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INTERNATIONAL JOURNAL OF CLIMATOLOGYInt. J. Climatol. 29: 605617 (2009)Published online 9 July 2008 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/joc.1727

Analysis of a 27-year rainfall data (19772003) in theSultanate of Oman

Andy Y. Kwarteng,a* Atsu S. Dorvlob and Ganiga T. Vijaya Kumaraa Remote Sensing and GIS Center, Sultan Qaboos University, Al-Khod PC 123, Oman

b Department of Mathematics and Statistics, College of Science, S ultan Qaboos University, Al-Khod PC 123, Oman

ABSTRACT: Analysis of variation of rainfall in space, time and amounts, and its attendant effect on the ecosystem is

vital in arid and semi-arid environments where the resource is scare, highly variable and unpredictable. In this study, we

analyse the characteristics of rainfall in the Sultanate of Oman using data recorded between 1977 and 2003. The data

is divided into six geomorphic compartments to represent the various topographic regions in Oman. The average yearly

rainfall varies from a low of 76.9 mm in the interior region to a high of 181.9 mm in the Dhofar Mountains, with an

average of 117.4 mm for the whole country. Mann Kendall statistics show a negative but insignificant rainfall trendsfor the datasets. In northern Oman, the main rainfall season occurs between December and April and that accounts for

57.882.9% of the annual rainfall. February and March record the highest rainfall accounting for 35.3 to 42% of the

yearly rainfall. The Dhofar Mountains and surrounding areas in southern Oman are dominated by the khareef season in

JulyAugust, which produces 44.3 to 67.5% of the rainfall in that area. The number of days of light rainfall (100 mm rainfall per day) approximately every 50 years. Copyright 2008 Royal Meteorological

Society

KEY WORDS arid environment; rainfall data; trend analysis; rainfall intensity; Oman

Received 8 April 2007; Revised 14 January 2008; Accepted 6 May 2008

1. Introduction

Rainfall is a scarce and an important hydrological vari-

able in dry land areas. The need for water in these

areas increases daily due to population growth, eco-

nomic developments, urbanization, and consequently,

water management using all the available resources is

becoming increasing crucial. In order to develop an effec-

tive management strategy, it is paramount to understand

and assess the impact of this resource on the ecosys-

tem. The study and understanding of climate, variation

of rainfall in space, time and amounts, and their atten-dant effects on the ecosystem is vital. Plant productivity

in these regions is primarily limited by the availability of

adequate water.

Arid and semi-arid zones are characterized by rainfall

which is highly variable in space, time, quantity and dura-

tion (Noy-Meir, 1973). The Sultanate of Oman is char-

acterized by hyper-arid (


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606 A. Y. KWARTENG ET AL.

Oman for several centuries. Water management in Oman

is vital for the continuous recharge of groundwater and

the maintenance of the natural ecosystem.

Rainfall in the Arabian Peninsula occurs primarily as a

result of four main meteorological conditions originating

from the Mediterranean, central Asia, the tropical mar-

itime regime of the Indian Ocean and tropical Africa.During different times of the year, these atmospheric

influences bring varying rainfall to different parts of the

peninsula. In general, the Arabian Peninsula is dominated

by two air masses, namely, the Polar Continental that

occurs from December to February and the Tropical Con-

tinental that occurs in summer from June to September.

Both systems are affected by minor incursions of Polar

Maritime and Tropical Maritime (Fisher and Membery,

1998). Rainfall patterns in Oman depend on these and

other meteorological conditions and their interaction with

the local topography.

In this study, we analyse the characteristics of rainfall

in the Sultanate of Oman using data recorded between1977 and 2003. The questions addressed are as follows:

(1) Are there any trends in the daily, monthly and yearly

rainfall for different parts of Oman? (2) Is there any

obvious cyclicity in the rainfall for the different parts?

(3) Are the rainfall patterns stable or variable? The

ability to shed light on these questions is essential in

the long-term economic and environmental planning for

the Sultanate. MannKendall tests are used to examine

trends in the rainfall data. The Sultanate of Oman is quite

diversified in terms of its northeastsouthwest extent,

geomorphology and climatic conditions. The corollary is

that rainfall spans from arid through semi-arid to hyper-arid conditions. The approach used for the rainfall data

analysis was to divide the dataset into six regions that are

representative of the different geomorphic compartments.

The average data for the different regions is considered

the most representative for Oman. To the best of our

knowledge, this study is the first to use such an approach

in Oman.

