Prospective of the utilization of rainfall in coastal regions in the context of climatic changes: case study of Egypt

Changes in climate drivers are projected to have a considerable impact on coastal freshwater supply and demand systems in future decades. The utilization of rainfall in coastal regions to secure sufficient freshwater to satisfy the rising demands of socioeconomic development has become a critical duty for decision-makers. This article assesses the present state of rainwater utilization in the water resources system for the coastal regions in Egypt. Volumes of annual rainfall that precipitate in 11 coastal Egyptian cities are estimated using ArcGIS maps and the run off rational equation. A future vision of using the estimated rainwater harvesting (RWH) from urban coastal cities in the water resources system is presented. Several scenarios are suggested and analyzed for using these volumes of water in the total water resources system in Egypt and also in the water resources system corresponding to each selected coastal governorate. The results indicated that over the areas under consideration, the annual maximum total amount of precipitation is limited to 1.8 km3. However, it is anticipated that 1 km3 of the average annual quantity of rainfall water is efficiently used for agriculture. These findings are intended to help managers, planners, and decision-makers to choose locations, invest in water resources, and employ RWH as a substitute for traditional water sources.


Introduction
Water resources should be properly managed to preserve water sustainability, especially in arid and semi-arid areas (ASARs), where population development, human activity, and the loss of fresh surface and groundwater are all ongoing trends. About 40% of the land area on earth is classified as ASARs (Tran et al. 2020;Alene et al. 2022). Arid regions receive about 150-350 mm of rain per year while rainfall in semi-arid regions ranges between 350 and 700 mm/year (Armanuos et al. 2020). Consequently, ASARs regions are characterized by low annual rainfall and varying distributions of temporal and spatial precipitation. As such, it is necessary to efficiently use the available rainfall in these regions as a freshwater source. Egypt has considered one of the world's arid regions and its average annual rainfall as a whole is very low (about 32 mm/year) but in some areas up to 200 mm/year (Gado 2020;Gado and El-Agha 2020). Both climate change and cities growing population have increased the stress on water resources availability in these cities. Such climatic change leads to an increase in the amounts of rainfall, especially in coastal cities. Only about 1.3 km 3 of rainfall is harvested from rural areas in Egypt that are either stored in lakes or used in irrigation and the rest quantities are lost. Rainfall precipitates a few days in the year with large intensity. Consequently, rainwater harvesting (RWH) in the winter season is necessary and it can be used in the Egyptian water resources system (Gabr et al. 2022a and b). There are several studies that showed the benefits of RWHs including water savings, mitigation of flood risks, efficient energy utilization, lowering carbon footprint compared to other water supply systems, addressing water scarcity problem, and reducing dependency on municipal water supply (El Zawahry et al. 2020). These studies classified the benefits of RWH into economic; environmental; technological, and societal. For economic benefits (Pelak and Porporato 2016) found that, when used RWH in Barcelona, Spain, RWH in dry and highly populated areas are not attractive with the current low water tariff. Tam et al. (2010) concluded that $240 per year per house could be saved from the annual domestic usage when RWH was applied in seven major cities in Australia. Fewkes (2012) deduced that the RWH benefit for water supply alone is economically less unless other benefits such as controlling the floods are taken into consideration. For evaluating the dual advantages of RWH, Snir and Friedler (2021) created a high-temporalresolution stochastic rainwater harvesting (RWH) model. Ranaee et al. 2021 recommended a framework for the water, energy, and food security nexus must be carefully considered when designing and implementing RWH projects [66]. In underdeveloped nations, the evaluation of the RWH project frequently emphasizes the need to conserve water, sometimes ignoring additional advantages that may be connected to the multipurpose character of RWHs. Shaaban and Appan (2003) found that up to 34% of domestic water uses can be satisfied and at the same time 10% reduction in peak discharge can occur when all houses in the residential area are installed with an RWH system. Gado and El-Agha (2020) tried to test the feasibility of harvesting rainwater in urban areas as a promising and sustainable solution to the water shortage problems in Egypt. They conducted a feasibility assessment of rainwater harvesting in more than 22 Egyptian cities. The results of the study showed that the annual volume of rainwater harvested could reach 142.5 MCM (million cubic meters) in the studied cities, provided that all the rain that falls on those cities is collected. The potential water from rain in the city of Alexandria, for example, can meet about 12% of the supplementary household water needs in the city. They noticed that the rainfall in central and southern cities is very little, compared to coastal cities such as Alexandria. Abdel-Shafy et al. (2010) reviewed and discussed the quantities and distribution of rainwater in different areas in Egypt. The National Institute of Oceanography, located on the Mediterranean, developed a Harvesting Pilot Plant for the first time in Alexandria. The harvested rain was used for irrigation and drinking purposes after being treated. Three declining roofs of areas of (38×4), (38×4), and (20× 6) m 2 with 2% slope were selected. The rain was collected through gutter pipes connected to the roofs and ends at a storage tank. Then, the rainwater was used to irrigate the surrounding landscape areas. Meanwhile, part of the water was, also, filtered using the sand filter. The filtered water was disinfected using a UV lamp. Then, the filtered water was examined to determine its suitability for drinking according to Egyptian guidelines (EEAA 1992). Generally, the flushing rain used to be directed to the nearby sea through a side pipe (Abdel-Shafy et al. 2010). Also, they expected the future quantity of rainfall to be 1.3 km 3 /year and it can be used as follows: 0.38 km 3 /year as a supplement for irrigation in the Nile Delta, 0.45 km 3 /year in Sinai, 0.2 km 3 /year in Red Sea Coast, and 0.27 km 3 /year in Alexandria and Marsa Matrouh. This research aims to evaluate the current situation of using rainfall in some coastal Egyptian governorates and investigate the possibility of using RWH from urban coastal Egyptian cities in the water resources system to decrease the gap between water demand and supply. To achieve this goal: (i) it is reviewed the water resources potentialities and uses in Egypt, (ii) rainfall data in the coastal Egyptian Cities are collected and analyzed from 11 stations: Al-Arish, Alexandria, Baltim, Dabaa, Elto, Hurghada, Kosseir, Marsa Matruh, Port Said, Ras Sedr, and Salloum Plateau, for rainfall record length ranges from 18 years at Hurghada station to 55 years at Alexandria station, (iii) RWH practices in Egypt's Governorates are assessed, (iv) water resources system for some coastal Egyptian Governorates are considered, and (v) use of total estimated RWH in the country water resources system are evaluated.

