Atmospheric water harvesting as a sustainable and resilient resource in arid climates: gaining insights from ancient techniques

Urbanisation, environmental injustice, and climate change are exacerbating water scarcity, with demand increasingly surpassing availability (He et al. 2021; Qadir et al. 2022). Particularly in semi-arid and arid climates, water security is at risk due to disrupted hydrological patterns, increased drought, reduced precipitation, and groundwater depletion (Arab Water Council 2009; Fessehaye et al. 2014). The agriculture sector faces the greatest risks of having to reduce their water consumption patterns significantly (Qadir et al. 2007; Arab Water Council 2009; Aldeek 2023). In this context, non-conventional water resources have the potential to secure a reliable, sustainable supply (Qadir et al. 2007; Domen et al. 2014; Tortajada et al. 2019). As precipitation and groundwater become scarcer in arid climates, and desalination often proves unsustainable or economically unfeasible, harvesting atmospheric moisture presents a promising non-conventional, complementary solution (Schemenauer & Cereceda 1994a; Wu et al. 2020; Farnum 2022). In rural regions where freshwater is transported over long distances, this offers a viable decentralised approach (Nikkhah et al. 2023). Atmospheric water harvesting, referred to in this article as AWH, is classified into two categories: fog and dew collecting, with the latter also referred to as a condensation method (Jarimi et al. 2020; Ahrestani et al. 2023). Fog is harvested through a passive, low-cost, low-maintenance system using a flat structure with a mesh whereon droplets coalesce (Schemenauer et al. 2022). Dew collection entails a (passive) condensation process requiring a surface with a temperature lower than the atmospheric water vapour (Jarimi et al. 2020). Both systems are inspired by organisms in arid climates that evolved to efficiently harvest atmospheric droplets. This field, known as biomimicry, seeks solutions in nature to solve human problems (Benyus 1997; Pawlyn 2011). Researchers study arid-based organisms to increase surface yield efficiency by implementing biomimetic materials (Jarimi et al. 2020; Verbrugghe & Khan 2023). However, ancient techniques are an overlooked source of inspiration, referred to as being remarkably sustainable (Koutsoyiannis et al. 2008; Luo et al. 2020). Achieving water resource sustainability involves ensuring water quality and quantity locally and globally for humans and ecosystems while protecting future life (Mays 2010). Ancient water management strategies were adapted to local environments with durable systems that allowed for sufficient replenishment (Mays 2017), in contrast to today's over-extraction issues causing water shortages (Ding & Ghosh 2017). Investigating ancient concepts for sustainability extends beyond hydraulic systems to archaeology, economy, agriculture, sociology, engineering, and architecture (Ranaweera 2010).

The Ancient Greeks were already concerned about climate change (Momete 2013), adopting holistic mitigation approaches in harmony with nature (Kakoty 2018). Extensive research exists on ancient water management strategies in Greece, Egypt, Jordan, the Incan Empire, and other large ancient societies, describing the use of aquifers, wells, terrace structures, and large water bodies (Koutsoyiannis et al. 2008; Mays 2010; Luo et al. 2020; Harvey 2021). Although many ancient civilisations settled near water bodies or migrated along them (Fang & Jawitz 2019), securing water in dry regions has been an ongoing challenge (Qadir et al. 2007). Qanats, a water management strategy dating back to 800 B.C., represents a pinnacle for arid areas. Tunnel wells were excavated on sloped surfaces to tap into the water table, delivering fresh water (Laureano 2012; Nasiri & Mafakheri 2015). Nonetheless, fog and rain exist in a liquid state within the atmosphere and were presumably among the first resources exploited in ancient times (Ortiz & Rao 2024). Findings from arid regions where civilisations thrived for over a thousand years offer insights into sustainable strategies that can positively affect socioeconomic developments (Vetter & Rieger 2019). However, no article comprehensively explores AWH techniques. Information is fragmented in the scientific literature, though some are still in use today. Consequently, this study reviews ancient techniques to enhance sustainable water management strategies using fog and dew as complementary resources in arid climates. The aim is to deduce the replication, revival, restoration, or redevelopment potential of ancient AWH techniques. In this context, ‘replication’ refers to spreading the technique to other regions, ‘revival’ involves reinstating the practice of obtaining water from fog or dew at its original location, albeit using modern structures, ‘restoration’ involves re-establishing existing infrastructure or using the same techniques to obtain potable water, and ‘redevelopment’ refers to learning from these techniques and applying their principles. Pre-modern water collecting approaches were intertwined with nature using natural materials and basic instruments as tools. The simplicity of traditional AWH techniques suggests a compelling solution to modern small-scale challenges.

