Selection of Dam Sites for Rainwater Harvesting in Western Desert of Iraq: A Case Study in Wadi Al-Mohammadi Basin

Abstract


Introduction
In dry areas, basins are usually exposed to unexpected and irregular rainstorms, sometimes resulting in floods that may endanger human areas.Flood risks rise as land cover and land-use change, particularly when vegetation is removed, causing an increase in surface runoff rates and a decrease in infiltration rates, resulting in an increase in the proportion of flood hazards in basins (Jasim, 2021).Although floods occur within a few periods, they are dangerous for humans and their farms, and on the other hand, long periods of drought occur, so it is necessary to harvest this water.Among the first of these solutions is the construction of dams.Dam construction is one of the primary water management strategies to cope with the effects of floods and droughts (Noori et al., 2019).Dams can protect the lands from the danger of floods or can be used to collect water in dried seasons (water harvesting).
The process of selecting water harvesting systems that are suitable for a given location is subject to several considerations, some of which are general and pertaining to the overall water harvesting process, and some are specific to the climatic, geographic, hydrological, and human conditions within the area of application (Manhi, 2021).Water harvesting is one of the most important aspects of investing in rainwater and surface runoff because it can deal with various circumstances related to the nature of the studied area.
Interest in rainwater harvesting has been renewed in recent decades, due to the growing demand for water for urban development and agriculture caused by climate change and higher population pressure.There are numerous approaches to improving water resource management in Iraq, including the use of rainwater harvesting techniques (Al-Khafaji et al., 2021).Increases in knowledge, application, implementation and administration (management) of rainwater harvesting systems are also a result of this interest (Ben Mechlia and Ouessar, 2004;Oweis et al., 2012).The sustainability of this harvested water is also necessary and is done through continuous evaluation of water in terms of quantitative and qualitative (Al-Kubaisi et al., 2021).The study area is undergoing the influences of the dry desert climate and to a lesser degree the influences of the climate of the Mediterranean and the Arabian Gulf (Al-Alwany et al., 2018).The climate has directly affected the abundance of water as a natural resource in rural and urban environments (Al-Kubaisi, 2020).
The Western Desert is rich in previous studies related to water resources, and the most important of these recent studies are: The study that was presented by (Awadh, 2018) studied the suitability of groundwater and surface water resources in the Western Desert.Determine the hydrogeologic conditions and characteristics of the Mulussa aquifer in Western Iraq were studied (Al-Kubaisi and Al-Kubaisi, 2018;Al-Kubaisi and Al-Kubaisi, 2022b).(Awadh et al., 2021;Hussien et al., 2022) focuses on the nature of the aquifers and characteristics of groundwater in the Western Desert of Iraq and Arabian Gulf countries.Concerning the study area in particular, the study of (Al-Kubaisi and Al-Kubaisi, 2022a;Al-Kubaisi and Al-Kubaisi, 2023b;Al-Kubaisi and Al-Kubaisi, 2023a), focused on hydromorphometric analysis and water harvesting.However, there are few studies related to water harvesting, despite the availability of many ephemeral valleys that flow into the Euphrates River, including a study conducted by (Al-Hadithi and Al-Araji, 2016) and (Al-Jawad and Al-Ansari, 2017).Where they studied Wadi Hauran, but many valleys need to be studied, including Wadi Al-Mohammadi, which this study focused on.
Geologically, the study area was characterized by the presence of several outcrop formations configured from the oldest to the youngest, Ms`ad, Euphrates, Fat`ha, Nfayil, Zahra Formations, and Quaternary Deposits (Sissakian and Mohammed, 2007).Structurally, The Abu-Jir Fault is the main structural phenomenon in the basin, as it cuts the eastern part of the basin extending towards northwestsoutheast.It is formed due to the rifting of the northeast passive margin of the Arabian Plate and suffered by right-lateral strike-slip movement.The Abu-Jir Fault System never continues northward, but it turns in Hit vicinity westward to meet Anah Graben (Abdul-Jabbar, 2013).The structural situation is very important and must be taken into account when choosing the location of the dams.
Suggesting dam sites for the Wadi Al-Mohammadi basin, as well as developing several scenarios for collecting water as lakes behind the proposed dams is the objective of this study.This objective was important because the study area had no previous studies related to water harvesting, and there were no dams established on Wadi Al-Mohammadi.

Study Area
Wadi Al-Mohammadi is located on the eastern edge of the Iraqi Western Desert, it is an ephemeral valley that drains into the Euphrates River, 15 km south of Heet city.The basin is bordered on the east by the Euphrates River and the south by the Abu-Jir depression, while from the north, the topographic boundary of the Wadi Al-Mohammadi basin is represented by Wadi Al-Hajiya of the Kubaisa basin, and the Kilo-160 area represents the western borders of the basin.The highway (Baghdad -Jordan, and Syria) passes through the basin.The basin is bounded by the following coordinates: Latitudes 33˚ 34ʹ 18.1ʺ-33˚ 4ʹ 44.4ʺ and longitudes 41˚ 55ʹ 3.8ʺ -42˚ 56ʹ 31.6ʺ (Fig. 1).The total area of the basin is 2286.8km 2 , while the basin's perimeter was estimated at 487 km.Its elevation above sea level ranges from 52 to 367 m.

