Climate

According to Köppen (1931) the wider area of interest can be classified as a Group B Dry (arid and semiarid) climate, since precipitation is less than the potential evaporation, but further subdivisions can be made. Morphology has the largest influence on the prevailing climate. Following the morphological division, a subdivision into three different climatic zones can be made: the Highlands area, the Western Slopes of the East Bank, and the Jordan Valley. The climate in the Highlands is of Mediterranean type. It is characterized by long, hot, dry summers and short, cool, rainy winters.

Towards the west, the climate undergoes a rapid change to semi-arid and arid climate in the Jordan Valley. The western Slopes act as a transition zone between the Mediterranean climate along the Highlands in the east and the arid climate in the Jordan Valley in the west.

Rain falls only during the winter months. Then the climate changes abruptly from dry hot summer conditions to humid, cold, and stormy conditions. Usually it starts to rain in November and rainfall continues until the end of April, whereby 70 % of the annual precipitation falls between November and February. Snow falls once to twice a year in the Highlands. In Fig. 2.1-4 the annual rainfall for the hydrologic years 1975/76 till 2001/02 for three selected sites together with their long- term seasonal average is given. The average annual precipitation on the western ridges of the Highlands (Jordan University) is around 505 mm/y, on the eastern part of the Western slopes (Naur) 395 mm/y, and in Jordan Valley (South Shuneh) the long term average is around 166 mm/y (Fig. 2.1-4). Thus a strong correlation with altitude and climate data for the area exists (Fig. 2.1-4). Strong variations regarding total annual amounts of rainfall is visible in Fig. 2.1-4. For the plotted period, the highest amount of annual rainfall for the Highlands is 1,135 mm/y in the hydrologic year 1991/92, whereas the lowest recorded amount was 234 mm/y in the hydrologic year 1988/89 (a difference of factor 4.85). This important nature of semi-arid areas is also one of the major challenge for water engineers. The water demand of the population increases or decreases constantly with time, while the water supply or input by rainfall is highly variable, both spatially as well as temporally.

The variability of annual rainfall and the necessity of long term records is addressed in Fig. 2.1-5 (data kindly provided by the Hashemite Kingdom of Jordan, Ministry of Water and Irrigation). The longest record for the wider area of interest was available for the Naur Station. Annual precipitation amounts are plotted for the period from the hydrologic year 1942/43 until 2002/03. Mean average values for time periods of 5, 10, 30, and 60 years are displayed. In order to undertake future water budget calculations long- term mean averages of at least 30 years should be used. A short period of only five years might lead to considerable over- or underestimations regarding water availability.

Fig. 2.1-4: Annual rainfall for the hydrologic years 1975/76 to 2001/02 together with the long-term average at three selected sites: Highlands (Jordan University), eastern part of the Western Slopes (Naur), and the Jordan Valley (South Shuneh) (MWI open files).

As stated at the beginning the annual mean potential evaporation exceeds by far the annual rainfall of the area. The mean annual potential evaporation in the Highlands is around 1,600 mm/y (Salameh et al. 2004), while the mean annual rainfall is not more than 505 mm/y. In the Jordan Valley (Jericho station) the mean annual potential evaporation increases to 2,082 mm/y for the period from 1968 till 1999, while, for the same period, the mean annual rainfall decreases to a mere 163 mm/y. Fig. 2.1-6 through Fig. 2.1-8 show mean average monthly climatic data for a period of around 30 years. No potential evaporation and temperature values for the South Shuneh area were available. Therefore two other stations located in Jordan Valley were used: Deir Alla, located 32 km to the north of South Shuneh, and Jericho, 16 km to the west of South Shuneh. The mean annual potential evaporation values of 2,240 mm/y and 2,082 mm/y respectively are very high. Rainfall decreases in the Jordan Valley from Deir Alla 291 mm/y in the north to South Shuneh 166 mm/y in the south.

