The main objective of this study is to evaluate the potential impact of Grand Ethiopian Renaissance Dam (GERD) and pumping scenarios on groundwater level by a three-dimensional groundwater model of the Nile Delta using MODFLOW software. The Nile Delta has highly intensive irrigation canal networks that share yearly about 35.5 km3 of water. In this study, an integrated three-dimensional groundwater model is built considering the actual condition of the irrigation canals and their recharges of the Nile Delta aquifer. The model was calibrated for estimating the vertical and hydraulic conductivity. The model was run for three scenarios: (1) reduction of water depth in canals, (2) increasing pumping discharge from the aquifer and (3) combination between the first and second scenarios. Results reveal that the effect of increasing the pumping discharge on groundwater level in the Nile Delta is more significant than decreasing the water depth of the canals network due to the fact of the existence of the upper clay layer which reduces the amount of water penetrating and reaching the groundwater in the aquifer. The last scenario presents the worst case as the average drawdown reached 1.26 m, 1.7 m and 1.35 m in the western, central and eastern parts of the Nile Delta respectively. The study results should be taken in account for studying the saltwater intrusion and climate change impacts on the Nile Delta region.
The Nile Delta receives almost 35.5 km3/year of the surface water from the Nile River. This also recharges the aquifer through the infiltration of excess irrigation water and seepage from canals (Anon 1980; Shahein 1985; Kashef 1983), which is the main source of recharge for the Nile Delta aquifer (Wahaab & Badawy 2004; El Ramly 1997). According to El Ramly (1997), the rate of groundwater recharge from the excess irrigation water ranges from 0.25 to 1.1 mm/day. The agricultural recharge to the Quaternary aquifer ranges from 0.8 to 1.1 mm/day for old lands and from 1.9 to 2.1 mm/day for the reclaimed area (Shamrukh et al. 2001). The groundwater recharge in the Nile Delta aquifer was studied by different authorities based on water balance equations, it ranges from 5 to 10% of the input value from about 2 to 4 km3/year (Shahein 1985). Armanuos et al. (2016a) estimated the groundwater recharge from rainfall in the Nile Delta aquifer for six different years by using the WetSpass model. The model was calibrated for the Nile Delta parameters based on crop classification. The groundwater recharge ranged from 0.0 to 134 mm/winter season in the years 2000 and 2010.
The water levels in the Nile River and 10 main canals have been assigned in building a three dimensional (3-D) model to study saltwater intrusion and climate change impacts on the Nile Delta. The freshwater recharge effect on the seawater intrusion in the Nile Delta aquifer is noticed in the upper layer around the Nile River and its branches (Sherif et al. 2012). The water and bed levels of only eight canals were taken in account for the Nile Delta groundwater modeling by Abdelaty et al. (2014). The seawater moved towards the Nile Delta aquifer with the decline of water levels in canals, Abdelaty et al. (2014). Armanuos et al. (2016b) used Google Earth Pro software to estimate the bank levels and upper water width of irrigation canals in the Nile Delta region.
The construction of the Grand Ethiopian Renaissance Dam (GERD) will affect the water quota of Egypt, as it will decrease the Aswan High Dam (AHD) discharges (International Panel of Experts (IPOE) 2013). The GERD construction will have negative impacts on Sudan and Egypt. Ramadan et al. (2015) used mathematical modeling to develop different operational scenarios of GERD and investigated its impacts on the security of Egypt water resources. The results showed that the negative effects on Egypt's water resources will be dominant. In cases of GERD impounding during 6, 3, and 2 years for the normal flow case, the Lake Nasser active storage (90.7 BCM) will be decreased by 13.287, 25.413 and 37.263 BCM per year. At minimum flow from the Blue Nile, Lake Nasser active storage (90.7 BCM) will be decreased by 44.398, 54.415 and 55.138 BCM per year. At minimum average flow from the Blue Nile, Lake Nasser active storage (90.7 BCM) will be decreased by 25.963, 37.814 and 45.105 BCM per year. The impacts of the GERD construction on the performance of AHD were assessed (Mulat & Moges 2014). In the case of 6 years of filling; yearly outflows of the GERD through the impounding period will not decrease more than 28.9 BCM per year (about 58% of the mean flow).
