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Gravity based estimates of modern recharge of the Sudanese area
Journal Pre-proof Gravity based estimates of modern recharge of the Sudanese area Ahmed Mohamed PII:
S1464-343X(19)30395-4
DOI:
https://doi.org/10.1016/j.jafrearsci.2019.103740
Reference:
AES 103740
To appear in:
Journal of African Earth Sciences
Received Date: 1 July 2019 Revised Date:
23 October 2019
Accepted Date: 13 December 2019
Please cite this article as: Mohamed, A., Gravity based estimates of modern recharge of the Sudanese area, Journal of African Earth Sciences (2020), doi: https://doi.org/10.1016/j.jafrearsci.2019.103740. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
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Gravity based estimates of modern recharge of the Sudanese area
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Ahmed Mohamed
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Geology Department, Faculty of Science, Assiut University, Assiut 71516, Egypt
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Author: Ahmed Mohamed, Geology Department, Assiut University, Assiut, 71516, Egypt,
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Phone: +20-100-839-0135,
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[email protected]
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Key words: Sudan – GRACE – Terrestrial water storage – Groundwater storage – Precipitation – Recharge
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Abstract:
The North Africa region is seeking water resources to develop agricultural expansions and land reclamations projects based on the groundwater resources. Monthly (April 2002–July 2016) terrestrial water storage (TWS) solutions of the Gravity Recovery and Climate Experiment (GRACE) along with other datasets were used to monitor and estimate the variations in groundwater storage over the Sudanese area and its sections. Results indicate: (1) the Sudanese area, Southern Sudan and Northern Sudan sections are receiving an average recharge of +4.15±1.07, +2.46±1.69, and +4.74±0.76 mm/yr, respectively during the analysed period, taking into an account the annual extraction rate of 0.67±0.067, 1.04±0.104 and 0.54±0.054 mm/yr from the Sudanese area, Southern Sudan and Northern Sudan sections, respectively, (2) and the average groundwater storage variations of +3.35±1.07, +1.21±1.69 and +4.09±0.76 mm/yr from the Sudanese area, Southern Sudan and Northern Sudan sections, respectively, and (3) the assumed natural discharge of –0.13±0.013, –0.21±0.021, and –0.11±0.011 mm/yr from the Sudanese area, Southern Sudan and Northern Sudan sections, respectively. (4) The average precipitation of Tropical Rainfall Measuring Mission data over the Sudanese area, Southern Sudan and Northern Sudan sections was estimated at 520.2, 1165.6 and 300.3 km3/yr, respectively. (5) The ground surface relief is forming northeastward streams taking the surface water away to the river. (6) The groundwater flows to the southernmost part of Egypt is impeded by the Uweinat-Aswan basement uplift and the thinned sedimentary cover in northern Sudan and southern Egypt, which in turn flows northeastward to the river. (7) The integrated study is informative and cost-effective model for best estimating the recharge rate of large areas.
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Introduction:
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More than 2 billion people around the world use groundwater as the only source of domestic and agricultural purposes (Alley et al. 2002). Groundwater represents a main component of the water cycle in sustaining lakes, streams, and aquatic communities (Alley et al. 2002). Groundwater resources are subjected, in many regions of the world, to stress due to many factors, including contamination, salinization, and heavy exploitation (Wada et al. 2010). Furthermore, climate change also affects the groundwater resources. Temperatures are globally going to rise and precipitation levels will fall (IPCC. 2007).
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The North Africa region is sensitive to climate change. It is an arid region and has limited water resources (Wingqvist, 2010). The increased population growth, developing economies, and landuse changes is leading to major stresses on the freshwater resources (Heathwaite, 2010) and causing great effects on the region’s socio-economic potential. There will be an increase of 40% in water demand by 2030 worldwide (Catley-Carlson, 2011). Moreover, this region ran out of fresh water in the 70s and depends on water from outside the region in the form of food imports (Allan, 2002). Based on the data of the World Bank (2005), the North Africa region will grow to a projected >430 million people in 2025 from the present 311 million and around 100 million in 1960. This is getting the per-capita water average to extremely worrying levels. The region is experiencing water scarcity combined with about a low efficiency of water use (about 40%) in agriculture purposes (FAO, 2003). Countries like Libya, Bahrain, Jordan are suffering from extreme situations of water scarcity (WWDR, 2003).
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Sudan, Egypt, Libya, and Chad share the Nubian Aquifer, the large extensive transboundary aquifer systems in northeast Africa, covering about 2.2x106 km² (CEDARE, 2002; Bakhbakhi, 2006). The shared aquifer is managed through an agreement formalized by the four countries. In Sudan, groundwater is used for agricultural expansions and domestic demands in many areas of Sudan, especially those away from the Nile valley. Surface water resources in these areas are affected by higher evaporation rates. These conditions make groundwater the only feasible source for water in these dry areas. Groundwater aquifers cover most of the Sudanese areas and includes: the Nubian aquifer, the Um Ruwaba formation, the alluvial aquifer and the hard rocks aquifers (Salih and Khadam, 1984). Satellite measurements of temporal changes of time-variable gravity from the Gravity Recovery and Climate Experiment (GRACE) mission (Tapley et al., 2004) are providing a method for observing the Terrestrial Water Storage (TWS) variations. The GRACE project was started in March 2002 with an original design lifetime of 5 years and was providing excellent data till 2017. GRACE-FO, successfully launched on May 22, 2018, will continue the goals of the previous GRACE mission and tracking Earth's water movement to detect changes in groundwater storage (GWS), the water reserve in large lakes and rivers, soil moisture (SM), ice sheets, and sea level variation caused by the addition of water to the ocean as well. GRACE provides monthly variations in Earth’s gravity field since April 2002. These temporal variations are caused by different climatic and atmospheric processes and natural events as well. GRACE data have been widely applied for attaining deeper knowledge to regional scale water mass variations (Andersen and Hinderer, 2005; Brown and Tregoning, 2010; Bonsor et al., 2010). They have been used to assist basin-scale water balance calculations (Rodell et al. 2004; Syed et 3
