Trial of fog and rainwater harvesting to alleviate water scarcity in the agricultural highlands of the Galapagos Islands

Despite being world-renowned for their endemic biodiversity, the Galapagos Islands face multiple obstacles in their progress towards the Sustainable Development Goal (SDG) Agenda – including the need to address severe water scarcity. On Santa Cruz – the archipelago's most inhabited island – limited water resources have long been overstretched (Reyes et al. 2016) and the rapid growth of the tourist and resident populations in recent decades has exacerbated the issue (Garcia Ferrari et al. 2021).

Improving the productivity of Santa Cruz's agricultural zone is seen as crucial for an archipelago which imports over 75% of its foodstuffs (Sampedro et al. 2020). However, the zone faces acute challenge in sustainable water provision, with extremely limited surface or groundwater resources and no distributed supply network (Nicholas et al. 2020). As such, the irrigation practices of farmers typically rely on either (i) rainfall-fed geomembrane-lined storage reservoirs (Jaramillo Díaz et al. 2023) or (ii) purchased tankers from the lowlands town of Puerto Ayora (Jaramillo Díaz et al. 2022). Both options are problematic for a region dominated by small-scale artisanal farming plots – the capital investment required for reservoirs is often prohibitive, while tankers are expensive, produce substantial CO₂ emissions and typically provide poor quality brackish water (which hampers crop yields and harms soil conditions) (Paltán et al. 2023).

Proposed centralised solutions, such as improved desalination technologies or tapping of the island's perched aquifer, have proved unworkable (Mateus et al. 2020). As such, there is a clear requirement to improve understanding of the potential capacity of off-grid mechanisms to meet the water demands of the island's smallholdings. One such mechanism is rainwater harvesting, widely established globally as tool for agricultural water provision (Velasco-Muñoz et al. 2019). This is already commonly used across Santa Cruz's agricultural zone for domestic purposes (Reyes et al. 2017), but often with poorly maintained systems that are inefficient or underused.

A second such mechanism is fog harvesting – the collection of atmospheric moisture using elevated ‘nets’ (Qadir et al. 2021) – which, while now increasingly used in agricultural contexts globally (Salem et al. 2017; Kanooni & Kohan 2023), has had very limited application in Galapagos. A recent 48-day trial of a standard collectors (i.e., 1 m² collecting area) on the neighbouring San Cristobal Island highlighted the possibilities of the technology, with yields of up to 7.9 L/m²/day at certain times of year (Echeverria et al. 2020). Other local trials have also demonstrated the promise of the technology, albeit without quantifying the potential for water yields.

By presenting the results of the largest-ever trial of fog and rainwater harvesting in the Galapagos Islands, this paper aims to demonstrate how these technologies could contribute to alleviating severe water scarcity across the agricultural zone of Santa Cruz.

In collaboration with local community leaders and NGOs, this study installed and monitored the yields from eight large fog harvesting collectors at farms across Santa Cruz's agricultural zone (see Figure 1 for their locations). Table 1 gives their individual characteristics. All were built using a double layer of 35% shade raschel mesh – a locally available material which, while not the most effective, is an established material for collection (Kaseke & Wang 2018). This mesh was hung (using 2.8 mm Ø galvanised wire) between 38 mm Ø galvanised steel pipes, which were founded on concrete-filled recycled tyres and braced using wire and ground anchors (see figure 2).

Yields from these collectors were measured using custom-developed ‘tipping flow gauges’ which used battery powered proximity sensors to measure the number of times water filled and emptied a small vessel. These gauges were also attached to eight roofs to measure yields of existing rainwater harvesting systems (see Figure 1 for the locations). Table 1 gives the sizes of the collection roofs. Care was taken to ensure the quality of these roofs, as well as the efficiency of each system. All gauges were checked weekly and installations maintained regularly. Data were collected from the beginning of March 2023 until the end of December 2023, thereby covering an entire ‘cold’ season.