The analytical framework classifies modern and ancient AWH techniques. To provide the necessary context, fog and dew are defined, followed by a description of modern passive techniques (Section 3). This expanded introduction allows for further comparison, where modern systems can potentially be supplemented or improved by ancient methods. The historical analysis involved reviewing ancient AWH depictions, contextualising these techniques within their time, and comparing them with contemporary research and modern methods (Section 4). The AWH techniques are grouped into two subsections based on the resource for which each technique was mainly developed: fog or dew. A comparative assessment of both the modern and ancient methods' yields and efficiency, with quantitative data where available, is presented alongside the historical development and contemporary relevance (Section 5). Lastly, the viability of ancient techniques to address modern-day challenges through replication, revival, restoration, or redevelopment is summarised and discussed (Section 6). Visual aids, created using Adobe Illustrator, are included throughout the review to enhance clarity. Section 7 concludes the findings and provides suggestions for advancing the field of collecting fog and dew as a complementary resource in arid environments.

The importance of atmospheric vapour, moisture, or water is its availability in dry climates (Jarimi et al. 2020). Approximately 98% of all liquid freshwater, or 13,000 km³, exists as vapour within the atmosphere (Nikkhah et al. 2023). This vapour is part of the hydrological cycle and is considered an unconventional resource, contributing to the renewal of conventional water sources such as rivers, streams, reservoirs, and aquifers through natural processes (Qadir et al. 2007).

Fog and dew are distinct in their form, formation process, and location. Fog is a cloud that touches the ground, significantly reducing visibility and containing water particles ranging from 1 to 40 μm. Fog forms when atmospheric moisture cools to saturation and condenses into visible water droplets suspended in the air. It is a location-specific phenomenon, mostly found on coastal cliffs and in valleys. There are various types of fog, but advection and orographic fog are accompanied by wind, making them most suitable for harvest. Advection fog forms when warm air masses move horizontally over a cooler surface, such as the sea. Orographic fog forms when humid air masses move uphill (Schemenauer et al. 2022; Verbrugghe & Khan 2023). In contrast, dew is non-existent in the air and results from water vapour condensing on a surface with a temperature lower than the atmospheric dew point. Dew formation is less restricted by geographical conditions, as water vapour is present across the earth (Tomaszkiewicz et al. 2015; Jarimi et al. 2020; Zhou et al. 2020). The meteorological conditions required for dew formation are high relative humidity and minimal wind. Greater surface cooling occurs under a clear sky with higher emissivity, facilitating condensation (Khalil et al. 2016; Carvajal et al. 2018). Natural dew formation is most common at night due to the earth emitting longwave radiation (Lobos-Roco et al. 2024). In built environments, dew is less common due to the urban heat island effect and reduced vapour supply (Richards 2004).

Both fog and dew have merits and limitations depending on location and needs. Passive dew collectors, also known as radiative condensers, theoretically yield a maximum of 0.8 L/m²/day, with typical averages ranging from 0.3 to 0.6 L/m²/day (Khalil et al. 2016; Kaseke & Wang 2018). Fog water yields vary significantly under fog-loaded climate conditions, with average yields between 3 and 10 L/m²/day and extremes up to 30 L/m²/day (Klemm et al. 2012). Although the average yield per square metre of fog is higher than that of dew, the latter offers a more widespread solution in terms of location.