Climate changes consequences
One of the most significant challenges of the twenty-first century is climate change. According to the Intergovernmental Panel on Climate Change (IPCC), there will be considerable changes in the Earth's climate over the next few decades, which will make climate-related dangers worse (IPCC 2018). According to the IPCC (2014) and Kleidon and Renner (2018), increasing greenhouse gas (GHG) concentration in the atmosphere is having a significant impact on the hydrological cycle. By the end of the century, huge areas may experience frequent flooding and persistent drought (EEA 2019). Global hydrological cycles are predicted to vary non-uniformly by climate models. In Northern Europe, there is expected to be an increase in yearly precipitation, while in Central Europe and the Mediterranean, there is expected to be a significant decrease in the number of wet days, which will raise the likelihood of drought periods (IPCC 2018). Water scarcity is predicted to worsen globally, and this tendency will be compounded by factors including climate change, population growth, socioeconomic development, rising demand for energy and raw resources, the expansion of populated areas, and advances in technology (Brooks 2012). The Mediterranean is anticipated to be a "hotspot" for water scarcity due to rising competition for water resources, which threatens a sustainable balance between water demand and supply (OECD 2017).

Description of the study area
Egypt is located between longitudes 25° and 35° E and latitudes 22° and 32° N with a total area of 1,001,450 km 2 (Fig. 1). The elevation of the land varies greatly between Qattara Depression (133 m below the mean sea level) and Mount Catherine (2629 m above the mean sea level) (Nashwan et al. 2018). Egypt climate's is classified as arid to semi-arid. The deserts (e.g., the Sinai Peninsula, the Eastern Desert, and the Western Desert) represent about 96% of its total area. River Nile is the main source of water (about 90% of all available water) (Abdel-Shafy 2010). The deserts suffer from an acute shortage of water supplies and the primary source of water is rainfall and underground water resources. All four seasons-winter, spring, summer, and autumn-have different climates in Egypt, with winter being the wettest. Summers are hot and dry, though. The other two seasons (spring and autumn) are transitional periods. The average daily temperature ranges from 22.3 to 35.0 °C (maximum) and 10.0-22.5 °C (minimum). The length of the Mediterranean and Red Sea coastlines of Egypt is roughly 3200 km, with the Mediterranean coastline being about 1550 km and the Red Sea coastline totaling about 1705 km. Coastal lagoons, salt marshes, mudflats, sand dunes, and beaches extending along the Mediterranean coast, as well as mangrove trees and coral reefs in the Red Sea coastal zone, as well as biodiversity associated with these marine and coastal habitats, are examples of the aesthetic ecosystems and natural habitats that most coastal areas are characterized by (Coastal migratory birds-turtles-fish). Egypt's coasts include about 40% of Egypt's population. This dense population is centered in Egyptian industrial and commercial cities (e.g., Alexandria, Port Said, Damietta, Rosetta, and Suez). Three coastal governorates (i.e., Alexandria, North Sinai, and Matrouh) are selected to study their water balance and their available water resources are analyzed.

Alexandria governorate
It is comprised of the main Alexandria city, the city of Burj-Al-Arab, and the new city of Burj-Al-Arab. The area of the governorate is about 2819 km 2 (CAPMAS 2019), and it is ranked seventeenth in terms of area between the governorates of Egypt. The population of Alexandria is about 5,299,718 inhabitants (CAPMAS 2019). Alexandria is bordered on the south and east by Beheira Governorate and to the west by Marsa Matrouh Governorate.

North Sinai governorate
North Sinai governorate is one of the two governorates in the Sinai Peninsula. Geologically, the peninsula is divided into different zones. The northern part comprises a strip of loose sand and dunes with width ranges between 16 and 32 km, from the coast. After that, it levels off to a flat barren plain extending about 241 km and it comprises gravel and limestone, rising at its southern farthest point to the Plateau of El-Tih. Then, a rough arrangement of mountains and wadis runs from this level toward the southern tip of the Peninsula (El Afandi G and Morsy 2020). North Sinai governorate is located in the northeast of Egypt between longitudes 32° and 34° east and latitudes 29° and 31° north, and it is bordered by the Mediterranean Sea in the north with a length of 220 km, while in the south is South Sinai Governorate and it is bounded on the east by the political boundary of Egypt with Palestine, as for the west by Port Said, Ismailia, and Suez Governorates. It includes six centers: Bir Al-Abd, Nakhl, Al-Hasana, Arish, Sheikh Zuweid, and Rafah. The area of the governorate is about 27,564 km 2 and its population is about 463,957 inhabitants (CAPMAS 2019).

Water resources in Egypt
Water resources in Egypt are divided into conventional and non-conventional resources (Gabr 2020). Conventional water resources include Egypt's share from the river Nile; the limited quantities of rainfall; groundwater pumped from the eastern and western deserts and Sinai; and Saltwater desalination. While, non-conventional water resources include agricultural water reuse; and the exploitation of shallow groundwater in the valley and the Delta, which is a source of water either from infiltration of the river Nile or seepage of canals and drains. The current total required water in Egypt is about 110 km 3 /year. Therefore, Egypt imports virtual water in the form of food and agricultural products of about 30 km 3 /year (The amount of water needed to produce these products if they are grown and produced in Egypt). The remaining required water is equal to 80 km 3 /year divided by non-conventional recourses, equal to 20 km 3 /year, and conventional water resources equal to 60 km 3 /year. The values of conventional water resources are estimated to be 55.5 km 3 /year for Egypt's share of the river Nile, 2.5 km 3 /year for deep groundwater pumping, 1.3 km 3 for rainfall in addition to 0.7 km 3 /year for desalination of saline and semi-saline water. The agriculture sector is the largest use of freshwater (61.35 km 3 /year) with a percentage of 76.7% of the total available water, the drinking water uses about 10.75 km 3 /year with a percentage of 13.4%, industry uses 5.4 km 3 /year with a percentage of 6.8%, and the remaining 2.5 km 3 /year (3.1%) represents the evaporation from irrigation canals (Gabr 2020;NWEP 2017).