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al. 2005), and to estimate the water storage changes on the basin and sub-basin scale (Crowley et al., 2006; Bonsor et al.,2010; Ahmed et al., 2011; Papa et al., 2015). GRACE data have been successfully used to estimate the depletion and recharge rates of the aquifers (Leblanc et al., 2009; Rodell et al., 2009; Tiwari et al., 2009; Voss et al., 2013; Gonçalvès et al., 2013; Joodaki et al., 2014; Döll et al., 2014; Chinnasamy and Agoramoorthy, 2015; Xiang et al., 2016; Alzyoud et al., 2015; Chinnasamy et al. 2015; Huo et al., 2016; Long et al., 2016; Jiang et al., 2016; Chinnasamy and Sunde, 2016; Mohamed et al., 2017; Ahmed and Abdelmohsen, 2018; Mohamed, 2019). Several studies have focused on estimating the variations in GWS over the Nubian Aquifer (e.g. Sultan et al., 2013, 2014b; 2015; Mohamed et al., 2014, 2015; Abdelmohsen et al., 2019) and monitoring the spatio-temporal variations in the total water storage of the Nile basin (Abdelmalik and Abdelmohsen, 2019). Sultan et al. (2019) have used an integrated approach of GRACE and geochemical data with other datasets to assess the sustainability, origin, age, and groundwater storage of the Mega Aquifer System in the Arabian Peninsula. Mohamed et al. (2017) have explained that the Northern Sudan sub-basin, which represents the recharge zone for the Nubian Aquifer in Sudan, has an average recharge rate of 0.78±0.49 km3/yr during January 2003– December 2013 period. Ahmed and Abdelmohsen (2018) have shown that the Egyptian part of the Nubian Aquifer is getting a cumulative recharge of 20.27±1.95 km3/yr through April 2002– February 2006 and April 2008–June 2016 periods. They have also shown that the recharge is occurring only under heavy rainfall events and/or a notable rise in Lake Nasser. Abdalla (2008) has estimated the average recharge rate of 4–8 mm/yr for the central Sudan rift basins based on groundwater flow modelling. Furthermore, Edmunds et al. (1988) have shown the central Sudan area is receiving an average recharge rate of about 5.8 mm/yr based on chloride tracer technique in the area. These studies have not quantified the recharge rate for the whole Sudanese aquifers. Moreover, the modeling and geochemical techniques are local studies and cannot be applied for the regional scales, given the uncertainties of model parameters (Gonçalvès et al., 2013) and the scarcity of the datasets used in these local methods. In the current study, temporal GRACE-derived TWS along with outputs of Global Land Data Assimilation System (GLDAS) were used to assess the GWS and the recharge rates over the different sections of Sudan. 2- Hydrogeological setting of the Sudanese area Four major aquifers cover about 50% of the surface area of the Sudanese region (Fig. 1). These aquifers are represented by four groups (Salama, 1976; Salih and Khadam, 1984; Abdo and Salih, 2012) as the Nubian aquifer, Um Rawaba formations, the alluvial aquifers, and the Basement complex. The Nubian aquifer is the largest and most extensive aquifer in Sudan, with its surface area covering more than 28% of the surface area of Sudan (Abdo and Salih, 2012). It is represented by sandstones and formed from nine major basins, including the Nile basin, the Sahara basin, Atbara basin, the Blue Nile basin, central Darfur basin, Gedaref basin, Nuhud basin, Shaggara basin and Sag El Naam basin. The saturated layers vary in thickness from 100 to 2000 m, with low to moderate permeability of good water quality.
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The Um Ruwaba aquifer is formed from three main basins: Atshan basin, Baggara basin, and Bara basin, covering more than 7% of the surface area of the country (Abdo and Salih, 2012). The aquifer is represented mainly by unconsolidated gravels sands and clays with a very large thickness up to three thousand meters. Well yield varies from 5 to 20 m3/h. The water quality is sufficient for domestic uses but tend to be saline in some areas. The alluvial aquifers are covered some regions as Gash, Azum, Wadi Nyala, Wadi Kutum, and Arbaat with high permeabilities, moderate to high transmissivity values (500-1500 m2/day) and well yield as well. The alluvial aquifers are recharged annually from seasonal streams and wadis. The aquifers are characterized by its moderate to high well yield. The water quality of the aquifers is very good with salinity less than 400 ppm. 3- Data and Methods 3.1. Gravity data The gravity field products from three different analysis groups were used in the current work: The Center of Space Research-University of Texas (CSR), the Jet Propulsion Laboratory (JPL), and the German Research Centre for Geosciences (GFZ).The available three solutions were downloaded, and the study areas were extracted to estimate the TWS variations. The associated scaling factors were averaged and applied to all three solutions to restore attenuated signals consistent with the employed filter parameters (Landerer and Swenson 2012). The scaled solutions were averaged, the mean of the three data solutions was calculated, and were used for the calculations. The ensemble mean (average of CSR, JPL, and GFZ fields) was found to be effective in reducing the noise in the gravity field solutions (Sakumura et al., 2014). 3.2. Climatic data Rainfall is one of the controlling factors that affect the mass variations within the geological formations. Rain gauges generally give very accurate point measurements of precipitation, but these stations are discontinuous and generally absent over the unpopulated desert areas of Sudan. Therefore, rainfall measurements from space were utilized in the current study. Two of these measurement types, Climate Merged Analysis of Precipitation (CMAP) data and Tropical Rainfall Measuring Mission (TRMM) data, were utilized to investigate the role of the climate on the temporal and spatial variations of GRACE-derived TWS and GWS over the Sudanese area and its sections. CMAP is a collection of precipitation datasets, with a 2.5° spatial resolution and monthly temporal resolution (Xie and Arkin 1997). This dataset provides global coverage merged precipitation estimations constructed from merging and analysis of different kinds of data sources with different characteristics (Xie and Arkin, 1997). TRMM was launched in 1997 to monitor and study rainfall for climatic research. It was acquired (1998–2018) with near-global coverage at a temporal resolution of 3 hours and with a 2.5° spatial resolution. TRMM was first proposed in 1984 (Kummerow, 1998; Huffman et al., 2007). It has been applied with GRACE data for estimating water storage changes and balances over Africa (Hassan and Jin, 2014). Monthly TRMM data, covering the studied period, was used to generate the Average Annual Precipitation (AAP) map (Fig. 2) over the study area. 3.3. Satellite soil moisture
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GRACE has no vertical resolution and cannot differentiate between the components of the TWS. The gravity signal is a cumulate of the mass changes within the atmosphere, oceans, and water stored on land (Wahr et al., 2006). Therefore, it is not possible to deduce whether a mass variation estimated for an area on land is caused by changes in the surface water storage (SWS), changes in the ground water storage (GWS), or changes in the soil moisture storage (SMS). To overcome this problem, other independent tools such as climatic model (CLM) outputs were used to separate those contributions. Monitoring and measuring of SMS at global scales can predict for flash flooding events. Although SM measurements often lack the reliability and consistency (Moiwo et al. 2011), satellite- and ground-based observational data products are now providing SM data with a high accuracy. One of these, is the Global Land Data Assimilation System (Rodell et al., 2004, 2009; Moiwo et al., 2011). It uses global land surface models in several different applications such as climate prediction, water cycle, and water resources. GLDAS has been used in: (1) estimation of TWS variations (Swenson and Wahr, 2006; Rodell et al., 2007; Henry et al., 2011; Moiwo et al., 2011; Long et al., 2013; Hassan and Jin, 2014); (2) evaluation of ground and satellite-based SM outputs (Dorigo et al. 2012, 2013); (3) analysis of regional environmental changes (Zhong et al., 2011; Gao et al., 2013); (4) estimation of agricultural developments and irrigated and dry areas (Romagura et al., 2012; Ghazanfari et al., 2013); and (5) estimation of global distributions of land surface fields such as SM (Rodell et al., 2004; Hogue et al., 2006). In the current study, GLDAS, version 1 (GLDAS-1), 0.25º 3-hourly data were downloaded from ftp://hydro1.sci.gsfc.nasa.gov/data/s4pa/GLDAS_V1/). The monthly data were generated through temporally averaging of the 3-hourly products (Fang et al., 2009). The used GLDAS data cover the same period as the GRACE observations. 