Fog collection entails a two-dimensional flat structure with a mesh suspended 2 m above the ground. The most widely installed and described fog collectors are FogQuest's Large Fog Collector (LFC) and Standard Fog Collector (SFC), with mesh surface areas of 40 and 1 m², respectively. The collectors are placed perpendicular to the dominant fog-loaded wind. Droplets are trapped on the mesh material where they coalesce and fall into a gutter. The amount of harvested water depends on wind speed, droplet size, the number of droplets within the passing cloud, and the mesh's properties for trapping and transporting the accumulated droplets (Schemenauer et al. 2022). The most widely used interception surface is a black double-layered polyethylene mesh with a shading coefficient of ∼35%, known as the Raschel mesh (RM), which has a triangular-weaved pattern that enables efficient run-off. Although effective in many fog collection projects globally, the RM can easily rupture in harsh weather conditions. More robust materials, such as stainless steel and wires, have proven more efficient in terms of maintenance and yielding abilities. A fog collector with vertically stretched wires or threads is referred to as a fog harp (Klemm et al. 2012; Verbrugghe & Khan 2023). Despite this, the RM is often advantageous due to its cost-effectiveness and availability. Its efficiency can be increased by, for instance, applying (super)hydrophobic coatings (Rajaram et al. 2016), or by using a white-coloured mesh to minimise evaporation (Suau & Zappulla 2015). Additionally, the two-dimensional structure is also prone to breaking (Holmes et al. 2015), which led to the development of three-dimensional designs (Verbrugghe & Khan 2023). This is particularly useful for increasing fog yields in areas with variable wind directions (Juvik & Nullet 1995; Klemm et al. 2012).

Dew collection involves an inclined plane condensing surface that allows droplets to run into a gutter. Contrary to collecting fog, dew collectors are placed close to the ground to avoid wind. Their efficiency highly depends on the surface properties of a material to maintain a cool temperature (Muselli et al. 2002; Beysens 2016). While an optimal height is not defined, Beysens et al. (2003) identified an optimal inclination angle of 30° to the horizontal, which favours dew formation and radiative cooling. The material's surface properties, such as high emissivity, low thermal mass, and surface wettability, also influence its efficiency (Tomaszkiewicz et al. 2015; Carvajal et al. 2018). A low-density, white polyethylene foil or OPUR foil is defined as a standard material by the International Organization for Dew Utilization (OPUR 2024). Its high reflectance reduces heating, prolonging dew formation during the morning, while integrated hydrophilic mineral fibres increase their wetting properties and droplet coalescence (Khalil et al. 2016; Tuure et al. 2020). However, OPUR foils, explicitly developed for dew recovery, are expensive (Corraide da Silva et al. 2021). Maestre-Valero et al. (2011) proposed a low-cost alternative. A cheaper black polyethylene foil demonstrated higher dew yields than the OPUR foil, suggesting that increasing surface emissivity is more effective than enhancing hydrophilic properties. Additionally, non-planar surfaces, such as an origami-shaped surface, have the potential to increase yields by 400% compared to planar collectors (Kaseke & Wang 2018).

Many other designs and methods for AWH collections have been developed over the last decades. Extensive reviews are available for fog collecting (Klemm et al. 2012; Domen et al. 2014; Fessehaye et al. 2014; Batisha 2015; Farnum 2022; Verbrugghe & Khan 2023); for dew collecting (Nikolayev et al. 1996; Tomaszkiewicz et al. 2015; Beysens 2016); and for AWH (Jarimi et al. 2020; Zhou et al. 2020; Ahrestani et al. 2023; Nikkhah et al. 2023).

Ancient AWH techniques primarily originated in (semi-)arid regions. This section is subdivided into three subsections for which the techniques were mainly developed: fog or dew, although some harvest both interchangeably.

Fog is naturally intercepted by plant and tree leaves. For some species and organisms along the Pacific coastline from northern Chile to California (USA) and in the Namib Desert, fog is the primary freshwater source (Weathers et al. 2020). A group of fog-harvesting tree species are collectively known as cloud forests (Schemenauer et al. 2022). The amount of harvested water depends on the canopy width and height, as well as the leaves' abilities to trap and retain droplets, regardless of age. When the leaf surface water exceeds storage capacity, liquid water drips into the soil (Katata 2014). The action of droplets falling from the tree's leaves after collection is referred to as fog drip (Schemenauer & Cereceda 1991). Schemenauer & Cereceda (1992) found that the vertical cross-section of an isolated tree yields around 10 L/m²/day. While originating in ancient times, Jarimi et al. (2020) and Ahrestani et al. (2023) classify fog drip methods as a modern technique (‘biomimicry inspired’ and ‘bioinspired’, respectively).