2. Materials and methods

2.1. Study area

Oman is located in the southeastern part of the ArabianPeninsula in both arid and semi-arid environments. With

an area of approximately 312 500 km2, Oman is bordered

in the northwest and west by the United Arab Emirates

and Saudi Arabia, respectively, and in the southwest by

Yemen (Figure 1). Oman has a coastline of 1700 km

stretching from the Strait of Hormuz in the north to

the borders of Yemen in the southwest and overlooks

the Arabian Gulf, Gulf of Oman and the Arabian Sea.

The land area is much diversified and can be divided

into three principal physiographic regions or geomorphic

compartments:

Mountain ranges, comprising the northern Oman or Al

Hajar Mountains and the Dhofar or Qara Mountains,

Figure 1. Landsat TM bands 7, 4 and 2 colour composite mosaicked

image of the Sultanate of Oman. This figure is available in colour

online at www.interscience.wiley.com/ijoc

occupy 15% of the country. The northern Omanmountains extend in an arc form for 700 km from

Musandam in the north and curve eastward towards the

coast to Ras Al Hadd, the easternmost part of Oman.

Jabal Shams with the highest peak of 3075 m above

mean sea level, is part of Al Jabal Al Akhdar that

forms the central part of the northern Oman mountains.

The Dhofar Mountains, located in southwestern Oman,

have peaks from 1000 to 2000 m above mean sea level.

Coastal plains comprise Batinah in the north and

Salalah Plain in the south, and account for 3% of

the land mass (Figure 1). These plains, which serve as

main agricultural areas, have elevations ranging from

0 to 500 m.

The interior region occupies the area between the

mountain ranges in the north and south, respectively,

and consists of sandy wasteland desert. The area has

elevations not exceeding 500 m and accounts for 82%

of the country.

The diverse topographic regions in Oman result in

a wide range of climatic conditions. The average tem-

perature in northern Oman from May to September is

between 32 and 48 C, and 26 and 36 C from October to

April. Coastal regions are hot and humid in summer with

high temperatures of 46

C and more than 90% humid-ity. In the interior plain, high temperatures in summer

can exceed 50 C. In winter temperatures are quite mild

Copyright 2008 Royal Meteorological Society Int. J. Climatol. 29: 605617 (2009)

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RAINFALL DATA ANALYSIS IN OMAN 607

between 15 and 23 C. The temperatures in the highlands

and southern Dhofar regions are moderate throughout the

year. The Dhofar region has a fairly steady year-round

temperature of 3035 C. The rainfall data used in this

study have been divided into six groups to represent the

different physiographic regions as described in the next

section.Rainfall in Oman is caused by four principal mech-

anisms (Roberts and Wright, 1993; Ministry of Water

Resources, 1995).

Convective rainstorms that are associated with local-

ized cells of strong convection can develop any time

of the year, but mostly during summer.

Cold frontal troughs that are most common from

November to April originate from North Atlantic or

the Mediterranean Sea and may bring seif rain to the

northern parts of Oman, and possibly to central and

southern Oman. Rainfalls in these areas vary depend-

ing upon the physiographic location. For example, theaverage rainfall in Muscat, situated on the coast, is

75 mm whereas the average rainfall in the Al Jabal

Al Akhdar, with elevations between 400 and 3000 m

above mean sea level is between 250 and 400 mm.

Tropical cyclones originate from the Arabian Sea

and tend to be distributed equally between two main

cyclone seasons, May to June and October to Novem-

ber. However, cyclones occasionally occur outside the

two periods. In general, the Arabian Coast of Oman

is affected by a frequency of one cyclone in every

3 years. These cyclones give rise to intense storms and

occur once in every 5 years in Dhofar Governorate andonce in 10 years in Muscat. Even though these storms

are uncommon, when they occur, they can bring heavy

rain to the Arabian Coast of Oman. One of such storms

crossed Masirah Island in 1977 when 430.6 mm of rain

was recorded in 24 h (Watts, 1978). Note that the aver-

age annual rainfall for Masirah Island is only 70 mm.

On-shore southwesterly monsoon currents occur from

June to September and bring humid conditions to

much of Oman accompanied by frequent drizzle, fog,

mist and rain (khareef) in Dhofar coast and bordering

mountain areas. Occasionally, the monsoon currents

penetrate further inland to produce convective storms.