Previous studies have considered groundwater recharge depending on 10 canals in groundwater modeling in spite of the actual irrigation network in Nile Delta region consisting of about 200 canals (MWRI 1954). Based on Ramadan et al. (2015) and Mulat & Moges (2014), the GERD construction will decrease the active storage of AHD by 14.8 to 60.7% and decrease the outflow from AHD to the Nile River and the Nile Delta irrigation canals network. This paper aims to evaluate the potential impacts of GERD and pumping scenarios on groundwater level by building a three dimensional model of the Nile Delta using MODFLOW software including the actual irrigation canals.
STUDY AREA DESCRIPTION
The Nile Delta is located in the Northern part of Egypt between latitudes 30°05′and 31°30′ N and longitudes 29°50′ and 32°15′E. It is about 25,000 km2(MWRI 2013). It is bounded by the Mediterranean Sea in the north, the Nile River in the south, the Suez and Ismailia Canals in the east and the El Nubaria Canal in the west (MWRI 2013), as shown in Figure A1 in the Supplementary Material (available with the online version of this paper).
The following four steps were considered to achieve the objectives: (1) identifying model parameters and data collection, (2) model building, model calibration, (3) the model testing for the studied scenarios and (4) analysis of results. The following sections describe briefly the different steps of methodology.
Identifying Nile Delta aquifer hydraulic parameters
The hydraulic parameters of the Nile Delta aquifer were identified based on previous research (see Tables A2 and A3 in the Supplementary Material, available with the online version of this paper). The directions of the irrigation canals and their names in the Nile Delta were identified based on MWRI (1954) and Roest (1999). The water level of El Raiah, the main canals and drains for the year 2008 in the Nile Delta were collected from previous studies (Morsy 2009; Abdelaty et al. 2014; RIGW 2015), see Table A4 in the Supplementary Material. The topography map of the Nile Delta and the base level of the Quaternary aquifer were collected from EGSA (1997).
Model calibration and validation
In the calibration process, the model was run many times to minimize the difference between the measured hydraulic head by RIGW in 2008 and the simulated head by the model. Sixty distributed observation wells have been selected in the study area for the calibration process until the maximum difference between the measured and simulated hydraulic head level became 0.60 m, as shown in Figure A4 in the Supplementary Material. For model validation, the observed hydraulic heads were compared to the simulated ones for an other 60 records, as shown in Figure A4 in the Supplementary Material. Table A6 in the Supplementary Material shows the calibrated values of the Nile Delta aquifer model parameter.
Studied scenarios and assumptions
To study the impact of flow reduction to the Nile Delta, due to filling the reservoir of GERD, on groundwater level in the Nile Delta aquifer, three scenarios were studied based on three assumptions. The first assumption is that the flow from High Aswan Dam to the Nile Delta will decrease with the same percentage of active water in Lake Nasser reservoir due to GERD construction. The second assumption is that the crop pattern in the Nile Delta is constant, so the water depth will be decreased with the same percentage in the canals according to the studied scenarios. Finally the decrease of the total surface water through the canals will lead to a further pressure on groundwater storage in the Nile Delta aquifer and increasing the groundwater pumping will be expected to compensate the reduction in surface water.
Ethiopia is planning to end the work in GERD by 2018. In case of impounding GERD in 2 and 6 years, the active storage of AHD will decrease by about 50 and 25%; as a result of that the outflow from AHD to the Nile River and the Nile Delta irrigation canals network will decrease (Ramadan et al. 2015). Two separate simulations of decrease in the water depths in canals were done by 50 and 25% in the years 2020 and 2024 respectively and two other separate simulations were done for increasing the pumping discharge from the wells by 50 and 25% in the same years consequently.
