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The United Nation 2023 Water Conference offers a critical opportunity to catalyse actions and innovations that bring increased water security to vulnerable communities across the globe. Researchers have an important role in supporting the delivery of needed on-the-ground impact, but their work must be informed by the priorities and necessities of Global South implementors.Too many of the images on our news feeds show the destructive power of water as droughts and floods ravage communities across the globe. The super-charging of the hydrological cycle from increased atmospheric greenhouse gas levels is adding stresses to water resource systems that are already challenged by over-exploitation, degradation and rising demand (Fig.1). Societies everywhere aspire to ‘water security’1, in which our management of water resources meets the diversity of human health, livelihoods, nature, and production needs, while reducing water risks to acceptable and manageable levels. The urgency of overcoming the gap between aspiration and the reality of water insecurity for billions of people was recognized by heads of government and heads of state who took part in the High-Level Roundtable on Water Security convened at the Conference of the Parties (COP) 27 in Sharm El-Sheikh, Egypt, in November 2022. They called for increased global cooperation, ramping up of investment, and higher political priority for water. Governments, for the first time, agreed explicitly at COP27 2 on the critical role of water systems in climate action. More

Ammonia (NH3) is a small and uncharged molecule typically used as a fertilizer, refrigerant, a fuel, and it is generated in wastewater processes1. Since ammonia is a fuel with a high energy density, it is possible to take this advantage by converting ammonia to nitrogen and electrical energy via the ammonia oxidation reaction (AOR). This reaction requires a catalyst to decrease the energy barrier that prevents the molecule from reacting and transforming into nitrogen. The AOR has been taken to the International Space Station (ISS) using an autonomous potentiostat system with electrode arrays, fluid pumps, and liquid reservoirs, and an autonomous potentiostat.The anodic electrochemical oxidation of ammonia was done on platinum nanocubes2 catalyst on screen-printed carbon electrodes (SPE). The cited literature suggests that under standard conditions, the products of the AOR on monocrystalline platinum (i.e., Pt{100}) is molecular nitrogen at an applied bias of 0.65 V vs. NHE. Nevertheless, other oxides of nitrogen may form at more positive potentials3,4. The gas molecules produced by the electro-oxidation of ammonia can detach from the catalyst interface due to the buoyancy effects that are exerted when in the presence of gravity. Below you may find an AOR mechanism developed by Gericher-Mauerer mechanism5.$$NH_{3(aq)} to NH_{3ads}$$
(1)
$$NH_{3ads} + OH^ – to NH_{2ads} + H_2O + e^ -$$
(2)
$$NH_{2ads} + OH^ – to NH_{ads} + H_2O + e^ -$$
(3)
$$NH_{xads} + NH_{y;ads} to N_2H_{x + y;ads}$$
(4)
$$N_2H_{x + y;ads} + left( {x + y} right)OH^ – to N_2 + left( {{{{mathrm{x}}}} + {{{mathrm{y}}}}} right)H_2O + left( {x + y} right)e^ -$$
(5)
$$NH_{ads} + OH^ – to N_{ads} + H_2O + e^ -$$
(6)
Under microgravity conditions the AOR has shown to have a lower current density because of the lack of buoyancy which allows the gaseous molecules to remain/stay near or at the electrode catalyst interface6,7,8. The lack of buoyancy for mass transfer convection affects the efficiency of the AOR at the platinum surface6. In a parabolic flight where a direct ammonia alkaline fuel cell (DAAFC) was used, the performance decreases up to 27% when using platinum nanocubes supported on Vulcan (Pt-V)8. This catalyst was selected for the ISS AOR study since it is robust and provides the means to achieve reproducibility in our experiments. In addition, it showed the highest AOR current densities9.The purpose of the Ammonia Electrooxidation Lab at the ISS (AELISS)10 project was to develop an autonomous electrochemical systems for studies at the ISS and to validate the previous results under parabolic flights6,7,8 and elucidate the factors affecting the ammonia oxidation reaction