Water resources management in Egypt
There are some examples of water resources management in Egypt but not especially in urban areas. Omar and Moussa (2016) assessed various scenarios for 2025 using the Water-Evaluation and Planning Model. Various alternatives for planning water resources have been tested by them. Finally, three scenarios for the future have been recommended. This research is a good piece of work but it does not cover stormwater management in urban areas of Egypt. Awadallah et al. (2017) have attempted to assess the rainfall intensity equations used for drainage network design in Egypt. This research partly helps in the management of stormwater but mainly it criticizes the Egyptian Code's equations for rainfall intensity. Mahmoud et al. (2015) have identified rainwater harvesting sites in Egypt, this is very useful research on stormwater management but it is very general and not specific to urban stormwater management. Ibrahim et al. (2019) have investigated flood risk management strategies for Alexandria Egypt. They have produced a highly useful piece of research for Egypt that can be used by engineers, planners, and managers for stormwater management in Egypt.

Rainfall water harvesting computations
The annual volume of RWH from coastal cities can be calculated from the rational equation for the runoff: in which, AVR is the volume of rainfall that can be harvested from an urban area (m 3 /s), AR is the average annual rainfall (mm/h), A is the urban area (hectare), and C is a runoff coefficient (it depends on the surface type, buildings, asphalt, parks, and bare soil, it can be taken equal to 0.8 Asphalt areas (Gabr et al. 2022a). The estimation of the effective coefficient of runoff is shown in Table 1. ESRI (2000) created and maintains the ArcGIS family of client, server, and online geographic information systems. The first version of ArcGIS, known as ARC/INFO, a command line-based GIS system for data manipulation, was launched in 1999 (ArcGIS Pro 3.0 system requirements, https:// pro. arcgis. com/ en/ pro-app/ latest/ get-start ed/ arcgispro-system-requi remen ts. htm; ArcMap will be replaced by ArcGIS Pro, GeoGeek. 2017-07-06. https:// www. trott er-inc. com/ arcgis-deskt op-repla ced). Google Earth maps, digital elevation model maps, and land-use maps for Sallum, Marsa Matrouh, Dabaa, Alexandria, Baltim, Port Said, Al-Arish, Ras Sedr, El-tor, Hurghada, and Kossier were all obtained using the ArcGIS software. The land topography slope is calculated using the digital elevation model maps, while the asphalt roads, parks, buildings, bare soil, and asphalt areas are calculated from the landuse maps (Gabr et al. 2022a).

Potential rainfall in Egypt
There are more than 15 destructive flash flood events that occurred during the period from 1972 to 2015 as a result of extreme rainfall events (El Afandi and Morsy 2020). Recently, Egypt has been subjected to flash floods, especially in Sinai, the northwest coast, and the Red Sea coastline, Fig. 2. The Sinai Peninsula is the most region exposed to heavy rainfall that leads to flash flooding causing severe damage. Also, Red Sea coast flash floods are being under consideration due to the rising population and tourists, and damages are impacting life in this region. The rainfall events in the Eastern Desert are infrequent with flash floods very rare. The harvested rainfall is considered an important source of water for small Bedouin

Rainfall data in coastal Egyptian cities
11 stations' rainfall data were studied (Fig. 3) (Gado and El-Agha 2020). The rainfall record length ranges from 18 years at Hurghada station to 55 years at Alexandria station, with an average of 29 years (Gado and El-Agha 2020). Monthly and annual rainfall data (total rainfall and the total number of rain days) were considered.