During the khareef season, parts of Dhofar region aretransformed into lush landscapes of green field and

verdant vegetation. The monsoon season in Dhofar

region brings 100400 mm of rainfall.

Even though rainfall in Oman, just like most arid

regions, is sparse and irregular, it is able to support

the plant ecosystem. However, the spectrum of plant

life and seasonality of flowering are more influenced by

the amount and distribution of rainfall than variations in

temperature (Ghazanfar, 1997).

2.2. Data description

The first rain gauge monitoring station in Oman was

established in Muscat in 1884, followed by Salalah

airport and Masirah Island in 1942 and 1943, respec-

tively. Systematic rainfall monitoring stations in the

whole country started after 1970 (Ministry of Water

Resources, 1999). The data used in this study were

obtained from the Ministry of Regional Municipali-

ties, Environment, and Water Resources (MRMEWR).

In 2002, there were more than 3000 systematic net-work for measuring parameters such as climate, rainfall,

groundwater levels and groundwater quality throughout

the country (Al Khabouri and Al Aata, 2002). Water

monitoring stations are designed according to World

Meteorological Organization (WMO) standards for an

arid region to acquire basic water resources data which

could be used to study, analyse and advice on the opti-

mum uses of water resources in the Sultanate. The rainfall

gauges automatically record rain on an hourly and daily

basis. In the majority of the coastal plains in Oman, rain

gauges are positioned at distances of less than 200 km 2.

In the mountainous ranges, there is one gauge positioned

per 400 km2. The network of rain gauges in the desertareas with less rainfall is much fewer.

In this study, 31 monitoring stations, with more than

27 years of continuous rainfall data coverage from 1977

to 2003, were used (Table I). Owing to Omans location

in both arid and semi-arid regions as well as the different

geomorphological compartments and climatic conditions,

rainfall data for a particular location may not be represen-

tative of other parts of the country. The stations have been

divided into six groups to represent the different geo-

morphic compartments. The categories are based on the

different geomorphologies including elevation, different

rainfall types and seasonal rainfall (see Section 2.1). Theyconsist of (1) the northern Oman Mountains (15 stations),

(2) Batinah coastal plain (2 stations), (3) northeast coast

(5 stations), (4) interior region (7 stations), (5) Salalah

coastal plain (one station) and (5) Dhofar Mountains (one

station) (Table I and Figure 2). The elevations are: north-

ern Oman mountains, 4201950 m above mean sea level;

Batinah Plain, 1070 m; northeast coast, 5175 m; inte-

rior region, 19320 m; Dhofar Mountains, 858 m; and

Salalah Plain, 20 m. Table I shows the characteristics

of the stations including Universal Transverse Mercator

(UTM) coordinates, elevation and average yearly rainfall.

2.3. Data analyses

Average monthly and yearly rainfall values were calcu-

lated for all the stations. The following parameters were

analysed for the stations: (1) total rainfall, (2) number of

rainy days, (3) maximum daily rainfall, (4) extreme rain-

fall events and (5) rainfall intensities. The following rain-

fall (R) intervals were used: light rainfall, R < 10 mm;

moderate rainfall, 10 R < 25 mm; heavy rainfall, 25

R < 50 mm; and extreme rainfall, R > 50 mm. Similar

approaches have been used in the analyses of rainfall data

in arid regions (Lazaro et al., 2001; Gong et al., 2004).

The MannKendall test was used to detect trends in thetime series data for the six geographic regions. Suppose

(x1, x2, . . . . . . . . . . . . . . ., xn) is a time series data. The


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Table I. Description of the 31 rain gauge stations used in this study.

Station

no.

Station ID Station name Geographical location UTM/

Eastings

UTM/

Northerns

Elevation

(m)

Average

yearly

rainfall

(mm)

1 AD889556AF Salalah Airport Salalah coastal plain 189 500 1 885 600 20 112.002 AE916185AF Zayk 1 Dhofar Mountains 196 900 1 911 800 858 184.58