Obtaining potable water from fog drip has also been observed on the Arabian Peninsula in the Sultanate of Oman, where inhabitants constructed cisterns under olive trees for domestic purposes. While this strategy supposedly was a freshwater resource for centuries up until the end of the 20th century, no exact date or location of origin is found (Cereceda et al. 2014; Fessehaye et al. 2014). Figure 4(b) shows a photograph taken by fog-harvesting expert Pilar Cereceda in Oman, displaying a large cistern placed next to two intertwined olive trees (Cereceda et al. 2014). In 1990, a project was set up to measure the amount of harvestable water from olive trees (Olea europaea) on a hilltop in the Dhofar region of Oman (Abdul-Wahab et al. 2009; Schemenauer et al. 2022). During the summer monsoon season, which lasts from mid-June to mid-September, Barros & Whitcombe (1989) described the region as almost constantly covered with fog and drizzle, accompanied by wind. In 1989, two small, intertwined olive trees dripped 860 L/day over 79 days, or approximately 70 L/m²/day. In comparison, a SFC placed in the region harvested an average value of 86 L/m²/day. Along with other comparative fog collection studies, these suggest that an SFC provides useful estimates of the collection ability of an isolated tree, and vice versa (Schemenauer & Cereceda 1994b; Schemenauer et al. 2022). In the same region, Abdul-Wahab et al. (2009) measured the fog-harvesting potential of a lemon, a tamarind, and a fig tree. The cross-sectional areas of the leaf surfaces and the canopy were normalised to square meters. The lemon tree exhibited the highest rate of fog collection, with 4.4 L/m²/day, followed by the tamarind (4.3 L/m²/day), and the fig (4.3 L/m²/day) over a period of 47 days during the monsoon season. The variations in efficiency are mostly attributed to differences in the leaf properties that influence the retention of incoming droplets (Abdul-Wahab et al. 2009). In the 1950s, Cook pine trees (Araucaria columnaris) were planted on the semi-arid Hawai'ian island of Lāna'i to recharge groundwater aquifers, hypothesising that trees harvest more water than they need. When the trees reached 10 m in height, initial measurements indicated that fog drip supplemented rainfall by 60% (1,250 mm/year rain and 762 mm/year fog). By 2006, a follow-up study reported increased water yields as the trees grew to 20 m and measurements became more thorough (Ekern 1964; Bruijnzeel & Scatena 2011; Juvik et al. 2011). These findings confirm the potential to obtain significant amounts of water from fog drip, sufficient to support ancient populations. In other regions prone to drought, many cultures used natural obstacles to harness water for agricultural and domestic purposes, yet documentation is scarce. For instance, the Incas living in the Peruvian Andes supposedly placed buckets under trees to collect drops (Farnum 2022; Furer 2022). While these methods primarily rely on fog, dew and occasional rain also contribute.

Although most of the Preceramic population in the Atacama Desert lived close to rivers, archaeological remains have been found far beyond water bodies. Fog oases along the Pacific coastline between ∼6 and 30°S enabled permanent settlement by Preceramic hunters between 6050 and 2550 B.C. (Beresford-Jones et al. 2015). Despite extreme aridity, these oases, referred to as ‘lomas’ in Peru and ‘oasis de niebla’ in Chile, are concentrated vegetated islands found up to 10 km inland receiving marine advective fog as their main water source (Río et al. 2018; Mikulane et al. 2022). The lomas and oasis de niebla are somewhat distinct, with the latter growing less diverse fauna and flora (Larraín et al. 2001b). Nonetheless, both served as agricultural lands, hunting areas, and freshwater resources in ancient times. For instance, regarding the Preceramic diet, the endemic fauna and flora of the Peruvian lomas included remains of the Andean potato, tomato, and papaya (Beresford-Jones et al. 2015). In Chile, archaeological findings from Alto Patache, an oasis de niebla located at 750 metres above sea level, suggest the role of fog water in supporting communities since the Chinchorro culture in 4500 B.C. (Stone Age). Remains of a presumed well with arranged stones and traces of mud were found. The vertical surfaces of the stones were most likely covered with lichens to intercept fog. Other remains indicate the use of vertically stretched long, thin hairs of guanacos and sea lions to accumulate and transport droplets, similar to modern fog harps. Findings of broken vessels and water jars indicate these functioned as water reservoirs (Larraín et al. 2001a). Water was also presumably harvested by strategically placing clay vessels under lichen-covered stone walls. In addition, lithic artefacts for hunting and preparing meat support the suggestion of permanent occupations. Within this context, the use of stones and rocks is referred to as ‘lithic’ (Larraín et al. 2001b). Even today, the local Chilean population and researchers in the Atacama Desert refer to the marine advective fog as ‘camanchaca’ (Schemenauer et al. 1988; Rivera et al. 2022). This term alludes to the close association ancient civilisations had with fog. According to a legend from the oasis of Pica, local fishermen followed the fog into the highlands in search of vegetation that was non-existent at sea level. Hence, they were the men of the ‘camanchaca’ and were called ‘camanchangos’, ‘changos’, or ‘camanchacos’ (Escobar & García 2017).