4. Water budget Figures 3 show the spatial distributions of the secular trend images in TWS over the Sudanese area and its sections from CSR, JPL, GFZ solutions (Figs. 3a, b, c) and the average of these solutions (Fig. 3d). Figure 4 shows a good agreement in amplitudes and phases between the different GRACE solutions and their averages over the investigated areas. A well correlation varying from 0.92 to 0.99 is among the different datasets over the Sudanese area (Table 1). For the Northern Sudan Section, the correlation coefficient varies from low as 0.83 to high as 0.97 between the different datasets, while it varies from 0.98 to 1 over the Southern Sudan section. Witnessing of Figure 3d indicates that the Northern Sudan section is experiencing average positive trend values concentrated on the central part of it. Whereas the Southern Sudan section is showing positive trend values at its southern part to negative trend values at its northern part. The Sudanese area, Southern Sudan and Northern Sudan sections have average TWS trend values of +2.25±1.15 mm/yr (+5.63±2.88 km3/yr), +0.39±1.83 mm/yr (+0.25±1.16 km3/yr), and +2.82±0.82 (+5.27±1.53 km3/yr), respectively (Fig. 5) throughout the investigated period. The temporal changes in GLDAS-estimated SMS and the secular trends over the Sudanese area, Southern Sudan and Northern Sudan sections (Fig. 6). The SMS is varying from –0.82±0.41 mm/yr (–0.52±0.26 km3/yr) over the Southern Sudan section to –1.28±0.13 mm/yr (–2.39±0.24 km3/yr), and –1.10±0.20 mm/yr (–2.75±0.5 km3/yr) over the Sudanese area and Northern Sudan section, respectively. The groundwater storage variation was estimated based on Equation 1,
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∆TWS = ∆GWS + ∆SWS + ∆SMS………………Equation 1 This equation has been previously applied by several authors (e.g. Rodell et al., 2009; Tiwari et al., 2009; Gonçalvès et al., 2013; Hassan and Jin, 2014; Mohamed et al., 2017; Ahmed and Abdelmohsen, 2018; Bhanja et al., 2018; Ahmed, 2019; Abdelmohsen et al., 2019; Sultan et al., 2019) to estimate the groundwater storage variations. The GWS trend values (Table 2) were calculated over the Sudanese area, Southern Sudan and Northern Sudan sections at +3.35±1.07, +1.21±1.69 and +4.09±0.76 mm/yr respectively (Fig. 7). The AAP of CMAP (AAPCMAP) data over the Sudanese area throughout the available years of the investigated period is low (2002–2011, CMAP: 399 mm) compared to the previous period (1979–2001, CMAP: 964 mm), and also over Southern Sudan section (2002–2011, CMAP: 762 mm; 1979–2001, CMAP: 1851 mm). The AAP over the Northern Sudan section is also decreasing form 1979–2001 period (CMAP: 561 mm) to the 2002–2011 period (CMAP: 233 mm). Comparison of the AAPs may indicate that the decreasing trend in GWS over the investigated periods with time is related to the decrease in the AAP from the preceding to the investigated period. Inspection of Figure 2 indicates that there is a general northward decrease in the AAP extracted from TRMM data. The acquired data was from April 2002 through July 2016. The Average Annual Precipitation of TRMM (AAPTRMM) data over the Sudanese area, Southern Sudan and Northern Sudan sections was estimated at 263.5, 596.7, and 149.8 km3/yr, respectively By adding the reported average annual groundwater withdrawal values (–0.66±0.066 km3/yr, FAO, 2016) and the assumed natural discharge (–0.13±0.013 km3/yr) to the GWS variation rate (+0.77±1.07 km3/yr) for the Southern Sudan section, the natural recharge was calculated at +1.56±1.07 km3/yr (+2.46±1.69 mm/yr) over this section (Table 2). The recharge rate of +8.85±1.42 km3/yr (+4.74±0.76 mm/yr) over the Northern Sudan Section was calculated by the summation of the artificial withdrawal (–1.01±0.101 km3/yr, FAO, 2016), the natural discharge (–0.21±0.021 km3/yr, and the GWS variation rate (+7.64±1.42 km3/yr). The Sudanese area is witnessing a recharge rate of +10.39±2.68 km3/yr (+4.15±1.07 mm/yr) by adding the groundwater extraction (–1.68±0.168 km3/yr) and the natural discharge (–0.33±0.033 km3/yr) to the GWS variation rate (–8.38±2.68 km3/yr). The recharge rates for the Northern Sudan section and the Sudanese area are similar to that estimated using GRACE and CLM4.5 model for the Northern Sudan sub-basin (4.01±1.19 mm/yr; Mohamed et al., 2017). Similar results to these values have also been achieved from groundwater flow simulation techniques in the rift basins of central Sudan at 4–8 mm/yr (Abdalla, 2008) and from using of the chloride tracer technique central Sudan at ~5.8 mm/yr (Edmunds et al., 1988). The calculated recharge is in a high correlation with the results of the local modeling and chemical techniques although the absence of information on the surface water volume by the recent constructed dams in northern and southern Sudan sections. Hence, the change in surface water (∆SWS) is neglected. The Southern Sudan section is receiving a natural recharge rate little lower than that of the Northern Sudan section, despite the high rate of precipitation. 5. Surface and groundwater flow 7
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Figure 8 shows that the ground surface map of the Southern Sudan section extracted from ETOPO1 Global Relief Model. It shows high relief of about 1000 m in its southern part and of about 700–800 m in its western part over the basement outcrops. It decreases north and northeastward to low value of less than 400 m it its northern part around the Nile River. The stream networks (Fig. 8) of the Southern Sudan section show that the surface water is draining away from the ground surface to the Nile valley. This supports the low recharge rate over this section. Moreover, the occupation of the massive basement rocks at the southern and southwestern parts of the area, which help take the water away northward to the Nile valley. On the other hand, the Nile River may help feed the aquifers along its stream in this area. The ground surface level of the Northern Sudan section varies from high values of more than 2700 m at the mountainous area, close to Chad in the west and decreases southward to values of about 400 m, close to the Northern Sudan section, forming streams joining with the streams from the Southern Sudan section and draining toward the Nile River. However, the ground surface elevation decreases northeastward to values of less than 250 m at the Nile valley, forming northeastward streams taking the surface water away to the river. The sediment thickness data was downloaded from NOAA National Geophysical Data Center (Divins, 2003). The sedimentary succession reaches high thickness of more than 6500 m at the northern part of Southern Sudan section and the southern part of the Northern Sudan section (Fig. 9). This may support the occurrence of high reserve of fluid in that area. The sediment thickness decreases northward to about 200–800 m in the Northern Sudan section. Thus, the lowlands in Sudan close to the Nile valley is apparently getting saturated from the heavy rainfall events causing flooding in these areas. The groundwater level in northern Sudan and southern Egypt is shown in Figure 9 (Hesse et al., 1987). In the western part, the groundwater flows northward to the Nile valley and southernmost part of Egypt close to the Sudanese border, which is in turn impeded by the Uweinat-Aswan basement uplift (Figs. 9 & 10). The uplift slows down the northward groundwater flow to southern Egypt, in addition to the thinned sedimentary cover in northern Sudan and southern Egypt (Fig. 9), close to the uplift. This was confirmed by presence of groundwater samples depleted in isotopic compositions (O, H) north of the uplift, in comparison to those enriched in isotopic compositions south of the uplift (Fig. 9). These isotopic differences across the uplift indicate mixing of the fossil groundwater with modern water from the rainfall south of the uplift (Sultan et al., 2013), whereas no modern water is present north of the uplift. However, in the eastern part, the groundwater flows northward to the Nile valley.
