Calculating the Water Balance for Kubaisa Basin by Simulating the SWAT Model, Western Iraqi Desert

Abstract


Introduction
Managing water resources is a crucial concern in arid and semi-arid areas, where water shortage is a pressing problem.Planning and managing water resources sustainably in these areas depends on having a solid understanding of the water balance of the stream basins.An effective modeling tool for determining the water balance of different basins, particularly those in arid regions, is the Soil and Water Assessment Tool (SWAT).To deal with several hydrological and/or environmental issues, the SWAT model has become one of the most extensively used models for basin studies (Gassman et al., 2014).
The SWAT model, which enables the interpretation and calculation of the water balance of an area, is one of the most influential computer simulation models that deal with hydrological operations, especially the water balance of a basin (Al-Kubaisi and Al-Kubaisi, 2023a).The SWAT model has been used recently in many studies related to hydrology, including those studies conducted in Iraq (Manhi and Al-Kubaisi, 2021;Jasim, 2021;Manhi, 2021;Sahar et al., 2021;Al-Kubaisi and Al-Kubaisi, 2022).The SWAT model becomes increasingly important as it proves its effectiveness in estimating long-term hydrological calculations, especially with regard to water balance, surface runoff, and water harvesting calculations.Therefore, this model was highlighted due to its importance in estimating the water balance in the Kubaisa Basin in Iraq.
Water is the secret of life and a natural renewable resource, and its importance and commodity increase in countries that suffer from scarcity and have a desert climate (Al-Kubaisi et al., 2023).These countries that are characterized by this climate are exposed to irregular rain storms, and sudden flow occurs in the basins, thus causing direct damage from floods (Al-Kubaisi and Al-Kubaisi, 2023b).The water balance of valley basins, which are typically distinguished by their distinctive topography and hydrological features, is significantly influenced by climate.The delicate balance between inputs (precipitation, inflows) and outputs (evapotranspiration, runoff, and groundwater recharge) determines the water balance in these basins.These factors are influenced mainly by the climate, and climate variations can significantly impact the water supplies in valley basins.The climate and hydrological conditions in a hydrological basin result from a complex interplay of natural elements, such as terrain and soil characteristics, alongside the influences of climate factors that directly impact the hydrological cycle (Al-Sudani, 2018).
Summers in Iraq are hot and dry, and winters are cold and rainy.Most of the yearly rainfall, or around 90%, falls in the winter months of December through March (Awadh and Ahmad, 2012).Spring and winter are more productive for improved drainage depending on the amount of precipitation (Jirjees et al., 2020).The region (Kubaisa Basin) is known for its arid climate (Awadh and Al-Ghani, 2014).SWAT modeling can predict how shifting temperatures and precipitation patterns may affect the water balance of the Kubaisa Basin as climate change exacerbates problems with water scarcity in desert areas.For long-term planning and adaptation methods, this information is crucial.To simulate hydrological processes, SWAT needs long-term meteorological data, such as rainfall, solar radiation, temperature, humidity, and wind speed.Historical weather data is essential to comprehend the regional climate and its impact on water balance.
Due to water scarcity, modern research is increasingly directed towards the Iraqi Western Desert, especially related to water (rainwater harvesting and management).Some of these studies are related to water: Al-Hadithi (2016) studied this region in terms of its groundwater suitability for irrigation Alhadithi and Alaraji (2016) and Al-Jawad and Al-Ansari (2017) studied rainwater harvesting.Awadh (2018) studied water suitability.Farhan et al. (2020) studied interaction groundwater with host rocks, etc.Finally, these studies followed the current study on the management of water resources in the region, and the primary objective is to accurately quantify the water balance within the Kubaisa Basin by simulating the SWAT model.
This includes estimating the various components of the water balance, such as precipitation, runoff, evapotranspiration, groundwater recharge, and streamflow.Thus, obtaining a comprehensive understanding of the hydrological processes and water availability in that specific area.Overall, the main objective is to support informed decision-making for sustainable water resource management in a region where water availability is a critical factor for both human and environmental needs.

Site Description
Kubaisa Basin, a Euphrates River System (ERS) component, is crucial to the region's water supply, especially for home and agricultural uses.However, the Kubaisa Basin confronts serious water-related difficulties (challenges) like many other areas of Iraq.These difficulties include the requirement for efficient water resource management, declining water availability, and conflicting water needs.