3 CM786116AF Al Buraymi 1 Interior region 376 800 2 683 200 300 65.15

4 DL455961BF Tanam Interior region 445 600 2 559 100 300 70.83

5 DL468825AF Ibri Interior region 448 200 2 568 500 330 90.36

6 DM204694AF Dank Interior region 424 900 2 606 400 320 75.81

7 DM260958BF Kitnah 2 Northern Oman mountains 420 500 2 669 800 650 109.67

8 DM264436BF Daqeeq 2 Northern Oman mountains 424 300 2 664 600 840 175.43

9 DM374569AF Al Far Far Northern Oman mountains 434 600 2 675 900 620 113.77

10 DM382737BF Hayl Al Adhah 2 Northern Oman mountains 432 300 2 687 700 430 103.29

11 DM459225AF Al Ghuzayfah near Al Khabat Northern Oman mountains 449 200 2 652 500 520 106.96

12 DM520025AF Falaj As Sudayriyin Northern Oman mountains 450 2 00 2 6 20 5 00 650 151.85

13 DM545506AF Haybi Northern Oman mountains 455 000 2 645 600 540 110.38

14 DM915258AF Majzi in Wadi Al Hawasinah Northern Oman mountains 495 500 2 612 800 475 98.37

15 DN259176AF Aswad Batinah coastal plain 429 700 2 751 600 70 132.67

16 DN436617AF Shinas Batinah coastal plain 446 100 2 736 700 10 118.13

17 EL144239BF Al Ghafat Northern Oman mountains 514 300 2 542 900 560 132.12

18 EL258618BF Al Hamra Northern Oman mountains 528 100 2 556 800 700 167.52

19 EL655171AF Sayq Northern Oman mountains 566 300 2 551 100 1950 330.15

20 FH988624AF Masirah Interior region 698 200 2 286 400 19 70.81

21 FK886394AF Al Ghabi Interior region 686 900 2 483 400 280 92.85

22 FL072718AF Samail Northern Oman mountains 602 100 2 577 800 420 84.88

23 FL245564BF Al Muqayhfah Northern Oman mountains 625 600 2 545 400 720 150.57

24 FL455969AF Al Bdayah Northern Oman mountains 645 600 2 559 900 595 181.92

25 FL516435CF Ibra Northern Oman mountains 656 300 2 514 500 425 122.12

26 FL585627AF Al Hajir Northeast coast 655 200 2 586 700 160 73.60

27 FL950429CF Mazara Northeast coast 690 200 2 554 900 150 110.08

28 FM300897AF Seeb Airport Northeast coast 630 900 2 608 700 15 80.4429 FM515309AF Mina Al Fahal Northeast coast 655 000 2 613 900 5 87.38

30 GK258642BF Al Wafi Interior region 728 400 2 456 200 175 69.92

31 GK594466AF Sur Northeast coast 754 600 2 494 600 14 79.37

MannKendall (MK) test statistic is defined as

T =i 0

0 if x = 0

1 if x < 0

Under the null hypothesis that there is no trend,

T is distributed as a Normal random variable with

mean zero and variance assuming no ties between (x1,

x2, . . . . . . . . . . . . . . . ., xn) (Hirsch et al., 1982). The

alternative used is no trend. All tests were at =

0.05 level.

3. Results

3.1. Yearly and monthly average rainfall

Yearly rainfall over Oman is quite variable and irregular,

and depends on the geographical location as depicted

in Figure 3. The average yearly rainfall recorded for all

the 31 stations between 1977 and 2003 is 117.0 mm.The highest average of 338 mm was recorded in 1997.

The years 1982, 1983 and 1995 to 1998 recorded

high averages. Subsequently, the averages dropped off

considerably. The lowest average of only 28.6 mm was

recorded in 1984. Other low yearly rainfall averages of

37.2, 50.9 and 52.9 mm were recorded in 2001, 1986 and

1991, respectively. Overall, there is no significant trend

in the rainfall pattern over the years. The MannKendall

statistic shows a negative trend (T = 0.31); however,

the trend is not significant (p-value = 0.75). Of the sixregions, the Dhofar Mountains have the highest yearly

average of 184.6 mm. The area measured the highest 1-

year average of 595.0 mm in 1985. The yearly averages

for the other region are the northern Oman Mountains,

141.4 mm; Batinah Plain, 100.8 mm; northeast coast,

89.8 mm; interior region, 76.6 mm; and Salalah Plain,

112 mm. For all the regions, there is a lot of variation

in the yearly averages as indicated by the coefficient of

variation which is over 50% for most of the years. The

lowest yearly average is recorded in the interior of Oman,

and it is not unexpected as this region is mainly dry sandy

desert. The data show that in the years that the rainfall ishigh, this applied to all regions. Conversely, low average

rainfall years are common to all the regions. In general,


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Figure 2. Geographical distribution of the 31 rain gauge stations used in this study.

yearly rainfall patterns appear similar, with the exception

of the Dhofar Mountains, suggesting potential similarities

in the mechanisms of rainfall.