Data Set
The map used in this study: is the Tectonic Map of Iraq (Jassim and Buday, 2006).The locations of the faults in the basin were extracted from this map.The faults are one of the important factors that must be taken into consideration when determining the location of the dam.
Digital Elevation Model (DEM): Essentially, the image interpretation of the Digital Elevation Model creates a very important tool for researching water harvesting solutions (F.A.O., 1993).Downloaded DEM provided by SRTM with a spatial resolution of 30 m from the USGS website in this study.

Software
Several criteria are taken into consideration to determine the construction sites for dams.This is done after determining the appropriate areas for the construction of dams.Using the Arc GIS 10.8 to process all the data set, as well as to analyze and interpret them.In addition to Spatial Analyst Supplemental tools (Storage Capacity tool) in ArcGIS, the storage capacity curves can be estimated.Through this tool, the water elevation and corresponding surface area and storage capacities for each reservoir can be generated.

Factors Controlling the Selection of the Appropriate Site for the Dam
The location of the barrier or dam is decided after taking into account several considerations.Hard limestone makes up the majority of the exposed rocks in the valley.They can be utilized as the foundation for a dam or barrier site and the investiture of the barrier's front side.The open area in front of the dam to store water and the size of the secondary drainage basin are also taken into account.Rainfall is the main source of recharge of runoff water.The distribution and intensity of rainfall are a prerequisite in selecting the site and design of the water harvesting system (Prinz and Singh, 2000).There are several suitability criteria were reclassified using the suitability degree presented in (Table 1).
Distance to roads (km) 0-1 1-15 15-25 25-45 >45 Stream*: There is a main stream on which a dam can be established Stream**: There is no main stream to establish a dam on it.

Suitability Index
A suitability index was calculated by adding the product of each criterion's relative relevance weight (percentage of influence) and its standard suitability score (Eq.1):  = ∑   *   (1) Where SI: is the suitability index, Si: is the standardized suitability score of criterion i, and Wi: corresponds to the relative importance of criterion i.

Dams Site Selection
The six criteria were reclassified using the suitability level presented in Table 1.This process resulted in six maps presented in Fig. 2 a, b, c, d, e, and f.Attribute scores and weights for the maps used in the dams site selection are presented in Table 2.
• Main stream factor: presence of a main stream received a score of 100, while the absence of a main stream received a score of 0. This factor was not given a certain weight because its presence is a stipulation for the construction of the dam to be achieved since the absence of a main stream means that a dam is not established in that location.
• Villages distance factor: a score of 0 was given to distances less than 1 km, while distances between 1 to 24 km received a score of 100.Other score values were given to distances mentioned in Table 2.The high weight of 50 was assigned to the village's distance factor because it affects directly the community, which should be given a priority in the planning for selecting the dams site.• Faults distance factor: a score value of 0 was given to a distance of 100 m or less, while a score value of 100 was given to distances of >100 m.A weighting value of 25 was assigned to the faults distance factor due to influencing the stability of the dam, which can lead to a negative effect on the community.• Elevation factor: a score of 100 was given to elevations less than 100 m, while the elevation of more than 350 m received a score of 0. Other score values were given to elevation mentioned in Table 2.A weighting value of 10 was assigned to the elevation factor due to its effect on the determination of water accumulation at the site.• Slope factor: a score of 100 was given to a slope less than 1%, while a slope more than 4% received a score of 0. Other score values were given to the slope mentioned in Table 2.A weighting factor of 10 was assigned to the slope factor, due to its significance to have a stable location for the surface runoff.
• Road distance factor: a score of 100 was given to distances less than 1 km, while distances more than 45 km received a score of 0. Other score values were given to distances mentioned in Table 2.
It was given a weighting value of 5, where it is less valuable because it does not directly affect it, in addition, it is possible to create additional roads for easy access to the dam site.

Suitability Index of Dams Site Selection
All of the criteria were classed into a single evaluation scale after determining their weights.In this approach, five levels of suitability were applied: Highly suitable, suitable, modestly suitable, Not suitable, and highly unsuitable.All factors are divided into the five categories above, except for the main stream and distance to faults.The main stream has been reclassified into two categories of suitability; the presence a main stream (highly suitable) and the absence of a main stream (highly unsuitable).The distance to faults was reclassified into two categories of suitability; more than 100 m (Highly suitable) and less than 100 m (highly unsuitable).Dams/reservoirs should not be built in highly faulted terrain, hence dam sites should be at least 100 meters away from faults (Othman et al., 2020).
From the equation (Eq.1), it is possible to obtain an index of suitability for each of the selected dams whose locations are on the main streams (Fig. 2a).Table 3 shows the results of the suitability index for each of these dams, with the score of each category and the weight of each factor taken into account when choosing a dam site.
The final results to locate the best sites for suggested dams took into account the distance to villages, distance to faults, elevation, slope, and distance to roads.As these results showed that the site of the first dam (Dam 1) is the best-selected site with a suitable index of 97.5.It is followed in the second degree by the site of the fifth dam (Dam 5) and then the site of the second dam (Dam 2) with a suitable index of 95 and 90, respectively.Hence it falls under the category of highly suitable.The third dam (Dam 3) site falls within the modestly suitable category.Finally, the sites of the fourth dam (Dam 4) could not be established, because they had a suitability index of less than 70 (Table 4).