Total rainfall
In Egypt, the spatial variation of the average annual total precipitation is shown in Fig. 4 (Gado and El-Agha 2020). The overall average annual rainfall is 80.45 mm (varies from 10 to 172 mm). On the North Coast, average annual rainfall exceeded 100 mm at Alexandria (172 mm), Baltim (140 mm), Marsa Matrouh (122 mm), and Dabaa (112 mm) stations. As such, a significant amount of rainwater can be harvested in these locations. The rainfall intensity decreases to the east at Port Said to be 99 mm/year. The average annual precipitation over the coastal part of the Sinai Peninsula, according to the available data, is generally less than 100 mm. In the northern zone at El-Arish, the average annual rainfall is equal to 87 mm/year. The yearly rainfall in other coastal cities in the Red Sea strip, according to the available data, ranges between 45 mm/year at Hurghada and 10 mm/year at Ras Sedr.  (23 days), and Al-Arish (20 days). The average monthly rainy days range from nine days at Alexandria and Baltim during January to less than a day in several cities, especially through the summer months. As such, Egypt's rainy season extends from October to March, with the peak precipitation during December, January, and February.

RWH practice in Egypt's governorates
In the year of 2014, the Egyptian government changed its view of floodwater, resulting from heavy rainfall, from an ordeal to a grant that could be used as an additional water resource. This can be done through the Egyptian government's five-year plan from 2014 to 2019. Several attempts are carried out to manage floodwater in a way that serves the water resources system in Egypt. Management of floodwater is also performed to overcome the day-after-day increase in water demand as well as to reduce the dangers of this destructive floodwater on both the constructions and lives. Flood water has been actually harvested from non-urban/  rural areas in several Governorates in Egypt using many techniques such as small dams, guide barriers, water transfer canals, artificial lakes, ground tanks, and flood outlets. These techniques (about 1070 constructions) can receive the incoming flood water and safely transfer it either to the river Nile or the canals and drains networks. The total amount of RWH using these techniques can be estimated equal to 1.3 Km 3 /year (Al-Zayed 2020).

Red sea governorate
There are the following techniques used for RWH in the cities of the governorate: 84 main flood outlets, and many secondary flood outlets in addition to 16 small dams, 12 artificial lakes, and 12 earthen barriers behind the lakes of full capacity equal to MCM of water. The most recent flood protection projects in the governorate are four storage dams with a total storage capacity of 2 MCM of water. These dams are Umm Samra Dam, Umm Harina Dam, 3 Dam, and Umm Dheis Dam which protect Sheikh Shazly village and its religious monuments from the risks of water torrents. Also, there are a guide barrier and an artificial canal in the village of Arab Saleh.

South Sinai governorate
There are several floodwater protection techniques used for RWH in the South Sinai governorate such as El-Awaj Lake in El-Toor city; the Mayer Dam, Habran Dam, and El-Jibi Dam in the city of Dahab; Lake 1 in Taba; El-Nakb lakes 1 and 2 in the city of Nuweiba; and Shabiha dam, and Lake 1 in Al-Sheikh Wadi in the city of Catherine.

Marsa Matrouh governorate
Marsa Matrouh is one of the governorates in Egypt that is exposed to severe rainfall, which can often turn into torrential rains. Several ground tanks are constructed to harvest rainwater, instead of wasting it and reusing it for several purposes such as: increasing the agricultural area and supplying drinking water.

Cairo and Beni Suef governorates
In Cairo Governorate, M1 and M2 dams were constructed to protect Wadi Degla from floodwater risks. A dam is constructed in a flood outlet in 15 May city, Beni Suef Governorate. There are also several flood outlets in the Governorate. Asyut and Aswan Governorates: there are several floodwater protection techniques used for RWH in Asyut Governorate such as M3 and M4 dams; and flood outlets in El-Badary Center. Also, there are several works for floodwater protection in Aswan such as artificial lakes and earthen barriers to protect Wadi Abbadi in Edfu; and an artificial lake in Wadi Abu Sabira used to protect the Wadi from the dangers of floodwater.