The figures demonstrate how both rain and fog harvesting yields are influenced by monthly rainfall. Both technologies had their lowest yields in March – a month with unseasonably modest rainfall. As the rainfall rates increase over subsequent months, yields from both technologies increase by over threefold to a peak in May, with over 8,500 L collected across the eight fog harvesting installations. During this period, fog harvesting collected yields that are, on average, 21% of those from the rain harvesting installations. Interestingly, during June and July – traditionally two of the driest months in the highlands – this proportion rises dramatically to almost 40%. In addition, the 75% drop in rainfall from July to August related to a 63% drop in rainfall harvesting yields but only a 10% drop from the fog collectors. These relative proportions demonstrate one of the key advantages of fog harvesting in the Galapagos context – the technology continues to offer yields during drier periods, thereby improving the resilience of smallholdings to the risk of purchasing tanker water at critical times of year.

By normalising the fog harvesting data and considering yields per square metre of net (i.e., L/m²/day), insight can be gained into how the characteristics and locations of installations can influence yields. The most influential factor is the altitude of the installation. The most effective net is F01 with yields reaching 5.74 L/m²/day which, while not quite the highest net, was situated on a farm ideally situated for fog harvesting – on an open crest of a prominent hill facing the prevailing wind. By contrast – the F08 net, which was the lowest net and situated on a more sheltered smallholding, reached yields of only 0.35 L/m²/day. Overall, the top three mean average rates were seen at the highest three locations (with a mean average across the entire study period of 1.8 L/m²/day), while the three lowest mean average rates were at the lowest three locations (with a respective average of 0.67 L/m²/day). These rates were substantially lower than that of 7.9 L/m²/day suggested possible by Echeverria et al. (2020). There are numerous explanations for this. First, Santa Cruz could offer substantially less potential for fog harvesting less potential for fog harvesting than neighbouring San Cristobal (where the original study took place) – though this seems unlikely given the hydroclimatic similarities between the islands (Violette et al. 2014). Second, these fog nets, when subjected to the rigours of agricultural environments, suffered comparably larger losses. Certain nets were disrupted periodically by livestock and other external influences. Third, the design of the fog nets – which was deliberately lightweight and transferable to suit farmers’ needs – often led to slight flexing of both the net and collection tubes during moderate winds, which would have also resulted in losses.

The normalised rates for rainwater harvesting (e.g. yields per square metre of roof) offer insights too. The highest installation, R02, was particularly efficient – reaching a high of 10.2 L/m²/day (in May), with a mean average across the study period of 4.1 L/m²/day. However, at lower altitudes reasonable rates were also found, with R04 averaging 2.6 L/m²/day across the study period. The lowest mean rate was found with R01, but this was primarily due to the farmer changing their water management infrastructure during the study period.

The results suggest that both fog harvesting and rainwater harvesting offer benefits in alleviating the water scarcity of farmers across the highlands of Santa Cruz. Rainwater harvesting produces larger yields more consistently – both temporally and spatially. Nevertheless, with the correct conditions, fog harvesting installations also offer the potential to provide a sizeable proportion of the existing water requirements of smallholdings on the island.

It is important to acknowledge that this short paper considers yield data alone and that there are several other critical factors in establishing the viability of these technologies in Santa Cruz. For instance, these yields cannot necessarily alleviate water scarcity without appropriate storage capacity to allow the required buffering to meet smallholdings' demand. Moreover, the installation and maintenance of these technologies creates an economic burden which – though modest – would need to be met by the farmers. These areas would all be fruitful areas for future study.

Considering both the region's severe water scarcity and low productivity, alternative mechanisms of water supply are urgently needed to improve the resilience of agricultural zones across the Galapagos Islands. While informal rainwater harvesting is already practiced in the region, this study suggests that the widespread implementation of well-maintained, effective rainwater harvesting systems could provide a substantial proportion of the water requirement in the highlands. Fog harvesting, while not providing as much yield, nevertheless offer a mechanism for collecting useful water volumes, particularly in remote locations or where rainwater harvesting is not possible.

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

The authors declare there is no conflict. This research was kindly supported by The Co-op Foundation's Carbon Innovation Fund.