of exotic fish species in LK is due to those which are not reproduced in LK and their feeding habits improve the water quality [14
4.1.6. SAT and Kinneret Incomparability
The conceptual prognosis design of the extreme SAT WL decline was similar: long-term enhancement of the irrigation water supply, whilst in Lake Sevan, hydroelectric production as well. The short-term local temporal exception of a 4–6 m WL decline in the deep and bottom steep Lake Kinneret is therefore not comparable to the SAT lakes. The dissimilarities between Kinneret and Sivan are the time duration: 71 years in Sevan and 1–3 years in Kinneret, as well as the WL decline amplitude range: 19.22 m in Sevan and 4–6 m in Kinneret. The morphometric structures of Tchad and the Aral Sea are significantly dissimilar in comparison with Sevan and Kinneret, which are deep and steep bottom lakes, whilst the Aral Sea and lake Tchad are shallow and flat bottom lakes. Therefore, the evaluated factors indicate a significant dissimilarity of implications. The rationale for the dramatic WL decline was the enhancement of food production through irrigation and domestic water supply improvement in all four lakes, the SAT and Kinneret. The morphometric shape of a lake can influence its biological production, whereas the difference between shallow flat and deeply steep bottom lakes creates incomparable conclusions. The significant difference between the morphometric structure of shallow flat (L. Tchad and Aral Sea) and deep steep bottom Lakes (Kinneret and Sivan) is presented by hypsographic curves (Figure 8
and Figure 9
). The depth–volume–surface area relations clarify the results of the WL decline in Lake Kinneret (Figure 8
) and theoretically in flat-bottom lakes (Figure 9
). One meter of WL decline in L. Tchad and/or in the Aral Sea exposes a very large bottom sediment surface, which is subject to dust storms and deposition, which enhance human health difficulties.
A speculative and hypothetical indication, partly based on international documents, about the ecological impacts of excessive WL fluctuations (WLF), particularly decline, on the nutrient dynamics and eutrophication in freshwater lakes was published [25
]. The consequences of nutrient dynamics in the Kinneret hypolimnion when the WL declines was mentioned earlier in this paper. Nevertheless, further consequent impacts on the water quality in the pelagic epilimnion, or even a trend of temporal eutrophication, are not evidently confirmed [25
] as well as in the present paper. Internal loading is tightly associated with thermal structure in lake Kinneret, but epilimnetic consequences are not confirmed [25
]. The long-term accumulation of nutrients in the Kinneret ecosystem was not confirmed. LK is not a closed system and intensively interacts with the terrestrial surroundings. Climate changes significantly affected nutrient loading dynamics, and the epilimnetic standing stocks in LK resulted from hydrological features (rainfall, river discharge), but the direct involvement of WLF in it is not confirmed. WLF is undoubtedly affecting the littoral ecosystem [25
]; nevertheless, its impact on the pelagic zone is insignificant. The dramatic change of the phytoplankton community structure in LK was widely documented, whilst its relation to WLF was found insignificant. Through the long history (~19,000 years) of the Kinneret ecosystem, WLF events were numerous within a very high range of amplitude, 207–217 [11
] and 6.57 mbsl (this paper), accompanied by anthropogenic and natural modification, but continuous eutrophication was not confirmed.