**2. Rainfall distribution in the KSA and recent problems**

The KSA climate is mainly arid and semiarid, except in the northern and southern areas. The arid and semiarid regions have an extreme continental climate with warm and dry summer and very cold winter especially in the central regions of the KSA. To assess rainfall pattern over the study area, it is necessary to define the dominant climatic patterns that have an influence on rainfall distribution over the western province of the KSA. The climatic pattern can best be described by considering the various air masses that affect the rainfall distribution over the KSA area. The influence of the different air masses and the rainfall patterns over the KSA has been discussed and mapped by several investigators [35–38].

Different air masses, which influence the Kingdom's climate, are illustrated in **Figure 1**. These air masses include (1) the monsoon front during the late autumn (maritime tropical air mass) reaches the area from the south, southwest, and southeast. This front that originates in the Indian Ocean and the Arabian Sea during the autumn brings warm and moist air. Outbreaks of westerly air become more frequent, characterized by medium to high intensity over the western and northwesterly regions of the country. This front often picks up further moisture while moving through the Red Sea Trough. (2) The continental tropical air masses are warm and moist coming from the Atlantic Ocean through the Central and North African continent. (3) The maritime polar air masses are derived from the Eastern Mediterranean

#### **Figure 1.**

*Air masses affecting the climate of the KSA: (1) maritime tropical air masses, (2) continental tropical air masses, and (3) maritime polar air masses [39].*

Sea. In early winter the Mediterranean-borne maritime air increasingly disturbs the monsoonal air movement and displaces it in the low altitudes. These maritime depressions draw the tropical continental air masses into the warm sectors, and extreme weather conditions occur that are associated with the passage of a hot sector. Both (2) and (3) move toward the east and prevail in the winter season. During this season, the western region, particularly the coastal area, is characterized by its relatively low rate of rainfall [38], whereas, due to the topographic effects, the highlands receive a considerable amount of rainfall. In spring, the impact of the Mediterranean air movement diminishes, whereas the monsoon from the south takes its place, penetrating the southern part of the Kingdom. During summer, the cyclonic flow sweeps along the Mediterranean Sea from the west toward the east and continues moving over the northern and central regions of the country preventing the maritime air masses of the northeasterly monsoon from penetrating the north regions of the Kingdom. Due to this, the summer season will be somewhat dry in the area considered.

The mean annual rainfall (from 1960 to 2018) over the KSA can be shown in **Figure 2**. The rainfall rate in the KSA (except for the Empty Quarter desert) is 109 mm per year. The southwestern region is characterized by a heavy rainfall compared with the rest of the Kingdom. Rainfall is more than 500 mm per year in some locations. The peaks of the Sarawat Mountains, which extend from the northern part of the Kingdom to the south, parallel to the Red Sea, are the dividing line for the distribution of surface water. The rains that fall in the east of this line take place in the valleys heading east, and in the west, the valleys are steep and flowing towards the Red Sea.

Recently, there is a definite increasing trend in rainfall in frequency and intensity in the KSA. Many areas in the KSA have shown an increasing trend in the annual rainfall and flood events. Most of the rainfall in the KSA occurs during the monsoon. Monsoon is a term derived from the Arabic word "mausim," meaning season. It was used to describe the seasonal winds of the Arabian Sea. The Jeddah area is characterized by wet and dry seasons that generally occur from November to May and from June to October, respectively. One example is Jeddah city which has shown an increasing trend in the rainfall events. These events cause disasters that result in human, property, and economic loss.

**Figure 2.** *Average annual rainfall values for 270 rainfall stations distributed across the KSA area (from 1960 to 2018).*

**21**

**Figure 3.**

*Study area and its meteorology station locations.*

*Statistical Analysis of Rainfall Patterns in Jeddah City, KSA: Future Impacts*

These unprecedented events have affected the KSA, causing considerable damage to highways, railroads, urban zones, and agricultural areas [39]. Most of the flash flood hazards in the KSA are caused by a combination of natural conditions (heavy rainfall and climate changes) and human interference (poor drainage systems and urban expansion). Recently, heavy rainfall events have triggered flash floods in various areas of the KSA (e.g., Jeddah city flash floods in the years 2009, 2011, 2015, 2017, and 2018 and Al Riyadh flash floods in 2015 and 2018). The severely hit areas are generally in the western part of the KSA, particularly in the city of Jeddah during November 2009 and January 2011 [39, 40]. These events were characterized by 70 and 111 mm of rainfall, respectively, within 3 h and were considered catastrophic flash floods for Jeddah city. They caused a death toll of 113 people in 2009, and, together, they damaged more than 10,000 homes and destroyed approximately 17,000 vehicles. Other areas impacted by flash floods are As Sail Al Kabir area, Taef; Najran city; Ar Riyadh city; Ha'il city; Makkah city; and Tabuk city [41–48].

The study area, the Jeddah area, is located in the western region of the KSA

and lies between latitudes 21°15′ and 21°57′N and

*DOI: http://dx.doi.org/10.5772/intechopen.86774*

**3. Study area and its characteristics**

covering an area of ~1731 km2

*Statistical Analysis of Rainfall Patterns in Jeddah City, KSA: Future Impacts DOI: http://dx.doi.org/10.5772/intechopen.86774*

These unprecedented events have affected the KSA, causing considerable damage to highways, railroads, urban zones, and agricultural areas [39]. Most of the flash flood hazards in the KSA are caused by a combination of natural conditions (heavy rainfall and climate changes) and human interference (poor drainage systems and urban expansion). Recently, heavy rainfall events have triggered flash floods in various areas of the KSA (e.g., Jeddah city flash floods in the years 2009, 2011, 2015, 2017, and 2018 and Al Riyadh flash floods in 2015 and 2018). The severely hit areas are generally in the western part of the KSA, particularly in the city of Jeddah during November 2009 and January 2011 [39, 40]. These events were characterized by 70 and 111 mm of rainfall, respectively, within 3 h and were considered catastrophic flash floods for Jeddah city. They caused a death toll of 113 people in 2009, and, together, they damaged more than 10,000 homes and destroyed approximately 17,000 vehicles. Other areas impacted by flash floods are As Sail Al Kabir area, Taef; Najran city; Ar Riyadh city; Ha'il city; Makkah city; and Tabuk city [41–48].