The average monthly rainfall distribution shows high

variability for the geographic locations (Figure 4). The

monthly average data for all the stations indicate that

the highest rainfall is recorded in February and March

and accounts for 35.9% of the yearly rainfall. The

lowest monthly rainfall occurs in November, October,

June and May, and these months collectively account for

only 18.2% of the yearly rainfall. The potential sources

and mechanisms for rainfall in the different months aredescribed in Section 2.1. Northern Oman Mountains

experience an average monthly rainfall of 11.8 mm,

with each month exceeding 3.8 mm. Approximately

53.9% of the rainfall occurs in the period from January

to April (seif rain), and summer rain (July August)

accounts for 23.3% of the rainfall. Rainfall patterns in the

Batinah Plain and the northeast coast are quite similar.

In both locations, the majority of rainfall (90.3% in

Batinah Plain and 81.2% in the northeast coast) occurs

between November and April (seif rain). Summer rainfall

(MaySeptember) is quite low and accounts for 6.8

and 15.1% of the rainfall in the Batinah and northeast

coast, respectively. For the interior stations, the highest

average monthly rainfalls are recorded between Januaryand April and that account for 64.5% of rainfall. The

rest of the months have relatively low average monthly


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Figure 3. Average annual rainfall in Oman. This figure is available in colour online at www.interscience.wiley.com/ijoc

rainfall. In the Salalah Plain, 78.4% of the rainfall is

recorded between April and August. Rainfall in the

months of NovemberMarch is relatively low (13.9%).

The majority of rainfall over the Dhofar Mountain ranges

(67.4%) occurs only in July and August (khareef rain).

Rainfall occurring in NovemberMarch is relatively low

(7%). Even though the Dhofar Mountains and Salalah

Plain are adjacent to one another, the amount and

distribution of rainfall vary.Average annual rainfall shows a positive relationship

with topography as depicted in Figure 5. In general, the

higher the elevation, the greater the average rainfall.

The correlation coefficient between elevation and rainfall

was 0.86 with data of the highest peak (1950 m above

sea level) and 0.69 without data of the highest peak.

The highest rainfall, 330 mm, was recorded at Saiq

with an elevation of 1950 m above mean sea level. The

Dhofar Mountains average 185 mm of rain annually. The

lowest rainfall, 76 mm, occurs in the interior region with

elevations ranging from 300 to 320 m. These stations are

located in dry desert conditions. For stations situated less

than 100 m above mean sea level, the average yearly rain

ranged between 71 and 133 mm only. This difference

in the yearly average rainfall may be attributed to the

different rainfall mechanisms.

3.2. Rainy days

A day is considered rainy when rainfall is greater than

1 mm. The annual average rainy days for the geographi-

cal locations and Oman are presented in Figure 6. The

average number of rainy days per year for Oman is

12.4. For the geographic locations the averages are:

northern Oman mountains, 13.0 days; Batinah coastalplain, 9.1 days; northeast coast 8.0 days; interior region,

8.2 days; Salalah coastal plain, 33.9 days; and Dhofar

mountain ranges, 46.0 days (Table II). The highest num-

ber of rainy days of 71, 70 and 68 days was recorded in

the Dhofar Mountains in 1986, 1985 and 1996, respec-

tively. On the contrary, some stations in the Batinah

Plain, the interior region and the northern Oman Moun-

tains did not record a single day of rain in 1985. On the

average, rain falls between 2 and 13% of the days in a

year. All the regions show a declining trend in the num-

ber of rainy days except Batinah coastal region and thenorthern Oman Mountains. These trends are however not

significant. The minimum observed p-value is 0.06.

There is a high correlation between the annual rainfall

and the annual rainy days as depicted in Figure 7. The

patterns are quite similar and indicate that a high number

of rainy days usually result in a high annual rainfall. For

example, the highest number of rainy days of 19821983

and 1997 correspond to the high rainfall years for all

the stations. The rainfall and rainy days in Salalah Plain

and Dhofar Mountains are different from the rest of the

country, suggesting a different mechanism of rainfall.