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5. Conclusion: The results indicate that during the analysed period (April 2002–July 2016), the groundwater storage variations were estimated at +8.38±2.68, +7.64±1.42, and +0.77±1.07 km3/yr for the Sudanese area, Northern Sudan and Southern Sudan sections, respectively. The Sudanese area, Northern Sudan and Southern Sudan sections are receiving high average annual precipitation estimated at 520.2, 300.3, and 1165.6 mm/yr, respectively. The AAPCMAP over these areas indicate that there is a general decrease in the precipitation during the investigated period in compared to the preceding period (1979–2001), where the AAPCMAP for the Sudanese area, Northern Sudan, and Southern Sudan sections were estimated at 964, 561, and 1851 mm/yr, respectively. Higher recharge rates were estimated at +10.39±2.68 and +8.85±1.42 km3/yr for the Sudanese area, and Northern Sudan sections, respectively. Minor recharge of +1.56±1.07 km3/yr was estimated for the Southern Sudan section, which is mostly occupied by massive basement rocks to the south and southwest. The streams over the Northern Sudan section are taking the surface water away to the Nile valley and lowlands, and this is also happening over the Southern Sudan section. However, the large area of the Northern Sudan section is giving the surface water enough time to feed the groundwater. The groundwater in Northern Sudan section is flowing northeastward to the Nile valley, while the northward flow to Southern Egypt is impeded by the Uweinat-Aswan basement uplift. The results show that GRACE and GLDAS data sets can give a good estimation of the water budget in the arid regions. Furthermore, their results are in a good agreement with those estimated from groundwater flow simulation techniques and chemical tracers.
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Table 1. A correlation matrix among the different datasets used in the current study for each area.
Sudanese area CSR JPL GFZ Avg
CSR 1
JPL 0.95 1
GFZ 0.95 0.92 1
Southern Sudan Section Avg 099 0.98 0.98 1
CSR JPL GFZ Avg
Northern Sudan Section
CSR
JPL
GFZ
Avg
1
0.99 1
0.98 0.98 1
1 0.99 0.99 1
CSR JPL GFZ Avg
CSR
JPL
GFZ
Avg
1
0.90 1
0.90 0.83 1
0.97 0.95 0.95 1
396 397 398 399 400 401 Table 2. TWS components over the Sudanese area. Area /section Sudanese Southern Sudan Northern Sudan
∆TWS
∆SMS
Qn
Qa
∆GWS
Rn
mm/ yr +2.25 ±1.15 +0.39 ±1.83
km3/y r +5.63 ±2.88 +0.25 ±1.16
mm/ yr –1.10 ±0.20 –0.82 ±0.41
km3/y r –2.75 ±0.5 –0.52 ±0.26
mm/ yr +3.35 ±1.07 +1.21 ±1.69
km3/y r +8.38 ±2.68 +0.77 ±1.07
mm/ yr –0.67 ±0.067 –1.04 ±0.104
km3/ yr –1.68 ±0.168 –0.66 ±0.066
mm/ yr –0.13 ±0.013 –0.21 ±0.021
km3/ yr –0.33 ±0.033 –0.13 ±0.013
mm/ yr +4.15 ±1.07 +2.46 ±1.69
km3/ yr +10.39 ±2.68 +1.56± 1.07
+2.82 ±0.82
+5.27 ±1.53
–1.28 ±0.13
–2.39 ±0.24
+4.09 ±0.76
+7.64 ±1.42
–0.54 ±0.054
–1.01 ±0.101
–0.11 ±0.011
–0.21 ±0.021
+4.74 ±0.76
+8.85± 1.42
∆TWS: Change in Terrestrial water storage. ∆GWS: Change in groundwater storage. ∆SMS: Change in soil moisture storage. Qa: Artificial withdrawal Qn: Natural discharge Rn: Natural Recharge
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Figure 1. Showing groundwater resources of Sudan (UNEP 2007).
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Figure 2. AAP (mm) extracted from TRMM data acquired from April 2002 to July 2016 for the study area, and surroundings.
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Figure 3. Color-coded secular TWS trend (mm/yr) map of monthly (April 2002–July 2016) TWS estimates created from CSR (a), JPL (b), GFZ (c) centers, and their averaging (d). 3
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Figure 4. Monthly series for TWS from CSR, JPL, GFZ centers and their averaging over the Sudanese area (A), Northern Sudan section (B), and the Southern Sudan section (C) from April 2002 to July 2016.
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Figure 5. Monthly series and secular trends for terrestrial water storage (TWS) over the Sudanese area (A), Northern Sudan section (B), and the Southern Sudan section (C) from April 2002 to July 2016. The brown line represents the overall trend in terrestrial storage changes for the investigated period.
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Figure 6. Monthly series and secular trends for SMS over the Sudanese area (A), Northern Sudan section (B), and the Southern Sudan section (C). The blue line represents the average trend in TWS changes for the investigated period.
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Figure 7. Monthly series and secular trends GWS over the Sudanese area (A), Northern Sudan section (B), and the Southern Sudan section (C). The blue line represents the average trend in GWS changes for the investigated period.
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Figure 8. A Digital Elevation Model (DEM) map over the study area showing the ground surface elevation. Also showing the Stream networks of the area.
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Figure 9. Sediment thickness map showing thickness of the sedimentary succession (m). Also showing the uplift, groundwater levels and directions in northern Sudan area. 9
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Figure 10. A geological cross section along the Uweinat-Aswan uplift (profile A-A`; Fig. 9) (Mohamed, 2016; Mohamed et al., 2017).
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Highlights:
•
Applications of satellite gravity data in estimation of the groundwater storage variations in Sudan.
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Modern recharge in Sudan based on geophysical data and climatic models.
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Partitioning of Terrestrial water storage in Sudan based on GRACE and GLDAS data.