Suggested Lakes for Each Dam
A small dam system is established to confine all or part of the valley's flow and is ponded within the streambed.This method provides quantities of water (lakes) that can be used later for agriculture, watering animals, etc., as well as providing the opportunity for water infiltration and recharging the groundwater.Also, the nutrients rich sediment that will accumulate can be used for agricultural purposes (Manhi, 2021).This system can be constructed in several places depending on the easiest to implement in terms of valley width and availability of wall construction materials.
One of the effective measures is to control surface runoff by holding large quantities of its water behind these dams.These dams are established with a height of several meters (often 10 -25 m) along the contour lines and perpendicular to the direction of surface runoff.Only four lakes can be established behind dams (Dam 1, Dam 2, Dam 3, and Dam 5), while the fourth dam (Dam 4) has been neglected because it has a suitable index of 45 and is in a very unsuitable category (Highly Unsuitable).
Figure 3 shows the locations of the suggested dams in addition to the lakes that are established behind them within the Wadi Al-Mohammadi basin.According to the height of the dam at each site, the capacity of storage was determined in Table 5. Figs. 4,5,6,and 7 show the areas of maximum inundation (lakes) and 3D views of the four suggested lakes in the Wadi Al-Mohammadi basin with the crosssection of dams.At the dam's suggested sites, dam bed elevation ranges between 65-260 m a.s.l. and due to the topography of the Wadi Al-Mohammadi basin, the dams create a lake with an area ranging from 3 km 2 (Lake 2) to 33 km 2 (Lake 1).The total storage capacities of suggested dams ranged between 13.39 -183.63Mm 3 while the depths of the lake were between 10 -20 m.

Storage Capacity of Suggested Lakes
Storage capacity curves are a vital parameter and their accuracy is a fundamental issue for water balance and strategic hazards management (Peng et al., 2006) and is considered one of the most important natural characteristics of the water reservoirs.Based on Spatial Analyst Supplemental tools (Storage Capacity tool) in ArcGIS, the storage capacity curves can be estimated.Through this tool, the table of water elevation and corresponding surface area and capacities of storage for each reservoir can be generated.Tables 6, 7, 8, and 9 show the elevation, volume, and surface area of each dam. Figure 8 shows the relationship between the water level of the lakes, storage volume, and surface area covered by each dam suggested.Depending on Fig. 8 a, b, c, and d and based on the assumption that the water level reaches the top of the suggested dams, there is a nearly linear relationship between the elevation of the water reservoir (lake), and storage capacity, and surface area.Each additional meter in the elevation of the water reservoir causes an increase in storage capacity and surface area for all suggested dams.Evaporation is an important factor that must be taken into consideration, as the increase in the surface area of the water reservoir leads to losses that may be large, especially in areas with arid and semi-arid climates.Therefore, the dam site that has high water elevation with maximum storage capacity and minimum surface area is an ideal location for dam construction.

Conclusions
Identification of suitable sites for the construction of dams/ lakes is essential for the study area, which is a semi-arid region with a significant water deficit.In this study, the approach followed based on GIS with Spatial Analyst Supplemental tools (Storage Capacity tool) in ArcGIS enabled us to obtain information about the relevant criteria (i.e., distance to villages, distance to faults, elevation, slope, lineaments, distance to roads, and storage capacity) to create a dam's/ lakes suitability map for the region of the Wadi Al-Mohammadi basin.
The results showed that the site of the first dam is the best-selected site.It is followed in the second degree by the site of the fifth dam and then the site of the second dam.Hence it falls under the category of highly suitable.The third dam site falls within the modestly suitable category.Finally, the sites of the fourth dam could not be established.The analysis also shows that the suggested lakes in the Wadi Al-Mohammadi basin have good storage capacity with the possibility of utilizing them to prevent the risk of floods, store water in drought seasons, irrigation capabilities, and take advantage of them as tourist areas.

Fig. 1 .
Fig. 1.Location map of the study area

Table 1 .
The criteria for site selection that are employed, as well as the degree of suitability

Table 2 .
Attribute scores and weights for the maps used in the dams site selection

Table 3 .
The suitability index of dams site selection

Table 4 .
Suitability index categories for suggested dam sites

Table 6 .
Storage capacity table of the elevation (m asl), volume (m 3 ), and surface area (m 2 ) of Lake 1

Table 7 .
Storage capacity table of the elevation (m asl), volume (m 3 ), and surface area (m 2 ) of Lake 2

Table 8 .
Storage capacity table of the elevation (m asl), volume (m 3 ), and surface area (m 2 ) of Lake 3

Table 9 .
Storage capacity table of the elevation (m asl), volume (m 3 ), and surface area (m 2 ) of Lake 5