Results

GROUNDWATER LEVEL

Groundwater level can be influenced by many factors, including precipitation, abstraction, lateral flow and intrusion or upconing of brackish or saline water, and return flow from irrigation, wastewater, or urban stormwater.

COASTAL AQUIFER

Figure 5. Map showing trends for groundwater levels in the Coastal aquifer during 1984–98.

Figure 6. Graphs showing groundwater levels for selected locations in the Coastal aquifer during 1984–98 (sea level = 0).

Trends for groundwater levels in the Coastal aquifer during 1984–98 are shown in figure 5. Several general patterns are obvious. Along the coast of the Mediterranean Sea, groundwater levels were mostly unchanged. Farther inland, water levels generally increased in the northern and central parts of the basin. In the southern part of the basin, water levels were unchanged to slightly decreasing. Time-series plots of water levels at a few locations are shown in figure 6 and illustrate some of the general patterns.

IHS cell 54 (WL–1 on figures 5 and 6) represents wells directly west of Tulkarm. Mean water level for wells in the cell increased 0.25 meter per year during 1984–98. Most of this increase occurred between 1991 and 1992 (fig. 6), when precipitation was very high in much of the region. The more gradual increase from 1985 to 1991 might result from increased recharge due to importing irrigation water into the area. A similar pattern is noticeable in cell 60 (WL–2), which represents wells in an agricultural area south of Tel Aviv. Here, the increase was greater (0.41 meter per year), but the probable causes are the same: high precipitation in 1992 and increased recharge from irrigation imports.

The area around Gaza City is densely populated, and abstractions for water supply are large. Groundwater levels have remained remarkably constant considering the density of wells and high abstraction rate. PWA well C/3C (WL–3) is located northeast of Gaza City. This area has a high infiltration rate and large inflows from the east. Water levels increased 0.24 meter per year during 1984–98, and this increase was fairly steady (fig. 6). The effect of precipitation in 1992 was not so pronounced as it was for the more northern locations.

PWA well P/66 (WL–4) represents an area of declining groundwater levels in southern Gaza (fig. 5). The decline during 1984–98 was 0.1 meter per year. Most of this decline occurred from 1984 to 1994; water levels were fairly constant in the later years (fig. 6). The decline was primarily due to high abstraction in this area of low precipitation.

OTHER AQUIFERS

Figure 7. Map showing trends for groundwater levels in aquifers other than the Coastal aquifer during 1982–91 (sea level = 0).

Figure 8. Map showing trends for groundwater levels in aquifers other than the Coastal aquifer during 1992–98.

For all aquifers other than the Coastal aquifer, groundwater data were divided into two time periods for trend analysis. This was necessary because of the large amount of recharge to many aquifers as a result of high precipitation during 1992. Trends in groundwater levels during 1982–91 are shown in figure 7, and trends during 1992–98 are shown in figure 8.

The results for most wells indicate groundwater levels declined during both periods, although for many wells there was a substantial increase in groundwater level between the two periods. Of 229 wells included in analysis for 1982–91, water levels declined significantly in 81 percent and rose in only 2 percent. The largest declines were concentrated in the Mountain Basins between Jerusalem and Jenin (fig. 1) and in the southern Jordan Valley (fig. 2). During 1992–98, water levels declined significantly in 79 percent of 272 wells and rose in only 2 percent. As in the earlier period, large declines were identified for many wells in the western Jordan Highlands and Jordan Plateau. Water levels were about constant or rising during both time periods only on the border of the Coastal and Carmel Basins, in Wadi Araba, and in the Wadi Hamad Basin. Some of the wells where water levels were about constant during 1992–98 might be abandoned production wells that were no longer affected by pumping.

Hydrographs of selected wells representing the general trends in the most significant aquifers are shown in figure 9.