The author declares that there is no conflict of interest.
S1464-343X(19)30395-4
DOI:
https://doi.org/10.1016/j.jafrearsci.2019.103740
Reference:
AES 103740
To appear in:
Journal of African Earth Sciences
Received Date: 1 July 2019 Revised Date:
23 October 2019
Accepted Date: 13 December 2019
Please cite this article as: Mohamed, A., Gravity based estimates of modern recharge of the Sudanese area, Journal of African Earth Sciences (2020), doi: https://doi.org/10.1016/j.jafrearsci.2019.103740. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
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Gravity based estimates of modern recharge of the Sudanese area
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Ahmed Mohamed
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Geology Department, Faculty of Science, Assiut University, Assiut 71516, Egypt
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Author: Ahmed Mohamed, Geology Department, Assiut University, Assiut, 71516, Egypt,
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Phone: +20-100-839-0135,
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[email protected]
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Key words: Sudan – GRACE – Terrestrial water storage – Groundwater storage – Precipitation – Recharge
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Abstract:
The North Africa region is seeking water resources to develop agricultural expansions and land reclamations projects based on the groundwater resources. Monthly (April 2002–July 2016) terrestrial water storage (TWS) solutions of the Gravity Recovery and Climate Experiment (GRACE) along with other datasets were used to monitor and estimate the variations in groundwater storage over the Sudanese area and its sections. Results indicate: (1) the Sudanese area, Southern Sudan and Northern Sudan sections are receiving an average recharge of +4.15±1.07, +2.46±1.69, and +4.74±0.76 mm/yr, respectively during the analysed period, taking into an account the annual extraction rate of 0.67±0.067, 1.04±0.104 and 0.54±0.054 mm/yr from the Sudanese area, Southern Sudan and Northern Sudan sections, respectively, (2) and the average groundwater storage variations of +3.35±1.07, +1.21±1.69 and +4.09±0.76 mm/yr from the Sudanese area, Southern Sudan and Northern Sudan sections, respectively, and (3) the assumed natural discharge of –0.13±0.013, –0.21±0.021, and –0.11±0.011 mm/yr from the Sudanese area, Southern Sudan and Northern Sudan sections, respectively. (4) The average precipitation of Tropical Rainfall Measuring Mission data over the Sudanese area, Southern Sudan and Northern Sudan sections was estimated at 520.2, 1165.6 and 300.3 km3/yr, respectively. (5) The ground surface relief is forming northeastward streams taking the surface water away to the river. (6) The groundwater flows to the southernmost part of Egypt is impeded by the Uweinat-Aswan basement uplift and the thinned sedimentary cover in northern Sudan and southern Egypt, which in turn flows northeastward to the river. (7) The integrated study is informative and cost-effective model for best estimating the recharge rate of large areas.
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Introduction:
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More than 2 billion people around the world use groundwater as the only source of domestic and agricultural purposes (Alley et al. 2002). Groundwater represents a main component of the water cycle in sustaining lakes, streams, and aquatic communities (Alley et al. 2002). Groundwater resources are subjected, in many regions of the world, to stress due to many factors, including contamination, salinization, and heavy exploitation (Wada et al. 2010). Furthermore, climate change also affects the groundwater resources. Temperatures are globally going to rise and precipitation levels will fall (IPCC. 2007).
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The North Africa region is sensitive to climate change. It is an arid region and has limited water resources (Wingqvist, 2010). The increased population growth, developing economies, and landuse changes is leading to major stresses on the freshwater resources (Heathwaite, 2010) and causing great effects on the region’s socio-economic potential. There will be an increase of 40% in water demand by 2030 worldwide (Catley-Carlson, 2011). Moreover, this region ran out of fresh water in the 70s and depends on water from outside the region in the form of food imports (Allan, 2002). Based on the data of the World Bank (2005), the North Africa region will grow to a projected >430 million people in 2025 from the present 311 million and around 100 million in 1960. This is getting the per-capita water average to extremely worrying levels. The region is experiencing water scarcity combined with about a low efficiency of water use (about 40%) in agriculture purposes (FAO, 2003). Countries like Libya, Bahrain, Jordan are suffering from extreme situations of water scarcity (WWDR, 2003).
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Sudan, Egypt, Libya, and Chad share the Nubian Aquifer, the large extensive transboundary aquifer systems in northeast Africa, covering about 2.2x106 km² (CEDARE, 2002; Bakhbakhi, 2006). The shared aquifer is managed through an agreement formalized by the four countries. In Sudan, groundwater is used for agricultural expansions and domestic demands in many areas of Sudan, especially those away from the Nile valley. Surface water resources in these areas are affected by higher evaporation rates. These conditions make groundwater the only feasible source for water in these dry areas. Groundwater aquifers cover most of the Sudanese areas and includes: the Nubian aquifer, the Um Ruwaba formation, the alluvial aquifer and the hard rocks aquifers (Salih and Khadam, 1984). Satellite measurements of temporal changes of time-variable gravity from the Gravity Recovery and Climate Experiment (GRACE) mission (Tapley et al., 2004) are providing a method for observing the Terrestrial Water Storage (TWS) variations. The GRACE project was started in March 2002 with an original design lifetime of 5 years and was providing excellent data till 2017. GRACE-FO, successfully launched on May 22, 2018, will continue the goals of the previous GRACE mission and tracking Earth's water movement to detect changes in groundwater storage (GWS), the water reserve in large lakes and rivers, soil moisture (SM), ice sheets, and sea level variation caused by the addition of water to the ocean as well. GRACE provides monthly variations in Earth’s gravity field since April 2002. These temporal variations are caused by different climatic and atmospheric processes and natural events as well. GRACE data have been widely applied for attaining deeper knowledge to regional scale water mass variations (Andersen and Hinderer, 2005; Brown and Tregoning, 2010; Bonsor et al., 2010). They have been used to assist basin-scale water balance calculations (Rodell et al. 2004; Syed et 3
