Optimization of floodwater redistribution from Lake Nasser could recharge Egypt's aquifers and mitigate its excessive floods

The study investigates how extreme precipitation events in the Nile Basin, which have produced excessive floods and elevated Lake Nasser (LN) to maximum capacity, can be transformed from a hazard into a water-resource opportunity for Egypt’s Western Desert. Two recent multi-year flood events (1998–2002 and 2019–2022) led to diversion of excess LN water to the Tushka depressions, forming Tushka Lakes (TLs) where most water evaporated rather than recharging the underlying Nubian Sandstone Aquifer System (NSAS), a large fossil aquifer currently being depleted. Climate model projections and observational data suggest increased interannual variability of Nile flow in the 21st century, potentially increasing the frequency of such extreme inflows. The research question is whether optimizing the redistribution of excess LN water to nearby lowlands can maximize aquifer recharge while minimizing evaporation and infrastructure impacts. The purpose is to evaluate natural, gravity-driven release scenarios to selected depressions and quantify partitioning among runoff, infiltration (recharge), and evaporation. The importance lies in providing an adaptation pathway that could mitigate climate-driven flood risks while sustaining groundwater resources crucial for Egypt’s development.

The paper situates its work within evidence that global warming is increasing the frequency and intensity of extreme climatic events, including floods, with teleconnections (e.g., ENSO) influencing Nile River flow variability, projected to increase by about 50% in the 21st century relative to the 20th. Prior studies have characterized the NSAS as a fossil aquifer with million-year-old waters, recharged in past humid periods and presently undergoing depletion under modern withdrawals. Methods for lake evaporation estimation (e.g., Priestley–Taylor, Penman, CRLE) and remote sensing approaches (GRACE/GRACE-FO for terrestrial water storage, Landsat for surface water extent) have been established in the literature. The study builds on these foundations by integrating remote sensing, evaporation modeling, and hydrodynamic simulations to propose an operational adaptation strategy for excess floodwaters.

The authors executed three tasks: Task I (groundwater depletion assessment), Task II (TLs storage, evaporation, depletion, and hydraulic parameters), and Task III (simulation and selection of optimum recharge scenarios). Task I: GRACE and GRACE-FO mascon solutions (CSR RL06.2 primary; GSFC RL06v2, JPL RL06.1 for uncertainty) were used to derive Terrestrial Water Storage (TWS) time series over the Dakhla subbasin (buffering 120 km from LN and TLs to avoid surface water leakage), yielding groundwater storage trends for 2002–2022. Projected 21st-century depletion combined linear trends from TWS and trends in groundwater extraction (2002–2021). Task II: Temporal variations in TL volumes were quantified using Landsat 5/8/9 imagery to delineate water extent via NDWI/MNDWI and COP30m DEM (1 m contours) to extract water surface elevations and compute volumes with RiverFlow2D volume functions. Uncertainties were assessed by ±1 m water level tests. Evaporation rates over TLs were estimated by the Priestley–Taylor model using ERA5 meteorological inputs; monthly rates ranged 3.4–9.1 mm/day (mean ~6.3 mm/day), annual ~2.32 m/yr, comparable to literature values. Using depletion histories of TLs after the 1st event (Aug 2002–Oct 2019) and seasonal depletion of Lake 6b (Nov–Jul for 2020–2021 and 2021–2022), the team calibrated Green–Ampt infiltration parameters (hydraulic conductivity, suction head, effective porosity) for key formations: Dakhla and Quseir shales, thin Paleocene–Eocene carbonates (Kurkur, Garra, Dungul), basement, and Kiseiba Sandstone. Calibration maximized agreement between simulated and observed TL volume declines, with model skill quantified via RMSE and NSE. Task III: Hydrodynamic modeling of excess LN water releases into two nearby lowlands (Depressions I and II) was conducted with RiverFlow2D (QGIS plugin), solving 2D shallow-water equations using a finite-volume scheme (AARS Riemann solver). Inputs included COP30m DEM topography, Manning’s n ~0.03, calibrated Green–Ampt infiltration parameters by substrate, Priestley–Taylor evaporation, and spillway boundary conditions with tested discharges (200, 500, 1000 m³/s), settling on 1000 m³/s to match observed TL filling during the 2nd event. Mesh resolution used 80–100 m cells (30–40 m in complex terrain). Scenario I released water toward Depression I (area ~2000 km²; max depth ~70 m; predominantly shale floor); Scenario II released water into Depression II (area ~850 km²; max depth ~50 m; predominantly Nubian Sandstone Kiseiba Fm. with some Paleocene–Eocene carbonates). Scenario performance was evaluated by partitioning of released water into surface storage, infiltration (recharge), and evaporation over time, while checking encroachment on infrastructure or farmlands.