3.3. Maximum daily rainfallMaximum daily rainfall is an important indicator of flash

flood hazards. Oman is characterized by highly diverse

low and extreme heavy rainfall as shown in Figure 8. The

highest daily maximum rainfall of 431 mm was recorded

in Masirah Island in the interior region in 1977. The

next three highest daily values of 184, 162 and 133 mm

occurred in 1983, 1982, 1995 and 1997. There is no

pattern to the maximum rainfall over the years. Fourteen

maximum daily rainfalls of greater than 100 mm were

recorded in all the stations. Of these, five occurred in the

northern Oman Mountains, three each in the northeast

coast and interior regions, and one each in the BatinahPlain, Salalah Plain and Dhofar Mountains. Such high

recordings of greater than 100 mm of daily rainfall are


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Figure 4. Monthly rainfall distribution in Oman.

Table II. Yearly average number of rainy days and no-rain days in Oman.

Interiorregion

Batinahcoastal

plains

NorthernOman

Mountains

Salalahcoastal

plains

DhofarMountains

Northeastcoast

Total

No-rain days Mean 356.8 355.9 352.0 331.1 319.1 357.0 352.6

Std. 5.2 6.6 9.4 7.6 17.9 6.5 10.9

Rainy days Mean 8.2 9.1 13.0 33.9 45.9 8.0 12.4

Std. 5.2 6.6 9.4 7.6 17.9 6.5 10.9

Days with rain 50 mm Mean 0.1 0.3 0.3 0.2 0.2 0.2 0.2Std. 0.4 0.6 0.7 0.4 0.5 0.5 0.6


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Figure 5. Relations between average yearly rainfall and elevations.

Figure 6. Average yearly rainy days in Oman. This figure is available in colour online at www.interscience.wiley.com/ijoc

observed only once in all the stations for the year in which

they occur. The exceptions are 1983, which recorded

three (northeast coast, interior, and Salalah Plain) and

1982, which recorded two (northern Oman Mountains

and Batinah Plain).

In the northern Oman Mountains, the maximum daily

rainfall ranged from a high of 162 mm in 1982 and

1995 to a low of 36 mm in 1980 and 1984, respectively.These represent 55% of the average yearly rainfall in

1982, 56% in 1995, 43.3% in 1980 and 96.4% in 1984,

respectively. In the Batinah Plain, the daily maximum of

100 mm was recorded in 1982, and the lowest of 12 mm

in 1984. These represent 50.4 and 9% of the average

yearly rainfall. The next maximum rainfall in the interior

region, after the 1997, was 184 mm in Masirah Island in

1983. This event represents 65.7% of the yearly rainfall.

The lowest maximum rainfall of 7 mm in the interior

region was recorded in 1984. In Salalah Plain, the highestdaily extreme rainfall of 127 mm was recorded in 1983

and it account for 35.8% of the yearly rainfall. In the


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Figure 7. Relations between annual rainfall and rainy days in Oman. This figure is available in colour online at www.interscience.wiley.com/ijoc

Figure 8. Maximum daily rainfall in Oman. This figure is available in colour online at www.interscience.wiley.com/ijoc

Dhofar Mountains, the highest rainfall of 120 and 90 mm

was measured in 2002 and 1983, and it represents 36.6

and 21.8% of the respective yearly rainfall.

3.4. Rainfall intensity

Rainfall intensity is investigated in two ways. First, thetotal yearly rainfall is divided by the number of rainy

days for the stations, and secondly, rainfall is analysed

in four groups, consisting of light rainfall (R < 10 mm);

moderate (10 R < 25 mm, heavy (25 R < 50 mm),

and extreme (R > 50 mm). From Figure 9, northern

Oman Mountains, Batinah Plain and northeast coast show

high intensity rainfall between 1981 and 1983. Rainfall

intensity in northern Oman Mountains and Batinah Plainappears uniform with the exception of 19801983 and

19951997 with above mean values. The northeast coast


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Figure 9. Rainfall intensity in Oman. This figure is available in colour online at www.interscience.wiley.com/ijoc

shows higher than the average rainfall intensities for

most of the years, perhaps indicating different mechanism

or source for the rain. Dhofar Mountains and Salalah

Plain indicate low rainfall intensities than the rest of the

country. Rainfall in the two locations consists of drizzle,

mist and light rain. No significant trends are shown in

any of the intensities over the years.

Figure 10 shows the average number of days for the

four intensity groups. Table II and Figure 10 indicate

that 6695% of the rainy days in Oman has only light

rainfall. In Salalah Plain and Dhofar Mountains, light

rainfall accounts for 95.4 and 92.9%, respectively, of the

rainy days. The next common occurrence in Oman is

moderate rainfall representing 323% of the rainy days.