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al. 2005), and to estimate the water storage changes on the basin and sub-basin scale (Crowley et al., 2006; Bonsor et al.,2010; Ahmed et al., 2011; Papa et al., 2015). GRACE data have been successfully used to estimate the depletion and recharge rates of the aquifers (Leblanc et al., 2009; Rodell et al., 2009; Tiwari et al., 2009; Voss et al., 2013; Gonçalvès et al., 2013; Joodaki et al., 2014; Döll et al., 2014; Chinnasamy and Agoramoorthy, 2015; Xiang et al., 2016; Alzyoud et al., 2015; Chinnasamy et al. 2015; Huo et al., 2016; Long et al., 2016; Jiang et al., 2016; Chinnasamy and Sunde, 2016; Mohamed et al., 2017; Ahmed and Abdelmohsen, 2018; Mohamed, 2019). Several studies have focused on estimating the variations in GWS over the Nubian Aquifer (e.g. Sultan et al., 2013, 2014b; 2015; Mohamed et al., 2014, 2015; Abdelmohsen et al., 2019) and monitoring the spatio-temporal variations in the total water storage of the Nile basin (Abdelmalik and Abdelmohsen, 2019). Sultan et al. (2019) have used an integrated approach of GRACE and geochemical data with other datasets to assess the sustainability, origin, age, and groundwater storage of the Mega Aquifer System in the Arabian Peninsula. Mohamed et al. (2017) have explained that the Northern Sudan sub-basin, which represents the recharge zone for the Nubian Aquifer in Sudan, has an average recharge rate of 0.78±0.49 km3/yr during January 2003– December 2013 period. Ahmed and Abdelmohsen (2018) have shown that the Egyptian part of the Nubian Aquifer is getting a cumulative recharge of 20.27±1.95 km3/yr through April 2002– February 2006 and April 2008–June 2016 periods. They have also shown that the recharge is occurring only under heavy rainfall events and/or a notable rise in Lake Nasser. Abdalla (2008) has estimated the average recharge rate of 4–8 mm/yr for the central Sudan rift basins based on groundwater flow modelling. Furthermore, Edmunds et al. (1988) have shown the central Sudan area is receiving an average recharge rate of about 5.8 mm/yr based on chloride tracer technique in the area. These studies have not quantified the recharge rate for the whole Sudanese aquifers. Moreover, the modeling and geochemical techniques are local studies and cannot be applied for the regional scales, given the uncertainties of model parameters (Gonçalvès et al., 2013) and the scarcity of the datasets used in these local methods. In the current study, temporal GRACE-derived TWS along with outputs of Global Land Data Assimilation System (GLDAS) were used to assess the GWS and the recharge rates over the different sections of Sudan. 2- Hydrogeological setting of the Sudanese area Four major aquifers cover about 50% of the surface area of the Sudanese region (Fig. 1). These aquifers are represented by four groups (Salama, 1976; Salih and Khadam, 1984; Abdo and Salih, 2012) as the Nubian aquifer, Um Rawaba formations, the alluvial aquifers, and the Basement complex. The Nubian aquifer is the largest and most extensive aquifer in Sudan, with its surface area covering more than 28% of the surface area of Sudan (Abdo and Salih, 2012). It is represented by sandstones and formed from nine major basins, including the Nile basin, the Sahara basin, Atbara basin, the Blue Nile basin, central Darfur basin, Gedaref basin, Nuhud basin, Shaggara basin and Sag El Naam basin. The saturated layers vary in thickness from 100 to 2000 m, with low to moderate permeability of good water quality.
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The Um Ruwaba aquifer is formed from three main basins: Atshan basin, Baggara basin, and Bara basin, covering more than 7% of the surface area of the country (Abdo and Salih, 2012). The aquifer is represented mainly by unconsolidated gravels sands and clays with a very large thickness up to three thousand meters. Well yield varies from 5 to 20 m3/h. The water quality is sufficient for domestic uses but tend to be saline in some areas. The alluvial aquifers are covered some regions as Gash, Azum, Wadi Nyala, Wadi Kutum, and Arbaat with high permeabilities, moderate to high transmissivity values (500-1500 m2/day) and well yield as well. The alluvial aquifers are recharged annually from seasonal streams and wadis. The aquifers are characterized by its moderate to high well yield. The water quality of the aquifers is very good with salinity less than 400 ppm. 3- Data and Methods 3.1. Gravity data The gravity field products from three different analysis groups were used in the current work: The Center of Space Research-University of Texas (CSR), the Jet Propulsion Laboratory (JPL), and the German Research Centre for Geosciences (GFZ).The available three solutions were downloaded, and the study areas were extracted to estimate the TWS variations. The associated scaling factors were averaged and applied to all three solutions to restore attenuated signals consistent with the employed filter parameters (Landerer and Swenson 2012). The scaled solutions were averaged, the mean of the three data solutions was calculated, and were used for the calculations. The ensemble mean (average of CSR, JPL, and GFZ fields) was found to be effective in reducing the noise in the gravity field solutions (Sakumura et al., 2014). 3.2. Climatic data Rainfall is one of the controlling factors that affect the mass variations within the geological formations. Rain gauges generally give very accurate point measurements of precipitation, but these stations are discontinuous and generally absent over the unpopulated desert areas of Sudan. Therefore, rainfall measurements from space were utilized in the current study. Two of these measurement types, Climate Merged Analysis of Precipitation (CMAP) data and Tropical Rainfall Measuring Mission (TRMM) data, were utilized to investigate the role of the climate on the temporal and spatial variations of GRACE-derived TWS and GWS over the Sudanese area and its sections. CMAP is a collection of precipitation datasets, with a 2.5° spatial resolution and monthly temporal resolution (Xie and Arkin 1997). This dataset provides global coverage merged precipitation estimations constructed from merging and analysis of different kinds of data sources with different characteristics (Xie and Arkin, 1997). TRMM was launched in 1997 to monitor and study rainfall for climatic research. It was acquired (1998–2018) with near-global coverage at a temporal resolution of 3 hours and with a 2.5° spatial resolution. TRMM was first proposed in 1984 (Kummerow, 1998; Huffman et al., 2007). It has been applied with GRACE data for estimating water storage changes and balances over Africa (Hassan and Jin, 2014). Monthly TRMM data, covering the studied period, was used to generate the Average Annual Precipitation (AAP) map (Fig. 2) over the study area. 3.3. Satellite soil moisture
5
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GRACE has no vertical resolution and cannot differentiate between the components of the TWS. The gravity signal is a cumulate of the mass changes within the atmosphere, oceans, and water stored on land (Wahr et al., 2006). Therefore, it is not possible to deduce whether a mass variation estimated for an area on land is caused by changes in the surface water storage (SWS), changes in the ground water storage (GWS), or changes in the soil moisture storage (SMS). To overcome this problem, other independent tools such as climatic model (CLM) outputs were used to separate those contributions. Monitoring and measuring of SMS at global scales can predict for flash flooding events. Although SM measurements often lack the reliability and consistency (Moiwo et al. 2011), satellite- and ground-based observational data products are now providing SM data with a high accuracy. One of these, is the Global Land Data Assimilation System (Rodell et al., 2004, 2009; Moiwo et al., 2011). It uses global land surface models in several different applications such as climate prediction, water cycle, and water resources. GLDAS has been used in: (1) estimation of TWS variations (Swenson and Wahr, 2006; Rodell et al., 2007; Henry et al., 2011; Moiwo et al., 2011; Long et al., 2013; Hassan and Jin, 2014); (2) evaluation of ground and satellite-based SM outputs (Dorigo et al. 2012, 2013); (3) analysis of regional environmental changes (Zhong et al., 2011; Gao et al., 2013); (4) estimation of agricultural developments and irrigated and dry areas (Romagura et al., 2012; Ghazanfari et al., 2013); and (5) estimation of global distributions of land surface fields such as SM (Rodell et al., 2004; Hogue et al., 2006). In the current study, GLDAS, version 1 (GLDAS-1), 0.25º 3-hourly data were downloaded from ftp://hydro1.sci.gsfc.nasa.gov/data/s4pa/GLDAS_V1/). The monthly data were generated through temporally averaging of the 3-hourly products (Fang et al., 2009). The used GLDAS data cover the same period as the GRACE observations. 