Lithic mulch agriculture, an ancient technique found in dryland environments, uses lithic materials to enhance crop yields. Pre-1900 remains have been discovered in desert areas of Israel, the European Mediterranean, the Canary Islands, the Peruvian Atacama, southern USA, central China, and in the Māori culture in New Zeeland. Lithic mulching involves placing gravel, rocks, volcanic ash, or pebbles to create a protective layer covering the soil. These lithic materials can also be piled around plants to create a wind buffer and minimise evaporation (Lightfoot 1996). A well-preserved example from the Negev Desert in ancient Palestine consists of low, circular honey-combed walls around grape and olive vines to exploit atmospheric water vapour (Dower 2002; Cho 2011; Fessehaye et al. 2014). Traces indicate its use from as early as 2000 B.C. While similar to the stone drip technique, lithic mulching entails the direct use of dew droplets to transmit water to the plant's roots. In Lanzarote, an arid island of the Canary Archipelago, lithic mulching was implemented in 1740 after a volcanic eruption (Lightfoot 1996). The agricultural fields are mulched with porous volcanic products referred to as ‘Picón’ (Graf et al. 2004). To this day, to capture and retain ‘moon water’, grape vines are planted within a half-moon-shaped stone assembly for wine production in La Geria (Figure 5(b)) (Borgia 2017). In Burkina Faso, this technique was successfully introduced in the 1980s to combat crop yield decreases and land degradation, with manure covering the soil (Nyamekye et al. 2018).

While building a wall around the plant (Figure 5(c)) seems to effectively aid in catching, condensing, and retaining atmospheric moisture (Fessehaye et al. 2014), covering the soil with lithic materials remains controversial. Researchers are sceptical about the atmospheric vapour deposition effects (Lightfoot 1994). Studies by Li (2002) in a semi-arid region of China and Graf et al. (2004) in Lanzarote showed that dew deposition on dry, loess sand soils is higher than on mulched surfaces. Nonetheless, Graf et al. (2004) also state that nocturnal dew depositions on cooled stone surfaces are higher. Kaseke et al. (2011) obtained similar results but argued that gravel mulch conserves soil moisture better, suggesting that this was the main reason for its historic use in drylands.

After soaking up the morning dew and occasional rain, under the influence of the sun and the moon (symbolising evening and morning, respectively), these linen cloths were wrung out to obtain the liquid water. The panel features a ram and a bull, symbolically indicating that dew was best harvested during April and May in the region (Möller 2008; Soentgen 2012; Dejeunes 2014). Subsequent panels illustrate the further treatment of dew water, with the emphasis on extracting magic salts to offer to the gods within an alchemical context rather than as a freshwater management strategy (Pérez-Pariente 2008).

Martin (1909) describes two categories of dew ponds distinguishable by their elevation and water source. Low-level ponds are likely sustained by drainage and run-off rainwater, while higher-level ponds are maintained by little rain and atmospheric moisture. During periods of drought and high temperatures, low-level ponds tend to dry up, while high-level ponds consistently contain significant amounts of water and are rarely observed to be completely dry (Martin 1909). Conflicting perspectives exist, with some dismissing the input of dew and attributing it solely to rain, while others suggest that dew contributes via deposition on bordering vegetation. Despite differing views on the role of atmospheric moisture, observations validate the presence of dew deposition near the ponds and the presence of fog (Nelson 2003; Wang et al. 2017; Maestre-Valero et al. 2021). This phenomenon suggests that the name ‘dew pond’ originates from this visual phenomenon. Nevertheless, the high relative humidity of the ground layer, due to the presence of lakes or ponds, attracts atmospheric moisture, leading to dense dew and fog formations with lower temperatures facilitating condensation (Herschy 2012). Therefore, in terms of thermodynamics, atmospheric moisture has an impact on maintaining water levels. Although many dew ponds have been abandoned or dried up, several organisations aim to restore their use in Yorkshire (northern England) for agricultural purposes to sustainably address local water stress (Yorkshire Wildlife Trust, 2024). Given the diverging scholarly perspectives and ongoing restoration efforts, further research on the effectiveness and practical aspects of dew ponds is beneficial to identify key faults and prevent ponds from drying out. Dew ponds were extensively studied at the beginning of the 20th century, and this research may now be outdated.

