Israel Water Context

5. Rainfall

In Israel, where the Mediterranean climate prevails, rains fall only in the winter, which extends from November until March-April. The source of most of the winter rains is in the barometric lows that move across the Mediterranean Sea from west to east. The lows, which are accompanied by clouds saturated with water, reach Israel on average once a week. The rains fall generally for three days in the north, two days in the center and one day (if at all) in the south. In the transition season the frequency of the lows decreases and the duration of their effect is shortened. On average, around  13-14 rainstorms occur in Israel every year, and the number of rainy days ranges from 70 in the north of the country to only a few days in the south.

                  Average annual precipitation, 1961-1990
                   (after the Meteorological Service, 1990).

In addition to the regional climatic system, the amount of rain in every region is also influenced by local characteristics such as proximity to the sea, altitude, aspect, and latitudes: the amount of precipitation increases with proximity to the sea, in the slopes facing westward, with the increase in latitude and in altitude. The largest amount of rain – up to 1300 mm/yr, is measured at Mount Hermon, the highest of Israel's mountains (2224+ m). Large amounts (700-900 mm) are measured in the Golan and the Galilee mountains and in the south Karmel (Carmel). In the Samarian, Jerusalem and Hebron mountains around 550-650 mm/yr are measured. In all of these regions one or two snowstorms a year are even possible. Small amounts of 300-400 mm/yr are measured in the northern part of the Jordan Valley. In the coastal plain the rain thickness ranges between ~600mm/yr in the north and ~300 mm in the south. A 200 mm isohyet, which denotes the "desert boundary", crosses the country from the sea in the west to west of Beer Sheva, where it takes a turn northward in the direction of the rift. The "desert boundary" separates the region with vegetation to the north of it from the dry region to the south. In the south of the Negev and in the Elat mountains amounts of rain even lower than 25 mm/yr are measured. The total volume of rainwater is, on average, ~7,900 million m³/yr, and ranges from12,000 million m³ in the very rainy years to less than 4,000 million m³ in the dry years. Most of the water (around 70%) evaporates back to the atmosphere, and only around 30% (about 2,400 million m³) is available to exploit, either as runoff in the streams or as groundwater.

Despite what is stated above, the amounts of rain change greatly from year to year. In rainy years the amounts of precipitation measured are double relative to the perennial average, and in dry years the amounts reach even less than half the average. In the past, several successive dry years occurred which, following the paucity of rain, resulted in low recharge to the aquifers and a resultant drop in the water levels there. The Israeli water economy must hence prepare also for a reliable supply of water under dry climatic conditions. This is being done by a perennial plan and collection of "surplus" water in rainy years, mainly in the coastal aquifer, for use in dry years.