Holocene marine transgressions are often put forward to explain observed groundwater salinities that extend far inland in deltas. This hypothesis was also proposed in the literature to explain the large land-inward extent of saline groundwater in the Nile Delta. The groundwater models previously built for the area used very large dispersivities to reconstruct this saline and brackish groundwater zone. However, this approach cannot explain the observed freshening of this zone. Here, we investigated the physical plausibility of the Holocene-transgression hypothesis to explain observed salinities by conducting a palaeohydrogeological reconstruction of groundwater salinity for the last 32 ka with a complex 3-D variable-density groundwater flow model, using a state-of-the-art version of the SEAWAT computer code that allows for parallel computation. Several scenarios with different lithologies and hypersaline groundwater provenances were simulated, of which five were selected that showed the best match with the observations. Amongst these selections, total freshwater volumes varied strongly, ranging from 1526 to 2659 km 3 , mainly due to uncertainties in the lithology offshore and at larger depths. This range is smaller (1511-1989 km 3 ) when we only consider the volumes of onshore fresh groundwater within 300 m depth. In all five selected scenarios the total volume of hypersaline groundwater exceeded that of seawater. We also show that during the last 32 ka, total freshwater volumes significantly declined, with a factor ranging from 2 to 5, due to the rising sea level. Furthermore, the time period required to reach a steady state under current boundary conditions exceeded 5.5 ka for all scenarios. Finally, under highly permeable conditions the marine transgression simulated with the palaeohydrogeological reconstruction led to a steeper fresh-salt interface compared to its steady-state equivalent, while low-permeable clay layers allowed for the preservation of fresh groundwater volumes. This shows that long-term transient simulations are needed when estimating present-day fresh-salt groundwater distributions in large deltas. The insights of this study are also applicable to other major deltaic areas, since many also experienced a Holocene marine transgression.

A comprehensive conceptual regional model for the Nile Delta aquifer has been composed in terms of the actual perspective of aquifer heterogeneity using all the new field data of drilled quality monitoring points in the Nile Delta aquifer. The study used the recently acquired configuration of the aquifer system which shows that the northern part the quaternary aquifer is devolved into multi-layered aquifer system while in the southern part; the aquifer consist of sand and gravel facies. The numerical modeling uses the finite difference SEAWAT program. The model is calibrated, both in terms of hydraulic heads and salt concentration, for the 2013 field data. The calibrated model is validated for the period 2013-2015. The model water balance reveals that seawater intrusion into the aquifer takes place at shallow to medium depths (up to 400 m), whereas groundwater fluxes in the deeper layers are moving toward the sea. The majority of the fluxes toward the sea help retaining old brine water in the deep zones and thus preventing seawater intrusion in these deep layers of the aquifer. This emphasizes the conclusion that the Nile Delta aquifer is not losing fresh groundwater flux to the sea. The study has achieved reliable necessary baseline simulation modeling and delineation of fresh/saline water interfaces as basis for decision-making and future management scenarios for controlled development of groundwater in the Nile Delta coastal aquifer.

Abstract. The Nile Delta is an important agricultural area with a fast-growing population. Though traditionally irrigated with surface water, the delta increasingly relies on groundwater. However, saline groundwater extends far land inward, rendering groundwater close to the coastal zone useless for consumption or agriculture. To aid groundwater management decisions, hydrogeologists reconstructed this saline and brackish groundwater zone using variable-density groundwater models with very large dispersivities. However, this approach cannot explain the observed freshening of this zone as observed by hydrogeochemists, who hypothesize that the coastal saline zone is the effect of the Holocene transgression. Here, we investigated physical plausibility of this hypothesis by conducting a palaeo-reconstruction of groundwater salinity for the last 32 ka with a complex 3D variable-density groundwater flow model, using state-of-the-art model code that allows for parallel computation. Several scenarios with different lithologies and hypersaline groundwater provenances were simulated, of which five were selected that showed the best match with the observations. Amongst these selections, total fresh water volumes varied strongly, ranging from 1526 to 2659 km3, mainly due to uncertainties in the lithology offshore and at larger depths. This range is smaller (1511–1989 km3) when we consider the volumes of onshore fresh groundwater within 300 m depth. Regardless of the variance, in all cases the total volume of hypersaline groundwater exceeded that of sea water. We also show that during the last 32 ka, the total fresh groundwater volumes significantly declined, with a factor ranging from 1.9 to 5.4, due to the rising sea-level. Compared to a steady-state solution with present-day boundary conditions, the palaeo-reconstruction improved our validation for the saline zone (5 g/L–35 g/L TDS). Also, under highly permeable conditions the marine transgression simulated with the palaeo-reconstruction led to a steeper fresh-salt interface compared to its steady-state equivalent, while low permeable clay layers allowed for the preservation of volumes of fresh groundwater. This shows that long-term transient simulations are needed when estimating present-day fresh-salt groundwater distribution in large deltas. The insights of this study are also applicable to other major deltaic areas, given the wide-range of lithological model scenarios used in this study and since many deltas also experienced a Holocene marine transgression.

The rising demands of groundwater for agricultural, due to the non-reliability of surface water sources have placed groundwater resource under serious pressure subjecting it to depletion and quality deterioration risks. Since, The Nile Delta aquifer is one of the most important renewable groundwater reservoirs in Egypt; it stands as the second source of water after the Nile, especially in the Nile Delta region. The Nile Delta aquifer is continuously recharged by irrigation water and through seepage from surface water. Moreover the Nile Delta aquifer is vulnerable to saltwater intrusion resulting from increasing groundwater abstraction. A 3D regional model for the Nile Delta aquifer system has been constructed to be used as a water management tool for different water management scenarios. The model was used to test the sensitivity of the aquifer to different extraction and recharge rates and investigate the system response on heads and salinity of the aquifer. The results confirmed that sensitivity of the aquifer to the tested parameters differs from one place to another.

As a result of population growth, economic development and climate change, feeding the world and providing water security will require important changes in the technologies, institutions, policies and incentives that drive present-day water management, as captured in Goal 6.4 of the Millennium Development Goals. Irrigation is the largest and most inefficient water user, and there is an expectation that even small improvements in agricultural water productivity will improve water security. This paper argues that improvements in irrigation water productivity involves a complex and comprehensive rural transformation that goes beyond mere promotion of water saving technologies. Many of the measures to improve water productivity require significant changes in the production systems of farmers and in the support provided to them.Looking forward, water use and competition over water are expected to further increase. By 2025, about 1.8 billion people will be living in regions or countries with absolute water scarcity. Demand for water will rise exponentially, while supply becomes more erratic and uncertain, prompting the need for significant shifts of inter-sectoral water allocation to support continued economic growth.Advances in the use of remote sensing technologies will make it increasingly possible to cost-effectively and accurately estimate crop evapotranspiration from farmers' fields. K E Y W O R D Sirrigation efficiency, water productivity, rural transformation RĖSUMĖ En raison de la croissance démographique, du développement économique et du changement climatique, nourrir le monde et assurer la sécurité de l'eau nécessiteront des changements importants dans les technologies, les institutions, les politiques et les incitations qui conduisent à la gestion actuelle de *Améliorer la productivité de l'eau dans l'agriculture en mettant l'accent sur la transformation des zones rurales.