The total annual groundwater abstraction from the Nile Delta aquifer has increased dramatically throughout the last 30 years. Research Institute of Groundwater in Egypt (RIGW) reported its increase from 1.6 × 109 m3/year in 1980 to 3.5 × 109 m3/year in 2003 and reached 4.6 × 109 m3/year in 2010 (RIGW 2003; Mabrouk et al. 2013). It is expected that the annual abstraction rate will exceed 0.20 × 109 m3 per year through the coming years (Mabrouk et al. 2013). Recent studies in simulating the saltwater intrusion in the Nile Delta by Sherif (2003) and Abdelaty et al. (2014) conducted different scenarios of pumping discharges increasing from the Nile Delta aquifer by 25, 50, 75 and 100% based on RIGW measurements (RIGW 2003) and (Mabrouk et al. 2013). The current model was run for three scenarios: reduction of water depth in canals by percentage 25 and 50%, increasing pumping discharges by percentage 25 and 50%, and the third scenario presents the combination between the first and second scenarios.
RESULTS AND DISCUSSION
Calibration and validation results
Scenario 1: Decreasing water depth of canal networks
Scenario 2: Increasing discharges of pumping wells
Scenario 3: Combination between scenario 1 and scenario 2
This paper presents the first study to build a 3-D model for the Nile Delta aquifer where the high intensive irrigation canal networks were included for accurate simulation of groundwater recharge from the canals network by using MODFLOW software. The impact of increasing pumping discharge from wells on groundwater level in the Nile Delta is much more significant than the impact of decreasing water depths of canals; this is due to the fact of the existence of the upper clay layer, which reduces the ability of the water to penetrate and reach the groundwater in the aquifer. The drawdown increases from the north towards the south. The reasons behind this are that the southern part of the Nile Delta has high pumping discharge values compared with the northern area and the effect of the constant head model boundary at the northern area along the shore line of the Mediterranean Sea. The last scenario presents the worst one where combination between increasing the drawdown related to increasing the pumping and decreasing groundwater recharge with respect to decreasing water depths of the canals.
The paper presents simulation of the effect of decreasing the water depth in the canal networks in the Nile Delta due to the impact of GERD and of increasing the pumping discharges on groundwater level by using MODFLOW software where the intensive irrigation network canals of the Nile Delta were included for more accurate simulation of groundwater recharge. The model was calibrated for hydraulic conductivity of the first and second layer using the observed head by RIGW (Morsy 2009). The model was tested for three scenarios: (1) reduction of water depth in canals due to the impact of GERD, (2) increasing pumping discharges of groundwater wells and (3) combination of the first and second scenarios. Under the conditions assumed in these scenarios, the effect of increasing the pumping discharges is much more significant than the effect of decreasing the water depth in canals network compared with the situation in 2008. The drawdown of groundwater head increases from the north towards the south in the Nile Delta region. Decreasing the water depth by 25% leads to drawdowns of 0.30 m, 0.5 m and 0.2 m in average in the western, central area and eastern parts of the Nile Delta while a further drawdown is expected to reach 0.6 m, 0.7 m and 0.4 m in average in case of decreasing the water depth by 50%. Increasing the pumping discharges by 25% leaded to drawdowns of 0.35 m, 1.0 m and 0.5 m in average in the western, central area and eastern parts of the Nile Delta while a further drawdown is expected to reach 0.7 m, 1.5 m and 1.27 m in average in case of increasing the pumping discharges by 50%. It is highly recommended to re-investigate the saltwater intrusion in the Nile Delta aquifer taking into considerations the different filling scenarios of the GERD reservoir.
The first author would like to thank the Egyptian Ministry of Higher Education (MoHE) for providing him with the financial support (PhD scholarship) for this research as well as the Egypt Japan University of Science and Technology (E-JUST) for offering the facility and tools needed to conduct this work.