during long-term μG conditions at the ISS. There is an interest on electrochemical processes in space for Environmental Control and Life Support System11,12.For the AELISS experiment an autonomous potentiostat needed to be developed for the ISS, a plug-and-play device. Autonomous potentiostats have been developed for wearable technologies13 and smartphones14. For the ISS, a 2-U Nanorack (Nanode)15 (4” x 4” x 8”) was connected to the ISS station equipment rack through a USB-b port. Inside the Nanode the AELISS was placed, which consisted of an autonomous potentiostat, two screen-printed electrode (SPE) Channel Flow-Cells (Metrohm DropSens), two Dolomite Microfluidics peristaltic micropumps, two liquid plastic containers, and a USB flash data storage drive. The autonomous potentiostat, designed and produced by NuVant Systems Inc., controlled all the AELISS components. The AELISS was launched to the ISS on a cargo resupply mission CRS-14/NG-14, in the vehicle Antares, at 9:38 p.m. EDT on October 1, 2020. The data acquisition followed is shown in Fig. 1.Fig. 1Ammonia Electrooxidation Lab at the ISS electrochemical experimental cycles summary.Full size imageThe aim of this research work is to create an autonomous electrochemical device able to improve the time and reproduction of multiple cyclic voltammetry and chronoamperometry experiments at the International Space Station. This will provide a better insight into the selected platinum nanocube catalyst performance for the ammonia oxidation reaction (AOR) and compare results with those generated on Earth gravity. More

Study populationA total of 39 volunteers participated in the study, including 24 women (60%), 14 men (37%), and 1 non-binary (3%), with an average age of 40.7 years (standard deviation (SD) = 10.2 years, range = 26–76 years). Educational level was university or more among 35 (90%) and a high school among 4 (10%). Average consumption of unfiltered tap, bottled, and filtered tap water were, respectively 0.6 (SD = 0.5, range = 0.1–1.5), 0.5 (SD = 0.4, range = 0.3–1.5), and 0.4 (SD = 0.5, range = 0.1–1.5) L/day, based on a self-reported water consumption questionnaire.PFAS, bisphenol A, and nonylphenol in tap waterIn total, 35 PFAS were analyzed in tap water, of which only perfluoroalkyl acids (PFAA; 7 carboxylates and 3 sulfonates) were above the quantification limits, mainly with a carbon chain length shorter than eight (≤C8); while C10, C11 and C12 carboxylates were only detected in one or two samples. Total PFAS detection rate for the first sampling was 79%, and 69% for the second sampling (Table 1). The most frequently detected ( >50%) compounds during the first sampling were perfluoropentanoate (PFPeA) (64%; median = 3.3 ng/L), perfluorobutane sulfonate (PFBS) (64%; median = 9.2 ng/L), perfluoroheptanoate (PFHpA) (52%; median = 3.0 ng/L), perfluorohexanoate (PFHxA) (31%; median = 13.0 ng/L) and PFOS (52%; median = 12.5 ng/L), while the other PFAS showed detection frequencies lower than 12% (Table 1, Fig. 1). Similarly, the most prevalent compounds during the second sampling were PFPeA (62%; median = 4.0 ng/L) and PFBS (45%; median = 6.8 ng/L), whereas PFOS and PFHpA were present in 4.8% and 24% samples, respectively (Table 1, Fig. 1). The PFAS composition profile in the first sampling was dominated by PFBS (25.9%), PFOS (22.1%), PFPeA (17.6%), PFHxA (16.2%) relative to the total PFAS concentrations (Fig. 2). In the second sampling, high contributions to total PFAS concentrations were observed for PFPeA (45.7%), and PFBS (39.2%) (Fig. 2). To our knowledge, this was the first study analyzing ether-PFAS (e.g., GenX, and ADONA) in drinking water of the Barcelona region, showing non-detected levels.Table 1 Number (%) of samples above the limit of quantification (≥LOQ), and concentrations (ng/L) of target compounds in unfiltered tap water samples collected in 42 locations in Barcelona, Spain, in repeated sampling campaigns (August–October 2020, and May 2021).Full size tableFig. 1: PFAS concentrations (ng/L) in tap water.Unfiltered tap water samples were collected in 42 locations in Barcelona, Spain, in repeated sampling campaigns (August–October 2020, and May 2021). The line within the box marks the median, the boundaries of the box indicate the 25th to the 75th percentiles, and the dots denote observations (samples) corresponding to PFAS concentrations.Full size imageFig. 