Monitoring the response of Saudi Arabia's largest fossil aquifer system to climate variability

Abstract

The study utilizes an integrated approach that involves GRACE and Global Precipitation Climatology Project (GPCP) data to analyze temporal water mass variations in Saudi Arabia (KSA). Recently, KSA has witnessed a historic wet period between 2018 and 2020 (i.e., 120–150 mm/year) following a prolonged dry period between 2002 and 2017 (i.e., 80–100 mm/year). Inspection of GRACE data showed a decline in groundwater recharge of −4 to −6 mm during the dry period. The depletion was reduced by 1 to 6.8 mm in the Northeastern side of KSA (e.g. Hail, Al-Jouf, Al-Hudud Ash-Shamaliyah areas) during the wet period. This impressive recovery has led to a groundwater recharge increase of 360 km³ in Hail, 174 km³ in Al-Jouf and 97 km³ in Al-Hudud Ash-Shamaliyah. The findings emphasize the notion that climate change will bring more frequent extreme climatic events to the Kingdom of Saudi Arabia and call for continuous monitoring of aquifers every year.

KEYWORDS:

• Geophysics
• groundwater sustainability
• groundwater recharge
• fossil aquifers
• GRACE
• Saudi Arabia

1. Introduction

Groundwater constitutes an integral part of the world’s water resources, particularly in hyper-arid regions where surface water resources are limited [1]. A prime example of such an area is the Saharan-Arabian Desert Belt, where sustainable utilization of groundwater from vast fossil aquifers has become a pressing need [2]. Examples of these fossil aquifers include the Northwestern Saharan Aquifer System (NWSAS; area: 1.2 × 10⁶ km²) in Algeria, Libya and Tunisia, the Nubian Sandstone Aquifer System (NSAS: area: 2.2 × 10⁶ km²) in Egypt, Sudan, Libya, and Chad, and the Mega Aquifer System (MAS; area: 2 × 10⁶ km²) in Saudi Arabia, Iraq, Jordan, Kuwait, United Arab Emirates, Qatar, Bahrain, Oman, and Yemen [3–5]. Agricultural development projects have been underway for the past few decades over large sectors of these aquifers leading to a significant decline of the groundwater table, which has been associated with the development of many deformational features over these areas that could challenge the sustainability of the development activities along these aquifers (e.g. [4,6–11]). Currently, the Saharan-Arabian Desert Belt is dominated by aridity with scarce precipitation and surface water resources leaving the groundwater as the major source of fresh water [12–16].

The Arabian Peninsula is widely known for its scarcity of water and is considered among the most sensitive regions to climate change [17–19] with a projected increase in the return periods of flooding and drought events in the upcoming decades [20–23]. As a result, understanding the effects of climatic variations and wind patterns, particularly the monsoon, on groundwater resources in the region is crucial. Despite receiving very little precipitation, the arid and hyper-arid deserts of the Sahara and Arabia have experienced higher precipitation levels in previous Pleistocene wet climatic periods, which recharged their aquifers [24]. Although these aquifers are now considered fossil, recent research has shown that they still receive modern recharge in areas with relatively high precipitation [25,26]. However, several aquifers have been overpumped leading to dramataical declines in the water levels [10,27–29]. Therefore, it is essential to identify sustainable management approaches for freshwater resources in arid areas, requiring a thorough understanding of the hydrologic and geologic settings of aquifers, groundwater sources and evolution, and recharge and depletion rates, as well as utilizing optimal management techniques such as appropriate location and rates of groundwater abstraction.

The GRACE solutions have proven effective in monitoring variations in TWS across various regions, including the Amazon basin [30–33], the Nile basin [34], the Mississippi basin [32], the Arabian-Saharan desert belt [4,21,35–39], the Middle East [40], India [41,42], Lake Volta [43], China [44] and Michigan [45]. The technique has also been utilized for measuring TWS variations in Lake Victoria [46], Lake Nasser [21] and the Three Gorges reservoir [47], and for estimating flooding [48] and drought events [49,50]. This manuscript demonstrates how an integrated approach using observations from traditional datasets meteorological, precipitation data from the Tropical Rainfall Measuring Mission (TRMM) and CHIRPS rainfall data and the Gravity Recovery and Climate Experiment (GRACE) mission can achieve these tasks.

