JKAU: Met., Env. & Arid Land Agric. Sci., Vol. 22, No. 2, pp: 59-78 (2011 A.D./1432 A.H.) DOI: 10.4197/Met. 22-2.4 Analysis of Rainfall over Saudi Arabia A. Mashat and H. Abdel Basset Department of Meteorology, Faculty of Meteorology, Environment, and Arid Land Agriculture, King Abdulaziz University, Jeddah, Saudi Arabia Abstract..In this study, rainfall over the Kingdom of Saudi Arabia (KSA) is analyzed using data from available meteorological stations. The analysis of these data shows that the highest amount of rainfall occurs during the spring over the southwest, middle and east of KSA. The second highest amount of rainfall occurs during winter over the east and northeast of KSA. The season of the lowest amount of rainfall over KSA is summer, when, except in the mountainous area in the southwest region, the amount of rainfall is very small. While the horizontal distribution pattern of rainfall during autumn is somewhat similar to that during spring, the amount of rainfall in autumn is smaller than that in spring. The most significant synoptic feature appears in spring when the cold air associated with secondary Mediterranean depressions meets the moist hot southerly air associated with Red Sea trough over KSA. 1. Introduction The amount of rainfall that falls over an area is an important factor in assessing the amount of water available to meet the various demands of agriculture, industry, and other human activities. Study of the distribution of rainfall over time and space is therefore very important for the welfare of a national economy. Many applications of rainfall data are enhanced by knowledge of the actual distribution of rainfall over a specified area rather than relying on simple summary statistics. In recent years, several studies have been devoted to the interannual and decadal variability of precipitation on regional, national and international scales. Different methodological approaches have been used, such as temporal analysis of 59 60 A. Mashat and H. Abdel Basset rainfall (Serrano, et al., 1999; De Luís, et al., 2000; Estrela, et al., 2000; Agnese, et al., 2002; Turkes, et al., 1996), analysis of extreme events in regional daily precipitation series (Brunetti, et al., 2002 and 2004), and use of the standardized precipitation index (Delitala, et al., 2000; Lana, et al., 2001; Bordi, et al., 2001; and Lloyd-Hughes and Saunders, 2002). Several researchers have analyzed hydro-meteorological time series in West Africa from Niger to Senegal (Carbonnel and Hubert, 1985 and 1994; Snijders, 1986; Hubert and Carbonnel, 1987; and Paturel, et al., 1997). They pointed out the non stationarity of the series and suggested a climatic change between 1965 and 1972, with the majority of the shifts appearing between 1969 and 1970. Precipitation in the Great Plains of the United States also changed significantly with an increase since the late 1960s and the last two decades of the twentieth century being the wettest of that century (Garbrecht and Rossel, 2000). A report from the Office of Technology Assessment (1993) pointed out that the anticipated intensification of the hydrological cycle would increase global rainfall by 7 to 15 % and evapotranspiration between 5 and 10%. An increase in extreme events that will be disruptive to natural and human systems is likely to occur (IPCC, 1995). However, whereas in certain areas an increase of rainfall is expected, other areas will suffer from decreased rainfall. The aim of this study is to analyze the climatology of monthly and annual rainfall over the Kingdom of Saudi Arabia (KSA). The remainder of this paper is organized as follows: In Section 2, data from meteorological stations in KSA and the homogeneity of these data are described. Section 3 presents the results of an analysis of monthly, annual and horizontal distributions of rainfall over KSA. Finally, conclusions are offered in Section 4. 