AVI 080 Project "ACROSS"
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AVI 080 Project "ACROSS" |
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"Analyzed Climatology of Rainfall Obtained from Satellite and Surface data
for the Mediterranean Basin"
by E. Salameh
Jordan is an arid to semi-arid country with a land area of about 88,500 km², of which only about 3% can be considered as Mediterranean. It is located in the eastern Mediterranean basin between latitudes 29° 30’ and 32° 31’ N, and longitudes 34° 50’ and 39° 15’ E. A mountainous range bordering the Jordan River on its eastern side runs from north to south with a width of 30-40 km. To the east, the land slopes gently towards the desert. To the west the land drops steeply towards the Jordan Rift Valley which extends from Lake Tiberias in the north (at an altitude of 210 m bls) to the Red Sea at the Aqaba (Map 1). About 100 km south of Lake Tiberias lies the Dead Sea, at the Southern part of the Rift Valley.
Jordan’s total average annual rainfall amounts to about 7.2 billion cubic meter (BCM/year). The average annual precipitation ranges between 50 and 600 mm. Only 1.3% of Jordan’s area receives and average annual precipitation of more than 500 mm, 1.8% between 300 and 500 mm, 3.8% between 200 and 300 mm, 12.5% between 100 and 200 mm and the remaining 80.6% receives less than 100 mm (Salameh 1993). About 85% of rainfall evaporates, 5-10% infiltrates to the groundwater and the rest flows along river and wadi courses leading to the Dead Sea, to the east to Saudi Arabia, collects to playas and evaporates, and in the southern part of Wadi Araba and Aqaba mountains to the Red Sea.
About 85% of rainfall in Jordan is usually associated with the atmospheric depressions which reach the eastern Mediterranean, and only 15% is connected with unstable weather conditions. Annual fluctations in rainfall reflect the variation in the annual number and intensity of atmospheric depressions.
Other factors contributing to annual fluctuations in rainfall include type of depression, its duration, its track vis Jordan and the humidity of air masses associated with it.
Spatial distribution of rainfall
Jordan’s average annual rainfall is 82 mm. Precipitation below 200 mm falls over 93% of the land area. Areas receiving more than 500 mm of annual is mainly concentrated in the high elevations of Ajloun and Balqa mountains. The major controls of the spatial distribution of rainfall is physiography and latitude. Islands of heavier rainfall appear in areas of high elevation, while most of the Jordan Rift Valley which lies below sea level receives less than 200 mm of rain. Rainfall decreases considerably from north to south and more rapidly from west to east. The former being attributed to the increased distance from the main tracks of the Mediterranean depressions and the small number of depressions which travel along the southern Mediterranean track. This explains why annual rainfall in the southern town of Shoubak which is 1365 m above sea level does not exceed 57% of the rainfall in the northern area of Ras Muneef which is 1150 m high. The eastward decrease however, is mainly caused by the adiabatic heating of the moist winds on the lee slopes of the eastern mountains. The rate of eastward rainfall decrease is often very rapid and considerable Annual rainfall in the University of Jordan is 474 mm, but decreases to 290 mm in Amman airport located less than 10 Km to the southeast.
Variabililty of Annual Rainfall
Like in other regions of similar climatic conditions, rainfall in the arid and semi arid parts of Jordan is characterized by its considerable annual fluctuation. Rainfall variability measured by the coefficient of variation exceeds 70% in these parts of the country, especially in the south and south-east. However, rainfall becomes less variable on the western slopes of Irbid mountains which is directly exposed to the Mediterranean through the gap of Marj Ibn Amer. The influence of this gap upon the increase of rainfall as well as the decrease of its varibility is also obvious in the northern part of the Jordan Valley, where the coefficient of variation of annual rainfall is less than 25%.
Distribution & Variability of Seasonal Rainfall
Despite the fact that the rainy season usually starts in October or November and lasts till May, the bulk of annual rainfall in Jordan is concentrated in the winter months However, the concentration of annual rainfall from December to March is higher in the northern and central parts of the country (about 80%) than it is in the southern and eastern regions where the contribution of Kamasin depressions to the annual rainfall becomes considerable. Early rainfall forms more than 13% of annual rainfall in the northern and central regions, and ranges between 14-19% in the south and east. Late rainfall which is of vital importance to the success of summer crops forms more than 5% of annual rainfall in the northern and central regions, but ranges between 6 and 13% in the southern and eastern regions. The monthly distribution of rainfall for five selected stations is shown in table 4. Variability of winter rainfall ranges from an average of 36% in the northern and central region to 55% in the mountains of Karak and Shaubak. This however, increases to more than 80% in the Badia, especially in the south east. Early rainfall is more variable, with an average coefficient of variation of 83% for most of the country and 128% in the Badia. Variation of late rainfall exceeds 125% in most of the country.