Luxor and Minya governorate
There are artificial lakes to protect Wadi El-Saidah and El-Umda (1, 2) from floodwater in Luxor Governorate. Also, flood outlets existed in El-Sheikh Ebada in Minya Governorate.

Sohag and Qena governorate
In Sohag Governorate, Protection works have been carried out in the village of El-Hajer in Al-Jalawiya, Saqulta district. Also, an artificial lake and an earthen barrier are constructed to protect the village of Hagazaa in the Qous Center, Qena Governorate.  Table 3 summarizes the water balance for Alexandria Governorate. In addition, Fig. 5 shows a schematic view demonstrating the available water resources for Alexandria Governorate and their consumption for several purposes including drinking and domestic uses; industry; and agriculture activities (Al-Zayed 2020). As can be seen, the Mahmoudia Canal and its branches get approximately 5.8 km 3 /year of water from the Nile. Marsa Matrouh and Beheira governorates each receive 3.4 km 3 /year of this water, but Alexandria governorate only receives roughly 2.4 km 3 /year of water from the Nile. Rainfall shares with 0.12 km 3 /year, evaporation losses are estimated equal to 0.1 km 3 /year, and consequently, the total net available water from both the Nile and rainfall is equal to 2.42 km 3 /year. 1.09 km 3 /year of this water is used for drinking and domestic uses with actual consumption of about 0.112 km 3 /year. Industrial water losses, pipe network leakage, and household sewage water losses are estimated equal to 0.055 km 3 /year, 0.29 km 3 /year, and 0.634 km 3 /year, respectively. The remaining water is about 1.33 km 3 /year mixes with reclaimed wastewater of about 0.014 km 3 /year and the total volume of available water becomes 1.344 km 3 /

Alexandria governorate
year used for both industry and agriculture activities. 0.196 km 3 /year of this water is used for the industry with actual consumption equal to 0.02 km 3 /year and 1.074 km 3 / year specializes for agriculture with an actual consumption equal to 0.601 km 3 /year. Total water drainage to the sea is estimated at about 1.343 km 3 /year [equal to 0.634 km 3 / year (untreated sanitation water) + 0.176 km 3 /year (industry losses) + 0.472 km 3 /year (agriculture losses) + 0.074 km 3 / year (ecological use) −0.014 km 3 /year (reclaimed wastewater)]. According to the water balance equation, water supply should equal water demand. It can be noticed from this table, the percentages of the River Nile, Rainfall, and drainage reuse from the total available water resources are equal to 94.71%, 4.73%, and 0.56%, respectively. Thus, the percentage of rainfall share is only 4.73% and it may come from RWH in some non-urban/rural areas in the governorate. It is known that there is no rainfall sewer network in urban areas of Alexandria Governorate and a large part of rainfall is useless and drainage to the sea. It can also be seen from this Table, the water uses in agriculture, drinking, and industry have percentages equal to 42.38%, 43.01%, and 7.73%, respectively, of total water resources. The percentage of both evaporation and ecological use is about 6.88%. For drinking and domestic uses, only 10.28% actually used the water specialized for this purpose and the remaining is lost in production, leakage, and sanitation from domestic. For agriculture, 55.96% actually used and the remaining is losses while 10.2% is only actually used for industry.  Table 4 summarizes the water balance for North Sinai Governorate. In addition, Fig. 6 shows a schematic view demonstrating the available water resources for North Sinai Governorate and their consumption for drinking and domestic uses; industry; and agriculture activities (Al-Zayed 2020). It is shown from Fig. 6 (Fig. 6), rainfall, and groundwater with percentages of 47.67%, 34.37%, and 17.96% of total water resources. Thus, the percentage of rainfall share is 34.37% of total water resources harvested from non-urban/rural areas. Figure 6 indicates the percentages of water use for agriculture; drinking and domestic; and industry of total water resources are 86.74%, 12.59%, and 0.67%, respectively. For drinking and domestic uses, only 9% is actually used from the water specialized for this purpose, and 47.53% of this water is drained to the desert while 43.47% is recharged to the groundwater. For agriculture activities, 80% is actually used and the remaining 20% is recharged to the groundwater while 45.66% is actually used for industry. The percentage of total water drainage to the desert is about 11.21%.

