This paper provides an overview of the Special Issue on water supply and water scarcity. The papers selected for publication include review papers on water history, on water management issues under water scarcity regimes, on rainwater harvesting, on water quality and degradation, and on climatic variability impacts on water resources. Overall, the issue underscores the need for a revised water management, especially in areas with demographic change and climate vulnerability towards sustainable and secure water supply. Moreover, general guidelines and possible solutions, such as the adoption of advanced technological solutions and practices that improve water use efficiency and the use of alternative (non-conventional) water resources are highlighted and discussed to address growing environmental and health issues and to reduce the emerging conflicts among water users.

The growing population, intensified anthropogenic pressures and climate variability have increased the demands on available water resources, and water reuse has become a high priority, particularly in areas of the world suffering from water stress. The main objectives of this review paper are to consider and identify the potential opportunities and challenges in the implementation of water reuse schemes worldwide by considering and analyzing different fields of interest in water reuse, the current and future global drivers of water reuse policies, the existing advances in treatment and reuse technologies promising elimination of environmental footprint and human health risk, an analysis of the trends in potable and non-potable reuse, and the development of quality criteria and issues related to transition circular economy. Moreover, the major knowledge gaps in critical issues on different domains of water reuse schemes are discussed. For this study, a thorough analysis of the current literature was conducted, using research and review articles, technical reports, specific national (and EU) proposals, guidance documents, and legislative initiatives and actions, as well as any validly disseminated findings by scientists around the world in the wider scientific area of (alternative) water resources, water supply, water management, sustainable development, and protection of public health. Water reuse practices are expected to increase in the future, mainly in developed countries and climate-vulnerable areas of the planet. Current advances in wastewater treatment and water reuse technologies can provide the opportunity for the foul exploitation of alternative water resources, increasing the potential of potable and non-potable water reuse systems worldwide, relying on pollutant/contaminant elimination, and improving economic and energy performances. Moreover, paradigmatic and technological switches based on an improved understanding of the relationships between the water cycle and the Water–Energy–Food (WEF) Nexus will increase the perspective of water reuse schemes. The benefits of the recovery of nutrients through sewage wastewater treatment are also highlighted, arising from reduced costs associated with their sheer removal and the supplement of fertilizers to the WEF Nexus. On the other hand, reduced nutrient removal may promote agricultural or landscape reuse practices, contributing to less energy consumption and reducing GHGs emissions. Regarding the management of water use schemes, a holistic approach (integrated management) is proposed, incorporating regulatory actions, actions increasing public awareness, interconnection among actors/stakeholders, and efficient control and monitoring. The establishment of quality criteria is paramount to preventing undesirable impacts on humans and the environment. The study considers the “one water” concept, which means equal water quality criteria independent of the origin of water, and instead differentiates among different types of water reuse as a means to facilitate implementation and management of potable and non-potable water reuse. Finally, it highlights the need to understand the impacts of water reuse systems on ecosystem services (ESs) and the consequences of achieving the global sustainable development goals (SDGs).

Crete, located in the South Mediterranean Sea, is characterized by long coastal areas, varied terrain relief and geology, and great spatial and inter-annual variations in precipitation. Under average meteorological conditions, the island is water-sufficient (967 mm precipitation; theoretical water potential 3425.89 hm³; and total water use 610 hm³). Agriculture is by far the greatest user of water (78% of total water use), followed by domestic use (21%). Despite the high average water availability, water scarcity events commonly occur, particularly in the eastern-south part of the island, driven by local climatic conditions and seasonal or geographical mismatches between water availability and demand. Other critical issues in water management include the over-exploitation of groundwater, accounting for 93% of the water used in agriculture; low water use efficiencies in the farms; limited use of non-conventional water sources (effluent reuse); lack of modern frameworks of control and monitoring; and inadequate cooperation among stakeholders. These deficiencies impact adversely water use efficiency, deteriorate quality of water resources, increase competition for water and water pricing, and impair agriculture and environment. Moreover, the water-limited areas may display low adaptation potential to climate variability and face increased risks for the human-managed and natural ecosystems. The development of appropriate water governance frameworks that promote the development of integrated water management plans and allow concurrently flexibility to account for local differentiations in social-economic favors is urgently needed to achieve efficient water management and to improve the adaptation to the changing climatic conditions. Specific corrective actions may include use of alternative water sources (e.g., treated effluent and brackish water), implementation of efficient water use practices, re-formation of pricing policy, efficient control and monitoring, and investment in research and innovation to support the above actions. It is necessary to strengthen the links across stakeholders (e.g., farmers, enterprises, corporations, institutes, universities, agencies, and public authorities), along with an effective and updated governance framework to address the critical issues in water management, facilitate knowledge transfer, and promote the efficient use of non-conventional water resources.