Study Area
Kubaisa Basin is situated in the eastern side of the Iraqi Western Desert (IWD) and is characterized as a transient valley that ultimately flows into the Euphrates River (ER).This basin is flanked on its eastern side by the Euphrates River, on its southern side by the Al-Mohammadi Basin, and is bounded to the north by the Khabbaz Valley.The geographic coordinates marking its boundaries are as follows: It extends between latitudes 33˚ 16ʹ 58.7ʺ to 33˚ 42ʹ 6.9ʺ and longitudes 41˚ 34ʹ 47.5ʺ to 42˚ 46ʹ 52.1ʺ (Fig. 1).Covering a total area of 2485.5 km2, the basin's perimeter has been approximated at 547 km.Its elevation above sea level varies within a range of 55 to 410 meters.
The location and area of the Kubaisa Basin give it great importance in terms of the water balance.It is situated in the eastern side (part) of the IWD, which is characterized by its valleys, provides great opportunities for harvesting water (Fig. 2).In this case, the water balance becomes more important, as it can be used to estimate the distribution and availability of water and plan to maintain sustainable water levels.The Kubaisa Basin extends over a wide area, meaning there are different areas within the basin, each of which may face other water challenges.The water balance helps analyze these changes and estimate each region's water needs.

Geological Setting
Kubaisa Basin is situated in Western Iraq.It is one of several sedimentary basins in the region and is a small part of the Arabian Plate.From a geological perspective, the study area exhibits several exposed rock formations, arranged in order from the oldest to the most recent, including the Msad Fn., Euphrates Fn., Fatha Fn., Nfayil Fn., Zahra Fn., and Depression Full Deposits (Quaternary sediments) (Sissakian and Mohammed, 2007) (Fig. 3).Regarding the region's structural characteristics, the prominent structural feature within the basin is the Abu-Jir Fault.This fault system traverses the eastern (E) side of the basin, extending in NW-SE (northwest-southeast) direction.Its formation can be attributed to the cracking and rifting of the northeastern margin that is passive of the AP (Arabian Plate) and is associated with right-lateral strike-slip movement.Notably, the Abu Jir (AJFS) Fault System does not continue northward but takes a westward turn near Hit, intersecting with the Annah-Graben (Abdul-Jabbar, 2013).
Geomorphological situation of this basin includes the perceptions and formations of the earth's surface and the geological features in it.Kubaisa Basin contains plains and multiple valleys that were formed as a result of geological and topographic influences over the ages.These plains can be flat or undulating and are influenced by river courses and water drainage.

Data from remote sensing and related sources
Remote sensing in hydrology is important for monitoring and understanding changes in water and water resources without being directly on-site.Most of the data utilized in this study is from the Landsat 8 satellite.The primary database obtained from this satellite can be divided into four groups necessary for the SWAT concept as a model.The first basic data is the Digital Elevation Model SRTM (Shuttle Radar Topography Mission) type; the website that was used to download it is USGS (https://earthexplorer.usgs.gov/)(Fig. 4).The second basic data is soil, which is necessary data for the SWAT model.A website that was used to download it is F.A.O.(http: // www.fao.org/home/en/) (Fig. 5).

Fig. 5. Soil distribution for the Kubaisa Basin
The third basic data is LU (land uses) and LC (land covers).This data must be downloaded at a time free of clouds.Also, the season in which plants grow must be considered (the best date is in the fourth month), so it was chosen on April 15th, 2023.Five classes as varieties emerged for the Kubaisa Basin, namely: Agricultural Land-Generic with an area ratio of 2.61%, Wetlands-Mixed with an area ratio of 6.47%, Pasture with an area ratio of 24.72%, Barren with an area ratio of 36.34%,Winter Pasture with an area ratio of 29.85% (Fig. 6).The fourth basic data is the climate data for three stations and 33 years, starting from 1990 to 2023 (Fig. 7).The website that was used to download it is POWER (Prediction of Worldwide Energy Resource) (https://power.larc.nasa.gov/).