Prospects of RWH from coastal Egyptian cities
During specific storm events, various coastal Egyptian cities are exposed to significant amounts of rainfall, which may lead to flash floods and inundation problems. As such, to overcome these problems, RWH from these cities has to be carried out. Also, this harvested rainwater can be used in the water resources system to decrease the growing gap between supply and demand, due to the increase in population. Several factors affect RWH: pattern and intensity of rainfall; variation of seasons; topography; evapotranspiration; type of soil and its holding capacity for water; attachment area and its location; and land use. The selected coastal cities are Sallum, Marsa Matrouh, Dabaa, Alexandria, Baltim, Port Said, Al-Arish, Ras Sedr, El-tor, Hurghada, and Kossier, Fig. 3  Volumes of the annual RWH from some selected Egyptian coastal cities and the 1st Department Region in Al-Arish city, respectively, that utilized in the RWH volumes. In addition, Fig. 9 demonstrates the calculated annual volumes of rainfall [using Eq. (1)] that can be harvested from the selected coastal cities. It can be noticed that the total annual volume of RWH from coastal cities in the Mediterranean Sea and the Red Sea is equal to 76.36 MCM/year, assuming that all rainfall precipitated on urban areas is collected. The minimum value is noticed at Ras Sedr (0.03 MCM/year) while, the maximum value is at Alexandria (59.6 MCM/year). Alexandria has the largest value (which represents 78% of total possibility of harvesting water) because it having the maximum average annual rainfall and urban areas. The percentage of rainwater that can be harvested from coastal cities on the Mediterranean Sea represents 97.17% of the total possibility of harvest water while the rest percentage of 2.83% is corresponding to the Red Sea cities. Consequently, the harvested rainwater from coastal urban cities on the Mediterranean Sea increases by a factor of 34 to other coastal urban areas on the Red Sea.
in the jurisdiction of each company and the required future increase in water supply. The percentages of satisfaction with future water needs depending on RWH are also listed. From Table 5, it can be noticed that the potential water savings ranged from 1.15% at the Red Sea Water Company to 11.9% at the Alexandria Water Company. Thus, 11.9% of the future additional domestic water can be fulfilled in Alexandria city depending on its urban RWH. 6.8% of the future additional domestic water can be satisfied by RWH from Salloum, Marsa Matrooh, and Dabaa. In Sinai, the urban RWH of Al-Arish city in the north and Eltor and Ras Sedr cities in the south can give 6.67% and 8.2% of the future extra additional domestic water needs of both North and South Sinai companies, respectively. The urban RWH of Port Said city only can satisfy 1.8% of the future domestic water increase in the company of canal cities.

Conclusions
In this study, it can be concluded that: 1. At the present time, rainfall contribution is minor to Egypt's water resources. However, it is expected to play an important role in the coastal zone which extends for more than 3500 km and houses more than 40% of the population. This dense population is centered around industrial and commercial cities (Alexandria, Port Said, Damietta, Rosetta, and Suez). As the population increases, the water supply per capita decreases. 2. Most rain events in Egypt are local, while events with high spatial variability are rare with short duration. Some heavy rainfall occasions, in autumn and winter, may cause flashflood. 3. These events are mainly happening over the Red Sea Mountains and the Sinai Peninsula. RWH can be implemented mainly on Egypt's north coast as this region have the amount of rainfall that can be collected. However, the rest of the country has rare rainfall.
4. Over the areas under consideration, the annual maximum total amount of precipitation is limited to 1.8 billion m 3 . However, it is anticipated that 1 billion m 3 of the average annual quantity of rainfall water is efficiently used for agriculture. 5. RWH can be implemented mainly on Egypt's north coast as this region have the amount of rainfall that can be collected. However, the rest of the country has rare rainfall.