Inspired by Zibold, bioclimatologist Leon Chaptal and inventor Achille Knapen continued his line of research in the Mediterranean climate of southern France. In 1929, Chaptal constructed a 2.5-m-high pyramidal air well near Montpelier (Figure 8(c)). Humid air entered through small vents, condensing on the interior porous limestone surface. The structure yielded 1–2.5 L/day from March to September 1930. However, measurements from April to September resulted in a total amount of 100 L in the adjacent water reservoir, suggesting that a large amount evaporated. That same year, Knapen built a 14-m-tall air well in Trans-en-Provence (Figure 8(d) and 8(e)). His design featured thick walls (2.5–3 m) to prevent heat radiation from the ground, a concrete core with air ducts, and a central pipe for night cooling, which successfully retained its low temperature during the day. Warm, moist air entered through the upper vents, condensing on the thick walls, and trickling down to a basin. However, this large structure performed poorly, occasionally producing a maximum of ∼19 L in one night (Nelson 2003). The thick walls of both designs stored the latent heat released during the condensation of atmospheric vapour into liquid water instead of dissipating, preventing the walls' temperature from falling below the dew point. In contrast, Zibold's condenser had areas open to the sky, allowing better radiative cooling and dew yields (Muselli et al. 2002).

A prominent example of a dew harvester is a sealed 16th century stone sarcophagus from the abbey of Arles in southern France. It was believed that the sarcophagus harvested hundreds of litres of water, functioning as a stone-pile condenser on a smaller scale (Beysens et al. 2001). The thick marble-walled structure, 40 cm high with an internal volume of 0.33 m³, was exposed to external atmospheric conditions of temperature and humidity. This enabled local convective movements and thermal variations within the sarcophagus, causing water vapour to condense inside. This condensation came either from its air content or from fresh air entering through slits (Batina & Peyrous 2021). After collecting data from 1997 to 2000, Beysens et al. (2001) discovered that the marble lid was porous, thus absorbing rainfall in addition to facilitating dew formation. They measured a 200 L annual output over 3 years, with dew contributing about 10%, or 20 L, identified during periods of non-rainfall.

Table 1 presents a comparison of the yields and purpose of the discussed fog and dew collection techniques, both modern and ancient, alongside the key factors influencing efficiency. While some lack quantitative data, efficiency is assessed based on their ability to capture water relative to their intended purpose.

The mesh-like structures used in pre-Hispanic Mexico City were presumably fog collectors made with tule nets and human hair. Tule nets resemble the RM used in modern fog collectors, which harvest 3–10 L/m²/day. Fog collectors with hairs, as in the fog oasis of Alto Patache using guanaco and sea lion hairs, resemble modern fog harps. Under light fog conditions, Shi et al. (2020) stated that fog harps harvest up to 78 times more than mesh-based collectors, showcasing their ingenuity. The efficiency of fog collectors varies significantly depending on the properties of the interception material in relation to local fog conditions. Celestial dew collectors were efficient regarding their alchemical purpose. The technique resembles modern dew collectors, with the use of linen fabrics showcasing an understanding of material science, given linen's high water absorption capability (Bilen 2021). Modern dew collectors' yields depend on the surface properties, inclination angle, and rise with lower wind speeds. Fog drip from trees varies based on species, fog conditions, and isolation, with isolated trees being exposed to more fog. Research has advanced the field of fog drip by identifying yields of various species. Trees are self-maintaining, three-dimensional structures yielding comparable amounts to flat-framed fog collectors. As demonstrated in Oman, an isolated olive tree yielded 70 L/m²/day and the SFC 86 L/m²/day. Stone drip methods are influenced by stone properties and vegetation cover, while lithic mulching and dew ponds primarily focus on water retention rather than direct harvesting. These methods lack specific quantitative data on atmospheric moisture inputs. Lastly, Zibold's stone-pile condenser in Crimea produced approximately 360 L/day, which is a notable output compared to the air wells, producing 1–2.5 L/day for Chaptal's construction and an occasional maximum of 19 L for Knapen's air well. The stone sarcophagus in southern France collected around 20 L of dew annually, demonstrating its effectiveness despite its small size.