In the cases of Deir Alla and Jericho monthly potential evaporation always exceeds precipitation.

Consequently, no infiltration of rainwater can be assumed. However, as can be seen from Fig. 2.1-9 and Fig. 2.1-10 rainfall is not evenly distributed during the months and rainfall often happens during rain storms, where large amounts of precipitation falls in a short period of time. Precipitation either infiltrates directly, or, in cases of high rainfall amounts, flows downgradient as Hortonian overland flow. No data regarding daily evaporation values in the Jordan Valley could be obtained, but it can be assumed, that these values during the rainy season are not always higher than the daily amounts of precipitation.

2. Description of the study area

Fig. 2.1-5: Annual rainfall 1942/43 – 2002/03 for the meteorological station in Naur with short (5 years) to long term (60 years) averages (MWI open files).

Fig. 2.1-6: Mean monthly rainfall, low temperatures and high temperatures in the Highlands (Jordan University), calculated for the period 1975 – 2002 (MWI open files).

Fig. 2.1-7: Mean monthly rainfall, potential evaporation, low temperatures, and high temperatures in Jordan Valley (Deir Alla), calculated for the period 1975 – 2002 (MWI open files).

Fig. 2.1-8: Mean monthly rainfall, potential evaporation, low temperatures, and high temperatures in Jordan Valley (Jericho), calculated for the period 1968 – 1999 (data kindly provided by the Israeli Hydrologic Survey).

Fig. 2.1-9: Daily rainfall for the hydrologic year 1996/97 and 1997/98 in the Highlands (Jordan University).

(MWI open files).

2. Description of the study area

Fig. 2.1-10: Daily rainfall for the hydrologic year 1996/97 and 1997/98 in the Jordan Valley (South Shuneh).

(MWI open files).

Temperature

As stated above the temperature within the study area varies significantly (Fig. 6 through Fig. 2.1-8). The lowest temperatures are found in the Highlands and the highest temperatures in the Jordan Valley. The average annual temperature in the eastern, middle, and western part are 17° C, 20° C and 23° C, respectively. August is usually the hottest month (up to 34° C in Amman and 40° C in Jericho), whilst the coolest month is January and February (down to 0.3° C in Amman and 5° C in Jericho).

The cool temperature in the Highlands results from the high elevation (up to 1,096 m a.s.l.) and relatively strong wind coming mainly from the Mediterranean Sea in west east direction. However, extremely hot and dry air, originating from southern Iraq and Saudi Arabia changes the weather drastically during the so-called ‘Khamaseen’. The name Khamaseen is derived from the Arabic word for “50” and refers to the approximately 50-day period in which the wind annually occurs. It is most often observed between April and June, but can also occur in other months. The Khamaseen takes place when a low-pressure centre moves eastward over the Sahara desert or the southern Mediterranean Sea. On its front side, the centre brings warm, dry air (temperatures can increase up to 40° C) northward out of the desert, carrying large amounts of dust and sand; on its rear side, it brings cool air southward from the Mediterranean (Encyclopædia Britannica 2002).

The temperatures in the Western Slopes are transient to the desert climate in the Jordan Valley.

The hottest temperatures are measured in the Jordan Valley. The Jordan Valley is situated at an extremely low elevation and is surrounded by a series of high mountains from both the east and west, which creates a natural greenhouse climate due to the absence (or lower degree) of cooling winds from the Mediterranean. Wind direction is mostly north south. In summer winds may come from the south (Salameh et al. 2004). The above-mentioned Khamaseen winds thereby can cause temperatures above 50° C at times.

After the cool winter months the temperature starts to rise, whereby the warming of the atmosphere in April and May is slower than the cooling in November (Fig. 2.1-6 through Fig. 2.1-8).

Dew

At night times, especially in windless nights, relatively large amounts of dew often precipitate in the Highlands and, to a lesser degree, in the upper parts of the Western Slopes. This moisture enables plants and animals to benefit from additional water outside the rainy season.