Recently, several attempts have been conducted to integrate different approaches to better understand the role of both climate change and anthropogenic activities on the dynamics of groundwater in the Saharan-Arabian Desert Belt. Examples include the use of in-situ hydrological data, land surface models and GRACE observations to quantify the modern groundwater recharge to the aquifers in the eastern region of the United Arab Emirates [51], in the northern Saudi Arabia [52] and in Saharan aquifers [53]. Moreover, global databases of in situ groundwater-level trends as well as statistical models have shown that the Saharan-Arabian Desert aquifers witnessed rapid decline in the groundwater heads attained (>0.5 m year⁻¹) in the twenty-first century with forecasted significant increase in the water deficits in these aquifers [16,54].

The Kingdom of Saudi Arabia (KSA) is largely arid and historically, it experienced a prolonged drought period from 2002 to 2017, during which the average annual precipitation was below the mean values [55]. Despite this challenging scenario, the hydrological landscape underwent a dramatic shift in the subsequent years. From 2018 to 2020, the region experienced some of the most intense precipitation events ever recorded [56–59], according to satellite-based data from the Global Precipitation Climatology Project (GPCP). The average annual precipitation increased during this period, leading to a remarkable expansion in surface vegetation cover and a boost in groundwater storage. However, very little studies have considered a long-term regional monitoring of the response of the large fossil aquifers to climate variability.

Climate change is expected to bring more extreme precipitation events to historically arid areas like the KSA, potentially altering the general pattern of precipitation. Understanding these changes will be crucial in managing the groundwater resources of the region for a sustainable future. Therefore, the present study integrates GRACE and rainfall data together with remote sensing observations to examine the response of the Mega Aquifer System in KSA to climate variability over the past three decades.

2. Materials and methodology

2.1. Description of the study area

The basement rocks in the western Arabian Peninsula (i.e. the Arabian Shield) crop out along the margins of the Red Sea, forming the Red Sea Hills [60–61]. The Red Sea opening-related uplift uncovered the basement complex and the overlying Paleozoic/Mesozoic strata, thus providing an opportunity to recharge aquifers within these sedimentary formations where they crop out [62]. These formations, which are made of thick limestone and sandstone sequences, form one of the largest multi-layered aquifers on Earth, which is commonly described as the Mega Aquifer system (MAS; Figure 1) [63]. The MAS is largely differentiated into two major units: a lower continental unit, namely the Lower Mega Aquifer System (LMAS) and an upper marine carbonate-dominated unit (i.e. Upper Mega Aquifer System (UMAS) [64]. While the LMAS is made up almost entirely of clastic sedimentary rocks, the UMAS is largely composed of fractured and karstified marine carbonates [65].

Figure 1. Areal extent of the study area with lateral change in the geological formation.

In Central Arabia, the UMAS includes the Biyadh, Wasia, Aruma, Umm Er Radhuma, Rus, and Dammam formations and the LMAS includes the Saq, Tabuk, Tawil, Minjur, and Dhruma formations. In northern and northwestern sections of Arabia, locally named as Wadi As-Sirhan Basin (WASB), the above mentioned UMAS units are absent. Instead, the Secondary-(Mesozoic)-Tertiary-Quaternary complex (STQ) represents the local productive carbonate aquifer in the basin. The major tectonic feature in the WASB is a graben structure with a displacement of more than 1,500 m [65], and is bound by the Kahf fault system from both east and west sides [66]. The faults have a major role in the aquifer connectivity, where they act as vertical conduits for groundwater between the aquifer layers as evidenced by the occurrence of salt lakes, and sabkhas within depressions and the spatial correlation with the fault distribution [67–68]. The potentiometric head data before the onset of excessive pumping of the groundwater in 1983 indicated higher hydraulic heads in the deep LMAS compared to those of the shallow UMAS [65]. Under these hydraulic conditions, artesian upwelling of the deep groundwater into the overlying shallow aquifers could occur along sub-vertical faults [69]. At the present hyper-arid conditions, it is estimated that the extraction from the MAS in northern and central regions of KSA exceeds (by 100 times) the annual recharge [70]. This situation apparently triggers continuous depletion in the groundwater levels and land subsidence [10].