The use of stone structures for capturing fog and dew deposition has been observed in various arid regions worldwide. The oldest dates to 5000 B.C. in fog oases on the Pacific West coast of Peru and Chile. Today, various fog and dew collection projects operate in these areas, functioning as decentralised water resources (Klemm et al. 2012; Carvajal et al. 2018). To address the impending water stress the Atacama Desert is projected to face by 2040, mainly due to overextraction for local mining industries (Cantillana & Iniesta-Arandia 2022), enhancing AWH initiatives is imperative. Similarly, lithic mulching is a technique developed independently worldwide, with pre-1900 remains found globally, the oldest being honey-combed walls from 2000 B.C. in Palestine. Lithic mulching remains an eco-friendly approach to mitigate global agro-ecological imbalances caused by rising temperatures. Despite contradictions among scholars, mulching tailored to local conditions and crop types is a cost-effective method to conserve soil moisture and reduce weed growth (Iqbal et al. 2020). Dew ponds in England, introduced during the 18th century, exemplify a pragmatic response to permeable undergrounds. With the UK facing medium water stress, dew ponds could help mitigate this situation. Fog drip from trees is valuable when vegetation is present, as in the Sultanate of Oman and El Hierro. The technique is unquestionably older than its documentation in the 1400s and remains under investigation and relevant today for livestock and aquifer recharge.

Farnum (2022) states that AWH methods combine traditional or ancient knowledge with advanced material science. This is exemplified by the mesh structures found in Tenochtitlan, reflecting the Aztecs' understanding of local geography, the hydrological cycle, and the physical properties of materials. This suggests the potential for yielding fog and dew during the dry winter season in Mexico City, which faces high water stress (Arias-Torres & Flores-Prieto 2016). Similarly, in La Rochelle (France), facing medium water stress, the alchemists' celestial dew collectors align with contemporary methods. The rediscovery of Theodosia's stone-pile condensers in the 20th century, whether they successfully supplied the ancient city or not, can be traced back to 600 B.C. Further studies undertaken by Nikolayev et al. (1996) confirmed favourable conditions for dew formation, highlighting the suitability for dew recovery given the high risk of water stress in the area.

Ancient techniques have laid the foundation for modern collectors, with materials being the key differences. Today, advancements in material science, engineering optimisation, and systematic research have significantly improved the reliability and efficiency of fog and dew harvesting. For instance, yields for both fog and dew collection have improved by testing alternative interception surfaces. Specifically for dew collectors, the transition from planar to non-planar surfaces has shown promising results.

While these techniques focus on alleviating water stress in (semi-)arid climates, they are also applicable in other regions. In the Netherlands, dew recovery has proven effective for agricultural purposes. Despite its temperate climate without a rain-free period, dew events are frequent and evenly distributed throughout the year (Jacobs et al. 2008). Similarly, in tropical highlands receiving abundant rainfall, such as Chiapas (Mexico), research on fog collection in forests demonstrated high potential to supplement rainfall and recharge underground aquifers (Schemenauer et al. 2022).