2. Data from KSA Meteorological Stations Because of its low annual precipitation, KSA is commonly regarded as a dry country. This is a climatic simplification; however, because precipitation varies over space and time and of the amounts of precipitation over particular regions of KSA are diverse. Besides spatial differences, interannual variations of precipitation also occur. The complexity of the rates of precipitation over KSA derives from the vast 2 area of the country (about 2,250,000 km ), its wide latitudinal expanse (15.5°N - 32.5°N) and its pronounced topographical relief. Monthly Analysis of Rainfall over Saudi Arabia 61 precipitation records are compiled for 28 stations managed by the KSA Meteorological Organization. The stations are distributed across KSA, although their spatial density is low and uneven in some parts of the country. Table 1 gives the name, position, elevation and the available period of data from each KSA meteorological station. Studies on data homogeneity are essential to climatology. Homogeneity is manifested differently depending on the climatic element. The values of climatic measures of these climatic elements could be used in estimating daily and monthly averages. Artificial lakes and reservoirs and other man-made changes to the local environment produce sources of inhomogeneity in historical records of climatic data. Homogeneity in rainfall over cities in KSA has been examined by means of the Bartlet test. Table 1. The name, position, elevation and the available data period for each KSA meteorological station. No Name Lat Long Elevation Available Data Years 1 Turaif 31.68 38.73 852.44 1978 --- 2003 26 2 Gurait 31.40 37.28 503.90 1985 --- 2003 19 3 Arar 30.90 41.14 548.88 1979 --- 2003 25 4 Aljouf 29.78 40.98 668.74 1978 --- 2003 26 5 Tabouk 28.37 36.60 444.10 1978 --- 2003 26 6 Hail 27.43 41.69 768.11 1970 --- 2003 34 7 Wejh 26.20 38.47 357.60 1978 --- 2003 26 8 Rafha 29.62 43.49 413.00 1978 --- 2003 26 9 Alqusoma 28.31 46.13 1001.52 1978 --- 2003 26 10 Hafrbaten 27.90 45.53 646.71 1985 --- 2003 19 11 Dhahran 26.25 50.16 16.77 1970 --- 2003 34 12 Ahsa 25.29 49.48 23.73 1985 --- 2003 19 13 Gassim 26.30 43.76 178.17 1978 --- 2003 26 14 New Riyadh 24.92 46.72 613.55 1985 --- 2003 19 15 Old Riyadh 24.71 46.73 619.63 1970 --- 2003 34 16 Wadi Aldwaser 20.50 45.25 635.60 1978 --- 2003 26 17 Madina 24.54 39.69 3.58 1970 --- 2003 34 18 Yenbo 24.14 38.06 1452.75 1978 --- 2003 26 19 Jeddah 21.71 39.18 240.35 1970 --- 2003 34 20 Makkah 21.43 39.79 701.02 1985 --- 2003 19 21 Taif 21.48 40.55 614.39 1970 --- 2003 34 22 Baha 20.29 41.64 1651.88 1985 --- 2003 19 23 Bisha 19.99 42.61 1161.97 1970 --- 2003 34 24 Abha 18.23 42.66 2055.93 1978 --- 2003 26 25 Khamis Mush 18.29 42.80 2093.35 1970 --- 2003 34 26 Najran 17.61 44.41 1212.33 1978 --- 2003 26 27 Sharura 17.46 47.10 724.65 1985 --- 2003 19 28 Gizan 16.90 42.58 7.24 1970 --- 2003 34 62 A. Mashat and H. Abdel Basset Table 2 presents Bartlet test (short-cut) results for mean annual rainfall at each station. The mean annual rainfall seems to be homogeneous except at the Tabouk and Sharorah stations when 22 (SSmax/ min ) is used with 95% significance as discussed by Mitchell et al. (1966). The mean annual rainfall amounts at the Aljouf and Abha stations indicate some inhomogeneities. Table 2. Bartlet test (short- cut) result for the KSA stations (n is the number of terms in each subperiod k, and k is the number of the subperiod). 95% Station N k Homogeneity Significant point Turaif 13 2 3.28 1.56 Guriat 9 2 4.43 2.17 Arar 13 2 3.28 3.25 Aljouf 13 2 3.28 3.36 Rafha 13 2 3.28 1.48 Tabouk 13 2 3.28 6.23 Alqaisoma 13 2 3.28 1.55 Hail 11 3 4.85 1.68 Qassim 17 2 2.76 2.69 Dhahran 15 2 3.16 2.33 Alwajh 13 2 3.28 3.17 Alahsa 9 2 4.43 3.25 New Riyadh 9 2 4.43 3.52 Old Riyadh 17 2 2.76 1.32 Madinah 17 2 2.76 1.59 Yanbo 13 2 3.28 2.59 Jeddah 17 2 2.76 1.73 Taif 17 2 2.76 2.77 Makkah 9 2 4.43 3.96 Albaha 9 2 4.43 1.85 Bisha 11 3 4.85 1.92 Khamis Moshet 11 3 4.85 3.62 Abha 13 2 3.28 3.31 Najran 8 3 6.94 1.15 Sharorah 9 2 4.43 6.32 Gizan 11 3 4.85 1.57 3. Results and Discussion 3.1 Monthly Average Rainfall Figure 1(a) shows the average monthly amounts of rainfall for each month of the year at the Arar, Turaif, Guriat, Aljouf, Tabouk and Hail stations. The maximum amount of rainfall at these stations occurs in January, and the average amounts of rainfall in January at all stations Analysis of Rainfall over Saudi Arabia 63 exceeds 10 mm except for Tabouk. The second rainiest month at these stations is December. Hail and Turaif receive the highest amount of rainfall during the rainy months of the year. The months from June to September are the dry months of the year, while the winter and spring months are the wet months at these stations.