2.2. Topography

Garbrecht and Martz’s Topographic Parameterization (TOPAZ) programme from 1995 was employed to derive the drainage network and watershed distribution from the digital elevation data, which in turn were obtained from the Shuttle Radar Topography Mission (SRTM) at a spatial resolution of 30 meters. The software functions by comparing the elevation of each cell with its neighboring cells, postulating that the flow direction is towards the cell having the lowest elevation. Additionally, it presumes that all cells that drain into a single outlet belong to the same watershed.

2.3. Precipitation

The average annual rainfall (AAP) and average monthly rainfall (AMP) for Hail, Al-Jouf, and Al-Hudud Ash-Shamaliyah between 2002 and 2022 was obtained using the GPCP combination of monthly observed and satellite-derived precipitation data. This GPCP product combines the estimates of rainfall amounts from different individual sources including rain gauges, geosynchronous satellites as well as low orbit satellites. The precipitation estimates using the above approach combine both the accuracy of rainfall gauge measurements as well as the regional capabilities of satellite data and avoid the limitations associated with the limited spatial extent of rain gauges, especially in arid environments. Therefore, the GPCP products enable the development of a comprehensive and accurate precipitation dataset on the global scale for various applications including climate modeling, hydrogeological studies and weather forecasts [20]. This amalgamated technique capitalizes on the higher accuracy of the low-orbit microwave data to improve the accuracy of the most common and more frequent infrared measurements from the geosynchronous orbits. Prior to the adoption of the microwave method, the infrared-only observations were adjusted using the microwave-based analysis of subsequent years.

2.4. GRACE (Gravity recovery and climate experiment)

Previous studies [39,71,72] have demonstrated the potential of the GRACE mission, launched in March 2002 as a joint venture of NASA and the German Aerospace Center (DLR), to measure mass changes in extensive hydrological systems such as the Arabian Peninsula. The mission’s primary objective is to map global gravity fields, and variations in these fields over time are directly linked to changes in TWS. The Center for Space Research (CSR) at the University of Texas provided monthly mass concentration (mascon) solutions in Release 06 (RL06). In this study, GRACE solutions from the Center for Space Research (CSR), Jet Propulsion Laboratory (JPL), and the German Research Centre for Geosciences (GFZ) were utilized to determine the time-related variations in Total Water Storage (TWS) over Hail, Al-Jouf, and Al-Hudud Ash-Shamaliyah. The breakpoints (shift in GRACETWS trend from dry 2002-2017 to wet period 2018-2020) were identified for each area using the Regime Shift Detection (RSD) method (Andersen et al., 2009) and the TWS trends were calculated using the statistically identified breakpoints.

3. Results and discussion

As illustrated in Figure 1, the Paleozoic and Cretaceous formation constitute the majority of the Mega Aquifer System, which is the primary recipient of precipitation in KSA. In the presented figure (2), variations in the total water mass, as measured by the examined three GRACE solutions, are depicted for three distinct regions within the northeastern KSA spanning from 2005 to 2020, namely Hail province, Al-Jouf province and Al-Hudud Ash-Shamaliyah (i.e., Northern Borders) province.