Moderate rainfall is quite substantial in northern Oman

accounting for 1723% of the days on which there is rain.

Heavy rainfall, mainly in northern Oman, occurs only 5.7

to 8.8% of the rainy days. Extreme rainfall events are

relatively rare and occur only 0.4 to 2.9% of the days on

which there is rain.

4. Discussion

The average yearly rainfall distribution indicates that

Oman experiences hyper-arid, arid and semi-arid con-

ditions. The rainfall patterns appear stable and pre-

dictable over the years. No major periods of drought are

observed and rain falls throughout the year in different

parts of the country. Rainfall in the northern and central

parts of Oman is dominated by the seif from Novem-

ber/December to April. This accounts for 61% of the

average rainfall for the country and 5883% of the rain-

fall in northern and central Oman. During this period,

cold frontal troughs originating from the North Atlanticor the Mediterranean pass over and bring rain to the

north and central parts of Oman. Rainfall over central

Oman is due to the influence of the Oman convergence

zone (Pedgley, 1970; Fisher and Membery, 1998). The

interior region shows high variability from year to year.

Some stations recorded zero rainfall over certain years.

The paucity of rain in parts of central and northern Oman

in JulySeptember is attributed to the restricted source of

low-level moisture and the shallowness of the monsoon

(Pedgley, 1970). Rainfall in southern Oman and, to some

extent, parts of the interior region is dominated by the

southwest summer (JuneAugust) monsoon that causes

frequent drizzle, fog and light rain in the Dhofar Coast

and the adjacent mountains. Yearly rainfalls at Salalah

and Masirah Island are affected by storms which are asso-

ciated with the northward migration of the inter-tropical

convergence zone (Pedgley, 1970).

The ability to observe trends in rainfall records is

quite important as the mechanism might be helpful in

predicting potential periods of drought or intense rainfall.

The number of years of rainfall record that is necessary

to detect any significant trends in rainfall in arid and

semi-arid regions is quite debatable. In general, areas that

show greater interannual variability may require longerterm records. This implies that due to the irregular and

unpredictability of rain, arid and semi-arid environments

may require long period data for any trends to be

observable. From the rainfall records between 1997 and

2003 no significant, positive or negative, trends are

observed in the Sultanate of Oman. Is it possible that such

trends could only be seen in datasets for several more

years? Or it is possible that such trends are not readily

predictable and that they are being obliterated/changed

over time by the ever changing local, regional and global

climatic conditions? Fisher (1994) observed that data

records of 10 years or more were sufficient to estimatemean total rainfall and other parameters. In a study on

the cyclicity of rainfall in Oman and the United Arab


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Figure 10. Distribution of rainfall (R) intensity for light rainfall (R < 10 mm); moderate (10 R < 25 mm); heavy rainfall (25 R < 50 mm)and extreme rainfall (R > 50 mm) in Oman.

Emirates, Brook and Sheen (2000) observed apparent 5-

year cycles for the rainfall in Muscat and Masirah Island

accounting for 15.6 to 22% of the data variance. The

study utilized rainfall records ranging between 50 and

101 years. Such predictive potential, if reliable, could

have significant impact on the management of crops,

livestock and people in Oman. From a 30-year rainfall

record analysis of the semi-arid southeast Spain, Lazaro

et al. (2001) observed neither trends nor abrupt changes.

However, they observed summer drought that indicatedstrong annual cycles. Trends and changes in rainfall

intensities in arid and semi-arid environments are not easy

to estimate and the same has been reported by several

researchers (Hulme, 1996; Gong et al., 2004).

Rainfall intensity measurements are valuable for the

prediction of catastrophic events. Rainfall exceeding

50 mm per day are uncommon in Oman; however, in

the event of their occurrence, they can cause serious

consequences such as flash flooding, human catastrophes

and land degradation. Such events represent only 0.4 and

2.9% of the rainy days of the year and appear to be

random. They can occur in any part of the country, butare most likely to occur in the northern Oman Mountains

and to some extent in the Batinah Plain. Nevertheless,


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they could still occur in the interior region (Masirah

Island), Salalah Plain and Dhofar Mountains, albeit at a

lower frequency. The highest average number of days of

extreme rainfall of 1.5 and 1.4% were recorded in 1982

and 1997, respectively, in the northern Oman Mountains.