• GRACE-derived groundwater storage trend for the Dakhla subbasin (2002–2022): overall depletion −0.98 ± 0.1 BCM/yr; earlier high-depletion phase (Apr 2002–Jun 2017): −1.75 ± 0.17 BCM/yr; recent recovery phase (Jun 2018–Oct 2022): +2.86 ± 1.1 BCM/yr.
• Expansion of agriculture and extraction in Dakhla: cultivated area grew from 398 km² (2002) to 1483 km² (2020); extraction rose from 0.4 BCM (2002) to 1.53 BCM (2020). Projected cumulative depletion under current trends: 8.4 BCM (2030), 18.9 (2040), 29.4 (2050), 39.9 (2060), 60.9 (2080), 81.9 (2100).
• Tushka Lakes inflows: 1st event (1998–2002) total 25.5 ± 1 BCM; 2nd event (2019–2022) total 53.5 ± 1.4 BCM, with max annual inflow 18.7 ± 0.8 BCM (2021). By 2019, ~90% (~25.0 ± 1 BCM) of 1st-event water had evaporated (avg 1.46 BCM/yr), leaving ~0.50 ± 0.03 BCM in Lake 4.
• Lake 6b seasonal dynamics during 2nd event: inflow ~3.9 ± 0.1 BCM (Nov 2020) and ~4.0 ± 0.1 BCM (Nov 2021); losses Nov–Jul were 2.51 ± 0.3 BCM and 2.41 ± 0.3 BCM (≈ −0.2 BCM/month).
• Evaporation over TLs by Priestley–Taylor: monthly 3.4–9.1 mm/day (mean 6.3); annual ~2.32 m/yr (2017–2018), consistent with literature (2.2–2.7 m/yr).
• Model calibration performance: High agreement between simulated and observed TL depletion with NSE ≈ 0.97–0.99 and low RMSE (e.g., Lake 1 RMSE 0.16 BCM, NSE 0.99; Lake 6b NSE 0.97–0.98).
• Substrate controls: TLs 1,3,4,5 underlain by massive limestone over thick Dakhla/Quseir shales or basement have low infiltration (~10%) and high evaporation (~90%), akin to clay-like behavior. Kiseiba Sandstone exhibits high infiltration (~92%) and low evaporation (~8%).
• Scenario I (Depression I; shale floor): capacity only ~2.85 BCM before water encroaches on Darb El Arba’in Road, farms, and El Kharga; very limited infiltration (≈5% after 2.5 years), with remaining water split between surface storage and evaporation; not optimal for recharge though it offers localized agricultural surface-water opportunities.
• Scenario II (Depression II; mostly Kiseiba Sandstone): release 53.5 BCM from Lake 1 over 800 days (39.7 BCM at 1000 m³/s over 460 days, then 13.8 BCM over 340 days with variable rates). At end of discharge (800 days): surface water 24.5 BCM (45.8%), infiltration 23.4 BCM (43.8%), evaporation 5.6 BCM (10.4%); max depth ~59.5 m; area inundated ~1550 km². After 2.5 years from end of discharge: infiltration 39.7 BCM (74.3%), evaporation 10.8 BCM (20.1%), surface water 3.0 BCM (5.6%); max depth ~41 m; surface water mostly over carbonate/shale patches. Only a small cultivated area (~100 km² near Lake 6a) was submerged during filling.
• Water-balance implications: Infiltration from Scenario II (39.7 BCM) could compensate projected Dakhla subbasin depletion to ~2060 (39.9 BCM). A second similar event before 2100 would yield ~79.4 BCM infiltration, nearly offsetting projected depletion to 2100 (81.9 BCM).
• Overall conclusion: Redirecting excess LN floodwaters into Depression II maximizes aquifer recharge and minimizes evaporative losses and infrastructure impacts, providing a sustainable adaptation option.