2: Average percentage contributions of individual PFAS concentrations relative to total PFAS concentrations detected in drinking water samples.First sampling (N = 42; S1 DW), second sampling (N = 42; S2 DW), and urine samples of the first sampling (N = 39; S1 Urine).Full size imageCompared to previous studies conducted in Barcelona, replacement PFAS (PFPeA, PFHxA, PFBS) and PFHpA were the most predominant compounds detected in the tap water samples, with observed increasing concentrations over the last 10 years (Supplementary Table 1)16,17.This dominance of PFAS with fewer than eight carbons ( More

Our results show that the most-affected types of infrastructure during the first three months of the armed conflict were dams and reservoirs, underground mines, urban water supply and wastewater treatment systems (overview of this infrastructure in Supplementary Information 2).Ukraine’s critical water infrastructure at riskOf special concern are large reservoirs along the Dnieper River, which are critical for energy production, cooling of nuclear power plants, sustaining agriculture and seasonal flow regulation. In addition, there is a high concentration of settlements along the Dnieper River, with flooding being an immediate threat if the dams would breach (Fig. 3a,b). During World War II, intentional damage to the 800-m-wide dam of the Dnieper HES holding water in the Dnieper Reservoir, near the city of Zaporizhzhia, affected 20,000–100,000 civilians and retreating soviet soldiers crossing the river17 (Fig. 3a). Details on a quantitative flooding-risk assessment for the cascade of Dnieper reservoirs, including those based on hydrological conditions observed in 2022, are presented in Supplementary Information 2.Fig. 3: Examples of impacts on water resources and infrastructure in Ukraine during armed conflicts.a, The dam on the Dnieper River near the city of Zaporizhzhia after reportedly being blown up by Soviet special forces in 1941 in an attempt to delay the offence of German troops. b, Demolition of the dam on the Irpin River on 26 February 2022 caused flooding near the village of Demidov in the Vyshhorod district of Kyiv region. c, Craters formed by shells on the floodplain of the Irpin River. d, Water in the Kamyshevakha River polluted by mine waters (picture taken in 2021). e, Damaged pipe near Kiselevka village in the Kherson region (picture taken in April 2022). f, People in a line for drinking water in Mykolayiv (picture taken in April 2022). Panels adapted with permission from: a, ref. 54, Taras Shevchenko National University of Kyiv; d, ref. 55, Deutsche Welle; e, ref. 56, Korabelov.info; f, ref. 57, Novosti-N. Credit: photographs in b,c, Vincent Mundy.Full size imageApart from flooding, breaching of dams along the Dnieper River poses a danger of secondary radioactive pollution due to uncontrolled release of radioactive material accumulated in the sediments and associated with colloidal materials in surface waters after the disaster at the Chernobyl nuclear power plant (NPP) in 198618,19. Following the accident, the reservoirs of the Dnieper Cascade acted as sinks for radiocaesium, with extensive accumulation recorded in the Kyiv Reservoir. As for radiostrontium, about 43% of the dissolved form that entered the Dnieper system from 1987 to 1993 reached the Black Sea20. Zaporizhzhia NPP, the largest NPP in Europe, is located on the shore of the Kakhovka Reservoir, 40 km downstream from the dam of the Dnieper HES. A sudden loss of water needed for the reactor’s active cooling system can lead to a scenario analogous to the accident at the Fukushima Daiichi NPP in Japan in 201121. The Kakhovka Reservoir also serves as a water source for the largest irrigation system in Ukraine and in Europe22 (for details, see Supplementary Information 2). The conflict raises a risk of either intentional or unintentional bombing posing threats to regional agriculture, food production and international food trade.Military actions and severe environmental pollutionAs a result of the armed conflict, multiple Ukrainian communities have been left without wastewater treatment, resulting in pollution of