AWH is often considered unreliable by policymakers for not consistently delivering large volumes of water due to its dependence on environmental factors. As population densities and freshwater resources are unevenly distributed, nations rely on macro-scale approaches, supplying various regions with water from one freshwater-rich area (Falkenmark et al. 1989; Baggio et al. 2021). However, population growth and the rising industrial demand are causing overexploitation and ecosystem degradation. Therefore, considering unconventional water resources to supplement conventional systems is crucial (UN Water 2020a). The exploration of every available alternative to minimise pressure is particularly important in dry areas, where water scarcity is intensifying more prominently (UN Water 2020b). This includes larger-scale supplies from, for instance, wastewater treatment and reuse, which also have ancient roots (Angelakis et al. 2018), and micro-scale catchments from fog, dew, and rain, which have proven effective in supporting communities with small-scale water shortages (ARUP 2018). For populations not adequately connected to conventional supplies, diversifying water resources is recommended, corresponding to need-based solutions (Modéer 2020). Additionally, AWH approaches align with biomimetic nature-based solutions, which are ecosystem-based and mimic natural processes to enhance water availability (UN Water 2018, 2024; Verbrugghe et al. 2023). Thus, employing local climate-based strategies in (semi-)arid regions can enhance reliability, equity, and sustainability. When confronted with a water-scarce area, the question should be how much water is available and how it can be best utilised, rather than how much is needed and where it can be sourced (Falkenmark et al. 1989). AWH is often described as a good micro-scale solution for rural areas (Fang & Jawitz 2019; Nikkhah et al. 2023). It also holds potential as a complementary solution in arid urban environments (Hossain 2021; Verbrugghe & Khan 2023). Within the context of urbanisation and the increasing pressure on conventional water supply networks, inspiration can be found in ancient Greek cities. Despite being located far from large water bodies, they developed advanced hydraulic systems for groundwater collection. Small constructions followed the growth of a city, successfully supplying each area on a micro-scale (Koutsoyiannis et al. 2008).

Finally, the quality of fog and dew as freshwater resources is a key consideration. Acting as atmospheric scrubbers, their chemical composition is a function of the air quality. Most research indicates that fog and dew water meet World Health Organization standards; however, further research is needed due to limited data and the potential presence of heavy metals (Klemm et al. 2012; Tomaszkiewicz et al. 2015; Kaseke & Wang 2018).

Fog and dew are unconventional resources that can supplement water supplies in dryland environments. While many articles briefly mention the historical origins of fog and dew collection, no article comprehensively reviews ancient AWH techniques. The study defined atmospheric moisture in the form of fog and dew and described modern collection techniques. Both entail simple, low-maintenance structures. Research has significantly improved the efficiency of modern collectors through advancements in materials and design. However, in arid regions, climate change and urbanisation are rapidly exacerbating water stress, demanding the exploration of every sustainable, decentralised, and climate-adapted solution. Ancient civilisations secured essential water supplies from fog and dew using simple structures and natural materials, either for drinking or agricultural activities. Through an analytical framework and assessment categorising both ancient and modern techniques, this paper explored the pre-1900 use of fog and dew collection, highlighting their viability for replication, revival, restoration, or redevelopment. Mesh-like structures in Tenochtitlan, alchemists' dew collectors, and stone-pile condensers primarily demonstrate potential for revival with modern adaptations. Dew ponds have proven their efficiency over time, but outdated research and scepticism regarding dew input call for further research. Research on fog drip from trees has advanced its potential. It offers an adaptable, green solution to modern needs for agricultural needs on a micro-scale and to replenish conventional aquifers on a macro-scale. While modern fog collectors draw inspiration from tree structures, further research could compare the potential of complex three-dimensional structures of trees to flat fog collectors in various areas. Regarding urban alternatives, both trees and the stone drip method suggest the potential for redevelopment. Lithic mulching remains a viable strategy for soil water conservation if tailored to local conditions, despite some controversies. In conclusion, implementing techniques using natural matter can avoid the need to transport (potable) water over large distances for small-scale purposes, thereby addressing localised issues with micro-scale, decentralised solutions. In regions facing increased water stress, policymakers could investigate areas by assessing local water needs and proposing both modern and ancient AWH techniques, respective to environmental conditions, in addition to providing education programmes to intrinsically encourage sustainable developments and empower local communities. AWH methods are not yet widespread but offer an alternative in arid climates where more common solutions, such as rainwater catchment, are not feasible but offer similar benefits. Both past and present strategies can contribute to water security in (semi-)arid climates, whether on a micro or macro scale.

This article received no external funding; however, it is part of a research funded by the Belgian Fund for Scientific Research F.R.S.-FNRS with grant number: 40016420.

N.V. rendered support in literature review, research and wrote the article; A.Z.K supervised the work. All authors have read and agreed to the published version of the manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare no conflict of interest.