Intense rainfall can account for a significant percentage of

the annual rainfall. For example, the 431 mm of rainfallon Masirah Island in 1977 represents 3 years of the

regional average yearly rainfall. The locations of high

rainfall events are essential to the recharging of the water

table. Several dams have been built in Oman to help

store water and recharge the water tables in case of such

episodic events. Any observable trends in such events

could help in the establishment of maintenance programs

for the dams.

Tropical cyclones and catastrophic rainfall events are

rare, but some have been recorded in Oman over several

years. A tropical cyclone that passed over the Batinah

Coast on 4th June 1890, dumped 300 mm of rainfall in

Muscat. The accompanying destruction caused the deathof 700 people and a massive loss of property. Northern

Oman and the United Arab Emirates experienced an

unusually heavy rainfall in 1995 surpassing the previous

years. For most of the locations, 1995 is the wettest year

on record and December 1995 is the wettest month. The

rainfall caused a lot of havoc including flooding, washing

away of roads as well as deaths (Membery, 1997).

The December rainfall for Muscat exceeded 100 mm. A

similar high rainfall occurred in December 1956 when

171 mm of rain was recorded in Muscat. The three

events in 1890, 1956 and 1995 suggest that Muscat

should expect such unusual high rainfall between 39 and59 years, with an average of about 52 years. The 1956

and 1995 events occurred in December, whereas the 1890

cyclone was in June.

Noy-Meir (1973) reported that the number of rainy

days in arid regions is between 10 and 50 days. Even

though the average rainy days of 12.4 for Oman (Table II)

satisfy that criteria, three other regions (Batinah Plain,

northeast coast and interior region) have average rainy

days between 8.0 and 9.1, which are slightly below

average. The high rainy days values of 45.9 and 33.9 in

the Dhofar Mountains and Salalah Plain are responsible

for the high national average value. Figures 3 and 6 and

the detailed datasets indicate positive relations betweenthe rainy days and monthly or yearly rainfall volumes.

However, such relations are completely altered/changed

with the occurrence of extreme rainfall (Section 3.3). The

average number of rainy days for the interior region is

higher than the northeast coast, but the reverse is true

for the average annual rainfall. With the exception of

the northern Oman Mountains and the Batinah Plain,

the other regions show declining trends in rainy days.

However, such trends are not significant.

5. Conclusions

The Sultanate of Oman is quite diversified in terms

of its northeastsouthwest extent, geomorphology and

climatic conditions. The approach used in the analysis

of rainfall data from 1997 to 2003 for 31 stations

was to divide the dataset into six regions that are

representative of the different geomorphic compartments.

Rainfall in the Sultanate of Oman is highly variable,

irregular and diversified. The yearly average rainfall

of 117.4 mm varies from a low of 76.6 mm in thehot, dry desert (interior region) to a high of 184.6 mm

in the Dhofar Mountains. In general, the monthly and

yearly patterns are stable and no significant trends were

observed over the study period. The majority of rainfall

in northern Oman occurs in the winter seif season from

November/December to April and accounts for 5883%

of the average annual rainfall. These are caused by mid-

latitude westerly depression by polar front jet stream.

Rainfalls in Dhofar Mountains and Salalah Plain are

dominated by the khareef in July and August and account

for 73 and 55%, respectively, of the yearly rainfall.

During this period, southwest monsoon causes frequent

drizzle, fog and light rain and transforms the area into

lush landscapes of green field and blossoming vegetation.

February and March are the most prolific rainfall months

in northern and central Oman and account of 35.3 to

45.5% of the annual rainfall. The annual average number

of rainy days varies from 8 in the northeast coast to 46 in

Dhofar Mountains with an average of 12.4 for the whole

country. With the exception of the Batinah Plain and

northern Oman Mountains, all the regions show declining

but no significant trends in the number of rainy days.

Rainfall in Oman is dominated by light rain ( 50 mm

rainfall per day) represents only 0.4 and 2.9% of rainfall.

However, such a rare event can occur in any part of

the country and can be catastrophic leading to loss of

life, flash flooding and land degradation. The records

indicate that 1983 recorded the highest rainfall in all

the geographic locations. The highest rainfall years of

1982 and 1983 were followed by the lowest rainfall in

1984. Similarly, high rainfall years of 19951998, which

were far above the yearly average, were followed by 4

consecutive years with rainfall of 20% or more below theaverages.

Acknowledgements

The data used in this study were obtained from the Water

Resources Division, Ministry of Regional Municipalities,

Environment and Water Resources, Oman.

References

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