Climate of the Kubaisa Basin
Daily climatic data of precipitation, relative humidity, air temperature (Min.and Max), wind speed, and solar radiation play a crucial in calculating parameters and paths related to water balance in an area.Precipitation does not include only rain but is a term that includes many forms of precipitation, such as snow, drizzle, frost, hail, etc. (Chow, 1964).In the study area, snow rarely falls, and the percentage of other forms of precipitation is very small (neglected).Therefore, it can be called precipitation by the term rainfall only.This element is considered one of the most important climate data because it determines how much water flows into the water basin.The distribution and amount of precipitation is affected by the location and time of precipitation and general climatic conditions.The total annual rainfall of the Kubaisa Basin was 145.9 mm in 2018 (the highest rainy year), while the year 2005 was the least, with a total annual rainfall of only 14.1 mm (Fig. 8).The quantity of water vapor in the air, stated as a percentage of the quantity required for saturation at the same temperature, is known as relative humidity (Guarnieri et al., 2023).Relative humidity plays a role in determining the rate of water evaporation from soil and water bodies.Air humidity affects the water balance in the environment.The mean annual relative humidity of the Kubaisa Basin was 42.7% in 2019 (the highest year), while the year 2008 was the least, with a mean relative humidity of only 31.3% (Fig. 9).Temperature is one of the climate elements that significantly and effectively affects evaporation and evapotranspiration (Ibrahim and Al-Dabbas, 2021), and this means that it significantly affects the water balance.Temperature determines the speed of evaporation and condensation processes and thus their effect on the water balance.Higher temperatures can increase the rate of evaporation.mean annual temperature of the Kubaisa Basin was 23.1°C in 2010 (the highest year), while the year 1992 was the lowest, with a mean temperature of only 19.1°C (Fig. 10).
Wind helps exchange air and moisture between places, and this can affect the distribution of rainfall and the rate of evaporation.According to (Viessman and Lewis, 2007), air pressure and temperature have a strong inverse relationship with wind.Wind speed rises with high temperatures.The mean annual wind speed of the Kubaisa Basin was 3.15m/s in 2013 (the highest year), while the year 1995 was the least, with a mean wind speed of only 2.83m/s (Fig. 11).Solar radiation affects heating and evaporation processes.Large amounts of solar radiation can increase water from water surfaces and plants.Electromagnetic waves that originate from the sun and travel at the speed of light to the earth are what make up solar radiation (Chalkias et al., 2013).The mean annual solar radiation of the Kubaisa Basin was 20.4MJ/m 2 /day in 2004 (the highest year), while the year 1991 was the least, with mean solar radiation of only 17.4MJ/m 2 /day (Fig. 12).
Using this data, professionals in the field of water balance and climate can estimate water flows, evaporation rates, and rainfall distribution and thus understand the water cycle in a given area.This helps determine how to use water resources and plan to manage them effectively.

Water Balance of the Kubaisa Basin
The initial step in using the SWAT model in the Kubaisa Basin involved collecting basic data.This data included climate, land use, soil characteristics, topography, and streamflow information.Highquality data is essential and critical to obtaining good, easily interpretable results.One of the components of the SWAT model is the water balance.The SWAT model solves the water balance equation using variables for surface runoff, precipitation, percolation, evapotranspiration, and flow (Al-Kubaisi and Al-Kubaisi, 2023a).Water increases and decreases are severe environmental problems that significantly impact the environment and society.
Flooding can occur when precipitation and water run-off exceed normal levels, destroying property and affecting wildlife and human safety.Understanding the water balance helps predict the occurrence of floods and take measures to prevent them.On the other hand, water scarcity can occur when there is a shortage of water due to excessive evaporation or excessive use of water.This can affect water availability for human uses, agriculture, industry, and the environment.Understanding the water balance helps in developing water resource management strategies.
Efforts can be directed to maintain a sustainable balance between water supply and demand.An increase or decrease in water can affect ecosystems in multiple ways, such as changing rivers and wetland environments and affecting biodiversity.Understanding water balance helps maintain the natural environment.
In short, knowing and understanding water balance is crucial to ensuring sustainable water use and to deal with water-related environmental challenges effectively.
The yearly water balance components from 1992 to 2023 at the Kubaisa Basin outlet are summarized in Table 1 and Figs. 13 and 14.They show that loss by ET was about 95.7% of the rainfall as annual, whereas percolation had a value of about zero (0.0028) and had no impact on the rainfall amount.

Conclusions
Using a SWAT model to estimate the water balance takes time and effort and requires numerous analyses and monitoring, but it provides a powerful tool for understanding and managing water resources in the Kubaisa Basin or any other region.The study calculated the elements of the water balance in the Kubaisa Basin and the distribution of water resources in it through climate data for thirtythree years , and all components of the hydrological cycle were also calculated.The study results help us manage water and its optimal use in the Kubaisa Basin and develop strategies and plans to reduce the risks of floods and sustainably conserve water.The study results are also considered an indicator for stakeholders in water resources and the general public; raising their awareness and making appropriate decisions by stakeholders in water resources enables water abundance and sustainability.
Finally, we can say that attention and focus on such applied research that concerns the lives and needs of people is necessary on the part of water resources specialists.It is considered economically feasible for the country and sustainable for future generations.

Fig. 1 .
Fig. 1.Map depicting the geographical location of the studied area

Fig. 2 .
Fig. 2. Location of Kubaisa Basin in the Iraqi Western Desert

Fig. 4 .
Fig. 4. DEM of the Kubaisa Basin, type of Shuttle Radar Topography Mission

Table 1 .
Mean monthly that represents the WB (water balance) of the Kubaisa Basin for years1992-2023 RN-P symbolizes rainfall; SR-Q symbolizes surface-runoff; LT-Q symbolizes flow as lateral; GW-F symbolizes flow as groundwater; ET symbolizes evapotranspiration as actual; PET symbolizes evapotranspiration as potential; WLD symbolizes the water_yield; PEC symbolizes of water that permeates past (percolates) the root zone.