4. Water budget Figures 3 show the spatial distributions of the secular trend images in TWS over the Sudanese area and its sections from CSR, JPL, GFZ solutions (Figs. 3a, b, c) and the average of these solutions (Fig. 3d). Figure 4 shows a good agreement in amplitudes and phases between the different GRACE solutions and their averages over the investigated areas. A well correlation varying from 0.92 to 0.99 is among the different datasets over the Sudanese area (Table 1). For the Northern Sudan Section, the correlation coefficient varies from low as 0.83 to high as 0.97 between the different datasets, while it varies from 0.98 to 1 over the Southern Sudan section. Witnessing of Figure 3d indicates that the Northern Sudan section is experiencing average positive trend values concentrated on the central part of it. Whereas the Southern Sudan section is showing positive trend values at its southern part to negative trend values at its northern part. The Sudanese area, Southern Sudan and Northern Sudan sections have average TWS trend values of +2.25±1.15 mm/yr (+5.63±2.88 km3/yr), +0.39±1.83 mm/yr (+0.25±1.16 km3/yr), and +2.82±0.82 (+5.27±1.53 km3/yr), respectively (Fig. 5) throughout the investigated period. The temporal changes in GLDAS-estimated SMS and the secular trends over the Sudanese area, Southern Sudan and Northern Sudan sections (Fig. 6). The SMS is varying from –0.82±0.41 mm/yr (–0.52±0.26 km3/yr) over the Southern Sudan section to –1.28±0.13 mm/yr (–2.39±0.24 km3/yr), and –1.10±0.20 mm/yr (–2.75±0.5 km3/yr) over the Sudanese area and Northern Sudan section, respectively. The groundwater storage variation was estimated based on Equation 1,
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∆TWS = ∆GWS + ∆SWS + ∆SMS………………Equation 1 This equation has been previously applied by several authors (e.g. Rodell et al., 2009; Tiwari et al., 2009; Gonçalvès et al., 2013; Hassan and Jin, 2014; Mohamed et al., 2017; Ahmed and Abdelmohsen, 2018; Bhanja et al., 2018; Ahmed, 2019; Abdelmohsen et al., 2019; Sultan et al., 2019) to estimate the groundwater storage variations. The GWS trend values (Table 2) were calculated over the Sudanese area, Southern Sudan and Northern Sudan sections at +3.35±1.07, +1.21±1.69 and +4.09±0.76 mm/yr respectively (Fig. 7). The AAP of CMAP (AAPCMAP) data over the Sudanese area throughout the available years of the investigated period is low (2002–2011, CMAP: 399 mm) compared to the previous period (1979–2001, CMAP: 964 mm), and also over Southern Sudan section (2002–2011, CMAP: 762 mm; 1979–2001, CMAP: 1851 mm). The AAP over the Northern Sudan section is also decreasing form 1979–2001 period (CMAP: 561 mm) to the 2002–2011 period (CMAP: 233 mm). Comparison of the AAPs may indicate that the decreasing trend in GWS over the investigated periods with time is related to the decrease in the AAP from the preceding to the investigated period. Inspection of Figure 2 indicates that there is a general northward decrease in the AAP extracted from TRMM data. The acquired data was from April 2002 through July 2016. The Average Annual Precipitation of TRMM (AAPTRMM) data over the Sudanese area, Southern Sudan and Northern Sudan sections was estimated at 263.5, 596.7, and 149.8 km3/yr, respectively By adding the reported average annual groundwater withdrawal values (–0.66±0.066 km3/yr, FAO, 2016) and the assumed natural discharge (–0.13±0.013 km3/yr) to the GWS variation rate (+0.77±1.07 km3/yr) for the Southern Sudan section, the natural recharge was calculated at +1.56±1.07 km3/yr (+2.46±1.69 mm/yr) over this section (Table 2). The recharge rate of +8.85±1.42 km3/yr (+4.74±0.76 mm/yr) over the Northern Sudan Section was calculated by the summation of the artificial withdrawal (–1.01±0.101 km3/yr, FAO, 2016), the natural discharge (–0.21±0.021 km3/yr, and the GWS variation rate (+7.64±1.42 km3/yr). The Sudanese area is witnessing a recharge rate of +10.39±2.68 km3/yr (+4.15±1.07 mm/yr) by adding the groundwater extraction (–1.68±0.168 km3/yr) and the natural discharge (–0.33±0.033 km3/yr) to the GWS variation rate (–8.38±2.68 km3/yr). The recharge rates for the Northern Sudan section and the Sudanese area are similar to that estimated using GRACE and CLM4.5 model for the Northern Sudan sub-basin (4.01±1.19 mm/yr; Mohamed et al., 2017). Similar results to these values have also been achieved from groundwater flow simulation techniques in the rift basins of central Sudan at 4–8 mm/yr (Abdalla, 2008) and from using of the chloride tracer technique central Sudan at ~5.8 mm/yr (Edmunds et al., 1988). The calculated recharge is in a high correlation with the results of the local modeling and chemical techniques although the absence of information on the surface water volume by the recent constructed dams in northern and southern Sudan sections. Hence, the change in surface water (∆SWS) is neglected. The Southern Sudan section is receiving a natural recharge rate little lower than that of the Northern Sudan section, despite the high rate of precipitation. 5. Surface and groundwater flow 7
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Figure 8 shows that the ground surface map of the Southern Sudan section extracted from ETOPO1 Global Relief Model. It shows high relief of about 1000 m in its southern part and of about 700–800 m in its western part over the basement outcrops. It decreases north and northeastward to low value of less than 400 m it its northern part around the Nile River. The stream networks (Fig. 8) of the Southern Sudan section show that the surface water is draining away from the ground surface to the Nile valley. This supports the low recharge rate over this section. Moreover, the occupation of the massive basement rocks at the southern and southwestern parts of the area, which help take the water away northward to the Nile valley. On the other hand, the Nile River may help feed the aquifers along its stream in this area. The ground surface level of the Northern Sudan section varies from high values of more than 2700 m at the mountainous area, close to Chad in the west and decreases southward to values of about 400 m, close to the Northern Sudan section, forming streams joining with the streams from the Southern Sudan section and draining toward the Nile River. However, the ground surface elevation decreases northeastward to values of less than 250 m at the Nile valley, forming northeastward streams taking the surface water away to the river. The sediment thickness data was downloaded from NOAA National Geophysical Data Center (Divins, 2003). The sedimentary succession reaches high thickness of more than 6500 m at the northern part of Southern Sudan section and the southern part of the Northern Sudan section (Fig. 9). This may support the occurrence of high reserve of fluid in that area. The sediment thickness decreases northward to about 200–800 m in the Northern Sudan section. Thus, the lowlands in Sudan close to the Nile valley is apparently getting saturated from the heavy rainfall events causing flooding in these areas. The groundwater level in northern Sudan and southern Egypt is shown in Figure 9 (Hesse et al., 1987). In the western part, the groundwater flows northward to the Nile valley and southernmost part of Egypt close to the Sudanese border, which is in turn impeded by the Uweinat-Aswan basement uplift (Figs. 9 & 10). The uplift slows down the northward groundwater flow to southern Egypt, in addition to the thinned sedimentary cover in northern Sudan and southern Egypt (Fig. 9), close to the uplift. This was confirmed by presence of groundwater samples depleted in isotopic compositions (O, H) north of the uplift, in comparison to those enriched in isotopic compositions south of the uplift (Fig. 9). These isotopic differences across the uplift indicate mixing of the fossil groundwater with modern water from the rainfall south of the uplift (Sultan et al., 2013), whereas no modern water is present north of the uplift. However, in the eastern part, the groundwater flows northward to the Nile valley.