Routing Lake Nasser’s excess waters to the Tushka depressions has historically resulted in large evaporative losses because these depressions are floored by low-permeability shales, massive carbonates, or crystalline basement, limiting infiltration to the NSAS. The modeling demonstrates that channeling future excess waters into Depression II, largely underlain by Kiseiba Sandstone, dramatically increases infiltration and reduces evaporation, thereby converting episodic flood hazards into effective groundwater recharge. Under realistic inflow magnitudes observed in 2019–2022, Scenario II yields sufficient recharge to offset multiple decades of projected groundwater depletion in the Dakhla subbasin, with only limited and manageable surface-water encroachment on cultivated land and no major urban/infrastructure conflicts. Scenario I, despite geographic proximity and larger areal extent, is not suitable for major recharge due to its shale substrate and encroachment risks, though it may serve localized agricultural uses along flow paths during wet periods. The results highlight an operational adaptation strategy that balances flood mitigation, water resource sustainability, and development needs, and they underscore the importance of geologic substrate selection for maximizing recharge efficiency. The approach is generalizable to other arid regions hosting fossil aquifers, where engineered measures could capture extreme runoff and enhance aquifer storage.

The paper demonstrates that optimized redistribution of Lake Nasser’s excess floodwaters to geologically favorable nearby lowlands can substantially recharge the Nubian Sandstone Aquifer System while minimizing evaporative loss. Using integrated remote sensing, evaporation modeling, and 2D hydrodynamic simulations, the authors show that releasing 53.5 BCM into Depression II (Kiseiba Sandstone) would result, after 2.5 years from discharge cessation, in ~74% infiltration (39.7 BCM) and only ~20% evaporation (10.8 BCM), leaving minimal surface water. This recharge could offset projected groundwater depletion through at least mid-century, and an additional similar event could extend benefits through century’s end. The study offers a scalable adaptation framework to turn climate-driven extreme floods into sustainable groundwater resources. Future work should refine frequency and magnitude estimates of extreme inflows under climate change, assess impacts of sedimentation on infiltration efficiency, evaluate GERD operations’ effects on LN inflows, and develop continuous basin-scale rainfall–runoff models to support operational planning.

• Event frequency and magnitude: Uncertainty in 21st-century frequency/intensity of extreme precipitation over Nile source regions and their translation to excess LN inflows (varies with ENSO/IOD and climate change projections).
• Upstream regulation: Effects of GERD construction/filling/operation on downstream flood peaks reaching LN are uncertain and require integrated hydroclimate–operations modeling.
• Sedimentation: Progressive buildup of silt/clay from suspended solids may reduce infiltration in LN and Depression II, altering partitioning over time.
• Model assumptions and inputs: Simplifications include fixed or simplified discharge rates; DEM resolution (30 m COP DEM) introduces bathymetric uncertainty; infiltration parameters inferred via calibration from analogous formations; spatial heterogeneity of substrate properties not fully resolved.
• Encroachment risk: Limited but non-zero inundation of cultivated land near Lake 6a during filling must be managed operationally.
• Data uncertainties: GRACE mascon solution differences, evaporation model assumptions (Priestley–Taylor parameters, meteorological inputs), and remote sensing water delineation thresholds contribute to uncertainty bounds.