The three GRACE solutions which were applied (from 2002 through 2020) in this study are reported relative to a 2004–2009 mean baseline. Given the higher signal to noise ratio of the CSR-M RL6.2 solutions relative to other solutions (JPL Rl6, GFZ and CSRRL6), the CSR-M solutions were selected as the primary dataset for extracting trends over the investigated subbasins. The CSR solutions were derived using Tikhonov regularization [73] and were resolved on a geodesic grid [73,74]. (Figure 2)

Figure 2. GRACE Total Water Mass variation over northeastern Saudi Arabia, (a) GRACE Total Water Mass variation over Hail province, (b) GRACE Total Water Mass variation over Al-Jouf province, (c) GRACE Total Water Mass variation over Al-Hudud Ash-Shamaliyah (Northern Borders) province.

GRACE total water mass variations over Hail Province (Figure 3) indicate that the province experiences a decline of −6.3 mm, transitioning to −9.2 mm later to lose 3 mm of storage. On the other hand, GRACE total water mass variations over Al-Jouf province (Figure 3) and Al-Hudud Ash-Shamaliyah province (Figure 3) showcase increase in storage in water mass but at varying degrees. Al-Jouf province witnesses a depletion of −5.22 mm then transitioned to −0.5 mm, which means a recovery of 4.7 mm. Similarly, Al-Hudud Ash-Shamaliyah province changed from of −4.5 mm to + 2.34 mm denoting a recovery of 6.8 mm of storage increase. These GRACE storage trend observations agree with the spatial pattern of trend map in Figure 5. The relatively moderate decline in these provinces indicates regional variations in water retention, possibly due to differences in local water management practices, geology, or climate.

Figure 3. CSR RL06.2 mass variation over northeastern Saudi Arabia, Hail province, Al-Jouf province and Al-Hudud Ash-Shamaliyah province. For the two periods (a) first period 2002 to 2017 (Trend 1, Trend 2), (b) second period 2018 to 2020 (Trend 3).

The data unequivocally reveal a concerning decline in the total water mass across all observed regions in northeastern KSA over the 15-year period. The variations in decline rates between the three regions emphasize the importance of region-specific water management strategies. It is also crucial to consider external factors such as changing climatic conditions, groundwater extraction rates, and local water conservation efforts when analyzing these trends. The consistent decline across all sources and regions calls for immediate interventions and sustainable water management practices to safeguard the future water security of these areas.

To better understand the aquifer dynamics and their response to climate variability in the northern KSA, the precipitation data were analyzed together with the GRACE trends. Inspection of the precipitation data (Figure 4) indicates that the study area can be classified into two periods in terms of the rate of precipitation, where the average annual precipitation exhibits a substantial difference between these two periods. During the initial timeframe from 2002 to 2017, the average precipitation rate is estimated at approximately 100 mm. In contrast, during the subsequent period (2018-2020), precipitation escalated to around 150 mm over the northeastern borders of KSA, as shown in Figure 3.

Figure 4. Average Annual Rainfall (Precipitation) (2002–2020) over the study area from GPCC rainfall data.

Figure 5. GRACE TWS Trend from (2002–2020) over the study area as extracted from GRACE CSR Rl0602.

Inspection of the GRACE trend (Figure 5) shows the rapid response of the aquifers to the climate variability. The trend transitioned from a depleted behaviour of more than −5 mm/year to an increasing behaviour of up to 5 mm/yr indicating a rapid recovery of the aquifer. It is also noted that the area covered by the basement rocks showed no significant changes between the two periods compared to the area covered by the sedimentary sequences (i.e. aquifer regions), which concurs with the notion of a rapid response of the fossil aquifers in the northern KSA to climate variability. This phenomenon is facilitated by the topographic and lithological settings of the area. As indicated in Figure 5, the Cretaceous formations, which represent the outcrops of the lower Mega aquifer system in the area, occur in the low-lying areas, where rainfall is intense and meets the surface runoff from the precipitation over the elevated basement terrains in the west.