8
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5. Conclusion: The results indicate that during the analysed period (April 2002–July 2016), the groundwater storage variations were estimated at +8.38±2.68, +7.64±1.42, and +0.77±1.07 km3/yr for the Sudanese area, Northern Sudan and Southern Sudan sections, respectively. The Sudanese area, Northern Sudan and Southern Sudan sections are receiving high average annual precipitation estimated at 520.2, 300.3, and 1165.6 mm/yr, respectively. The AAPCMAP over these areas indicate that there is a general decrease in the precipitation during the investigated period in compared to the preceding period (1979–2001), where the AAPCMAP for the Sudanese area, Northern Sudan, and Southern Sudan sections were estimated at 964, 561, and 1851 mm/yr, respectively. Higher recharge rates were estimated at +10.39±2.68 and +8.85±1.42 km3/yr for the Sudanese area, and Northern Sudan sections, respectively. Minor recharge of +1.56±1.07 km3/yr was estimated for the Southern Sudan section, which is mostly occupied by massive basement rocks to the south and southwest. The streams over the Northern Sudan section are taking the surface water away to the Nile valley and lowlands, and this is also happening over the Southern Sudan section. However, the large area of the Northern Sudan section is giving the surface water enough time to feed the groundwater. The groundwater in Northern Sudan section is flowing northeastward to the Nile valley, while the northward flow to Southern Egypt is impeded by the Uweinat-Aswan basement uplift. The results show that GRACE and GLDAS data sets can give a good estimation of the water budget in the arid regions. Furthermore, their results are in a good agreement with those estimated from groundwater flow simulation techniques and chemical tracers.
9
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Table 1. A correlation matrix among the different datasets used in the current study for each area.
Sudanese area CSR JPL GFZ Avg
CSR 1
JPL 0.95 1
GFZ 0.95 0.92 1
Southern Sudan Section Avg 099 0.98 0.98 1
CSR JPL GFZ Avg
Northern Sudan Section
CSR
JPL
GFZ
Avg
1
0.99 1
0.98 0.98 1
1 0.99 0.99 1
CSR JPL GFZ Avg
CSR
JPL
GFZ
Avg
1
0.90 1
0.90 0.83 1
0.97 0.95 0.95 1
396 397 398 399 400 401 Table 2. TWS components over the Sudanese area. Area /section Sudanese Southern Sudan Northern Sudan
∆TWS
∆SMS
Qn
Qa
∆GWS
Rn
mm/ yr +2.25 ±1.15 +0.39 ±1.83
km3/y r +5.63 ±2.88 +0.25 ±1.16
mm/ yr –1.10 ±0.20 –0.82 ±0.41
km3/y r –2.75 ±0.5 –0.52 ±0.26
mm/ yr +3.35 ±1.07 +1.21 ±1.69
km3/y r +8.38 ±2.68 +0.77 ±1.07
mm/ yr –0.67 ±0.067 –1.04 ±0.104
km3/ yr –1.68 ±0.168 –0.66 ±0.066
mm/ yr –0.13 ±0.013 –0.21 ±0.021
km3/ yr –0.33 ±0.033 –0.13 ±0.013
mm/ yr +4.15 ±1.07 +2.46 ±1.69
km3/ yr +10.39 ±2.68 +1.56± 1.07
+2.82 ±0.82
+5.27 ±1.53
–1.28 ±0.13
–2.39 ±0.24
+4.09 ±0.76
+7.64 ±1.42
–0.54 ±0.054
–1.01 ±0.101
–0.11 ±0.011
–0.21 ±0.021
+4.74 ±0.76
+8.85± 1.42
∆TWS: Change in Terrestrial water storage. ∆GWS: Change in groundwater storage. ∆SMS: Change in soil moisture storage. Qa: Artificial withdrawal Qn: Natural discharge Rn: Natural Recharge
402 403 404 405 406
10
407 408
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Figure 1. Showing groundwater resources of Sudan (UNEP 2007).
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Figure 2. AAP (mm) extracted from TRMM data acquired from April 2002 to July 2016 for the study area, and surroundings.
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Figure 3. Color-coded secular TWS trend (mm/yr) map of monthly (April 2002–July 2016) TWS estimates created from CSR (a), JPL (b), GFZ (c) centers, and their averaging (d). 3
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Figure 4. Monthly series for TWS from CSR, JPL, GFZ centers and their averaging over the Sudanese area (A), Northern Sudan section (B), and the Southern Sudan section (C) from April 2002 to July 2016.
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Figure 5. Monthly series and secular trends for terrestrial water storage (TWS) over the Sudanese area (A), Northern Sudan section (B), and the Southern Sudan section (C) from April 2002 to July 2016. The brown line represents the overall trend in terrestrial storage changes for the investigated period.
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Figure 6. Monthly series and secular trends for SMS over the Sudanese area (A), Northern Sudan section (B), and the Southern Sudan section (C). The blue line represents the average trend in TWS changes for the investigated period.
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Figure 7. Monthly series and secular trends GWS over the Sudanese area (A), Northern Sudan section (B), and the Southern Sudan section (C). The blue line represents the average trend in GWS changes for the investigated period.
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Figure 8. A Digital Elevation Model (DEM) map over the study area showing the ground surface elevation. Also showing the Stream networks of the area.
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Figure 9. Sediment thickness map showing thickness of the sedimentary succession (m). Also showing the uplift, groundwater levels and directions in northern Sudan area. 9
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Figure 10. A geological cross section along the Uweinat-Aswan uplift (profile A-A`; Fig. 9) (Mohamed, 2016; Mohamed et al., 2017).
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Highlights:
•
Applications of satellite gravity data in estimation of the groundwater storage variations in Sudan.
•
Modern recharge in Sudan based on geophysical data and climatic models.
•
Partitioning of Terrestrial water storage in Sudan based on GRACE and GLDAS data.
The author declares that there is no conflict of interest.