Cretaceous aquifers

IHS well 20615201 (WL–5 on figures 7–9) represents the northern part of the Upper Cenomanian/Turonian aquifer in the Western Mountain Basin, and IHS well 7312701 (WL–8) represents the southern part of this aquifer in the Negev Basin. PWA well 15–15/002 (WL–6) and IHS well 13116401 (WL–7) represent the Lower Cenomanian aquifer in the Western Mountain Basin, and PWA well 18–16/001 (WL–9) repre-sents this aquifer in the Eastern Mountain Basin. Water levels in all these wells slightly declined or were about constant during both time periods (fig. 9). During 1982–91, the maximum water levels occurred in the wet year 1983 and the maximum drawdowns were in 1986, 1990, and 1991, which were dry years. All wells showed the effect of recharge from the high precipitation in 1992. Water levels rose about 8–15 meters, but the response took longer in the southern well (WL–8). This indicates the direct effect of the rainfall on the water level in these aquifers. Pumpage is the other main factor affecting the water level. The effect of rainfall is regional in scale, the effect of pumpage is local.

Water-level declines were somewhat larger for wells in the Lower Cenomanian aquifer, and recoveries following the high precipitation of 1992 also were somewhat greater. This is probably due to the high permeability of the aquifer, which is characterized by karst structures. The largest decline during either time period occurred for well WL–9 during 1982–91. This could be related to the high volume of pumpage from this well.

Figure 9. Graphs showing groundwater level for selected locations in aquifers other than the Coastal aquifer during 1982–98.

Water levels in the Cretaceous B2/A7 aquifer, east of the Jordan Rift Valley, are not necessarily affected by precipitation because of the presence of aquitards between the subaquifers. This aquifer is represented by MWI well 11 (WL–13 on figures 7–9) in the Side Wadis Basin, MWI well 16 (WL–14) in Amman-Zarqa Basin, and MWI well 42 (WL–15) in the extreme south of the Dead Sea Basin. Water levels declined in these wells during both time periods. Recovery following the high precipitation of 1992 was slow in well WL–13 because the aquifer at this location is semiconfined. Water level continued to increase through 1995. Recovery was faster in well WL–14, which is in an unconfined part of the aquifer. The rapid decline in this well after 1992 was a result of overpumping to supply water for the basin’s large population, which increased after the gulf war. Water levels in well WL–15 declined gradually throughout both time periods, with no recovery during 1992. This well is in a semiconfined part of the aquifer and an area that receives little precipi-tation; thus, recharge is low

Tertiary aquifers

PWA well 17–20/016Q (WL–10 on figures 7–9) represents the Eocene aquifer in the Northeastern Basin. Here the pattern of water-level change is similar to that in the Cenomanian aquifers: declining during 1982–91, recovering in 1992, and declining again during 1992–98 (fig. 9). For wells further southeast in the Eocene aquifer, in the Eastern Mountain Basin, water levels were more constant during both time periods. Rainfall does not directly recharge the aquifer in this area. Most recharge is from inflow from other aquifers or infiltration of return flow from irrigation. In addition to the small effect of rainfall, pumpage has a significant effect on water level.

MWI well 66 (WL–17) represents the B4/B5 aquifer in the Wadi Hamad Basin. This aquifer is unconfined and is affected directly by precipita-tion. Except for the dry year of 1984 and the wet year of 1992, water level stayed within a range of less than 2 meters during 1982–98 (fig. 9). This approximately constant trend is probably a result of low demand for water in the area.

Quaternary aquifers

The aquifers of the southern Jordan Valley Basin are represented by PWA well 19–13/015 (WL–11 on figures 7–9) and MWI well 4 (WL–12). Well WL–11, on the west side of the Jordan River near Jericho, is in the Pleistocene aquifer. Water level declined during 1982–91, then recovered about 10 meters after the high precipitation in 1992, and declined again during 1992–98 (fig. 9). The noticeable rise in the water level in 1985 corresponds to a 1-year cessation in pumping of the well. This indicates the effect of pumping on the water level of the Pleistocene aquifer in the Jordan Valley Basin. Well WL–12, on the east side of the river, is in the alluvial aquifer. Water level increased during 1982–91 (fig. 9), possibly due to infiltration of imported irrigation water. Also, the alluvial aquifer is affected by inflow from the Jordan River. The aquifer is unconfined and directly affected by precipitation, as indicated by the more than 10-meter increase in water level in 1992.

Paleozoic aquifers

MWI well 53 (WL–16 on figures 7–9) represents the Ram aquifer group in the Disi Basin. This is a non-replenishing aquifer; thus it is not affected by precipitation. Water level declined continuously during 1982–98 (fig. 9) due to overpumping of the aquifer.