This study has shed critical light on the response of fossil aquifers in northern KSA to climate variability and thus the potential for sustainable use of groundwater resources in large fossil aquifers, employing a new integrated approach that combines data from the Gravity Recovery and Climate Experiment (GRACE) and Global Precipitation Climatology Project (GPCP). Through this approach, the study has tracked substantial shifts in the hydrological patterns within the Kingdom of Saudi Arabia (KSA), a region historically characterized by arid conditions and now witnessing significant changes in precipitation (Figure 6).

Figure 6. The areal extent of the study area in Saudi Arabia, the general distribution of elevation and Stream network.

The drought period from 2002 to 2017 experienced a remarkable transition to a period of increased rainfall from 2018 to 2020. This transition has resulted in a positive impact on surface vegetation cover and groundwater storage, indicating the resilience and adaptability of these hydrological systems in the face of climate variability. Analysis of GRACE data has furthermore provided an invaluable perspective on groundwater recharge dynamics in Saudi Arabia, allowing us to quantify the substantial increase in groundwater recharge in several key regions.

However, while these results present a promising trend in the short term, it is imperative to approach them with caution. The impact of climate change, anticipated to bring more frequent and extreme precipitation events, could dramatically alter the typical precipitation patterns over the Kingdom of Saudi Arabia. Therefore, continued monitoring and sophisticated data analysis are crucial for understanding these trends and developing effective and sustainable water management strategies.

The complexity of the groundwater systems, the uncertainties linked to climate change projections, and the enormous value of water resources for the Kingdom of Saudi Arabia and similar regions underscore the importance of this research. As we move forward, we recommend leveraging the full potential of the integrative approach presented here to assess groundwater resources, monitor changes, and inform sustainable water management policies in the face of a changing climate. However, the presented approach provide a valuable cost-effective approach to monitor groundwater dynamics in the Arabian Peninsula and in similar hydrogeological settings worldwide, yet the approach does not account for the aquifer heterogeneity, which in part can be attributed to natural lithological changes that would locally affect aquifer characteristics and second to anthropogenic forcings such as excessive irrigation practices and artificial recharge that also can locally affect aquifers. Therefore, a recommended improvement of the presented approach is to integrate these satellite-derived datasets with in-situ groundwater measurements to improve both regional and local analyses of groundwater dynamics in such aquifers.

4. Summary and conclusion

This study utilizes an integrated approach to assess the sustainable use of groundwater from large fossil aquifers in the northern regions of KSA. Combining data from GRACE satellite and precipitation from GPCP, the study tracks changes in water mass over seasons and years. Following a prolonged drought from 2002 to 2017, KSA experienced significant increases in precipitation from 2018 to 2020, resulting in an augmentation in groundwater storage. GRACE data analysis reveals a decrease in groundwater recharge during the drought, followed by substantial recovery during the wet period, particularly in Hail, Al-Jouf, and Al-Hudud Ash-Shamaliyah A significant correlation was found between the intensity of precipitation and the level of aquifer recharge, primarily in low-lying areas with the Cretaceous formations, which suggested a rapid response of the aquifers to climate variability. Therefore, this study underscores the adaptability and resilience of hydrological systems, which are crucial for the sustainable use of groundwater resources.

However, the study also highlights the necessity for continued monitoring and emphasizing the use of in-situ hydrological data to reach a better understanding of the aquifer response to climate variability. Climate change is expected to bring more frequent and severe precipitation events that may dramatically alter the typical precipitation patterns over KSA. Understanding these trends and developing effective water management strategies requires a holistic approach, as presented in this study. The research stresses the importance of leveraging comprehensive data to assess groundwater resources, monitor changes, and inform sustainable water management policies, especially given the uncertainties linked to climate change projections and the critical value of water resources in KSA and similar regions.

Acknowledgments

The author extends his appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project number: IFP22UQU4360865DSR245.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia [grand number: IFP22UQU4360865DSR245].