Floods are one of the most dangerous natural disasters, causing great destruction, damage, and even fatalities worldwide. Flooding is the phenomenon of a sudden increase or even slow increase in the volume of water in a river or stream bed as the result of several possible factors: heavy or very long precipitation, melting snowpack, strong winds over the water, unusually high tides, tsunamis, or the failure of dams, gages, detention basins, or other structures that hold back water. To gain a better understanding of flooding, it is necessary to examine evidence, search for ancient wisdom, and compare flood-management practices in different regions in a chronological perspective. This study reviews flood events caused by rising sea levels and erratic weather from ancient times to the present. In addition, this review contemplates concerns about future flood challenges and possible countermeasures. Thus, it presents a catalogue of past examples in order to present a point of departure for the study of ancient floods and to learn lessons for preparation for future flood incidents including heavy rainfalls, particularly in urbanized areas. The study results show that ancient societies developed multifaceted technologies to cope with floods and many of them are still usable now and may even represent solutions and measures to counter the changing and increasingly more erratic weather of the present.

Since prehistoric times, water conflicts have occurred as a result of a wide range of tensions and/or violence, which have rarely taken the form of traditional warfare waged over water resources alone. Instead, water has historically been a (re)source of tension and a factor in conflicts that start for other reasons. In some cases, water was used directly as a weapon through its ability to cause damage through deprivation or erosion or water resources of enemy populations and their armies. However, water conflicts, both past and present, arise for several reasons; including territorial disputes, fight for resources, and strategic advantage. The main reasons of water conflicts are usually delimitation of boundaries, waterlogging (e.g., dams and lakes), diversion of rivers flow, running water, food, and political distresses. In recent decades, the number of human casualties caused by water conflicts is more than that of natural disasters, indicating the importance of emerging trends on water wars in the world. This paper presents arguments, fights, discourses, and conflicts around water from ancient times to the present. This diachronic survey attempts to provide water governance alternatives for the current and future.

Egyptian and Greek ancient civilizations prevailed in eastern Mediterranean since prehistoric times. The Egyptian civilization is thought to have been begun in about 3150 BC until 31 BC. For the ancient Greek civilization, it started in the period of Minoan (ca. 3200 BC) up to the ending of the Hellenistic era. There are various parallels and dissimilarities between both civilizations. They co-existed during a certain timeframe (from ca. 2000 to ca. 146 BC); however, they were in two different geographic areas. Both civilizations were massive traders, subsequently, they deeply influenced the regional civilizations which have developed in that region. Various scientific and technological principles were established by both civilizations through their long histories. Water management was one of these major technologies. Accordingly, they have significantly influenced the ancient world’s hydro-technologies. In this review, a comparison of water culture issues and hydro-structures was adopted through the extended history of the ancient Egyptians and Greeks. The specific objectives of the work are to study the parallel historical cultures and hydro-technologies, assessing similarities and differences, and to analyze their progress since primitive times. The tools adopted for the research include visits to historical aeras and museums, comments, consultations, correlation and exhibitions available in the cyberspace. Review results herein showed that dams and canals were constructed in ancient Egypt to manage the flood of the Nile river and develop irrigation systems from ca. 6000 BC. In the second millennium BC, Minoans managed the flow of the streams via two dams, to protect arable land from destruction after intense rainfall and to irrigate their farms. Additional results showed that ancient Egyptians and Greeks invented many devices for lifting water for plant irrigation such as the shadouf, sakia and tympanum and pumps, of which some were already in use in Mesopotamia for irrigating small plots. The ancient Egyptians were the first who discovered the principle and the basis of coagulation (after ca. 1500 BC). They used the alum for accelerating the settlement of the particles. Additionally, the ancient Greeks developed several advanced water treatment technologies since the prehistoric times. To sum up, the study captured many similarities between two civilizations in water technologies. In addition, it confirmed the sustainability and durability of several of those hydro-technologies since they are still in use up to now in many places.