surface waters. For example, remote-sensing images showed that polluted wastewater was released into the Kakhovka Reservoir when the wastewater treatment plant near Zaporizhzhia ceased operation23. Rivers and networks of irrigation channels that are natural barriers for movement of troops have also become a burial place for military objects (for example, Figure 3c). The underwater decomposition of ammunition leads to release of heavy metals and toxic explosive compounds, with impacts that may last for decades2. This can be critical in the southern regions of Ukraine where an extensive network of irrigation channels exists. Low quality of irrigation water affects the agricultural cropping and the quality of food production24. In the pre-conflict period, the concentrations of heavy metals in waters of the Kakhovka Canal were in compliance with water-quality standards25, but there is concern that the conflict will lead to a deterioration of water quality.In June–July 2022, for the first time, traces of oil products were reported within the area of the surface drinking water intake in the basin of the Siverkyi Donets River, together with exceeded concentrations of mercury, ammonium nitrogen, nitrites, polyaromatic carbons, heavy metals and the insecticide cypermethrin in some rivers within the basin26 (for details on the state of the Siverskyi Donets River since 2014, see Supplementary Information 3). In addition, multiple electrical blackouts within Donbass region have increased the threat of pollution of water sources with mine waters because of failures in operation of pumping equipment. Overflooding of geologically connected mines, a problem present in the region for a long time (for details, see Supplementary Information 2), leads to increase in the concentration of salts in mine water up to 20–70% (except for chloride) and can double concentrations of organic substances and hydrocarbons27. High concentrations of sulfates, chlorides and heavy metals in mine waters pose severe risks for groundwater and surface-water quality (for example, the Kamyshevakha River has become severely polluted by mine waters since 2018; Fig. 3d).Access to safe water resources and the danger of epidemicsDuring the armed conflict, water supply infrastructure has been subjected to repeated attacks, with limited time and few opportunities for repair and recovery. By 20 April 2022, the United Nations reported that 6 million people in Ukraine were struggling every day to get access to drinking water, with 1.4 million people being reported to lack access to safe water in the east of the country and another 4.6 million people having only limited access28. For the period between March and December 2022, the UN estimates that some 16 million people in Ukraine will need water, sanitation and hygiene assistance29. In the city of Mariupol, more than 40% of the water supply system is reportedly damaged, and on 17 May 2022, the World Health Organization raised concerns about the danger of a cholera epidemic in the city due to mixing of sewage and drinking water30. In Mykolayiv, the population was left without a centralized water supply for more than a month (Fig. 3e,f), and water supplied with interruptions from an alternative source later had excessive concentrations of chlorides, sulfates and other mineral salts even after treatment31. The population of Donetsk is reportedly receiving water for only two hours once every 3–4 days, and all specialists capable of addressing problems with the water system are mobilized in the armed conflict, limiting the ability to repair the system32. The Luhansk region, with a pre-conflict population of 2.1 million, was left completely without water supply in the beginning of May, and delivery of water was possible only externally through humanitarian organizations. The lack of access to clean water poses a serious threat of epidemic outbreaks, which was worsened by both extremely hot temperatures observed during the summer in 2022 and reduced capabilities of the medical system33. According to UNICEF, children living through prolonged conflicts are more likely to die from water-borne diseases than from the military conflict itself34.Caveats and uncertaintiesExpert evaluation of reported and projected impacts of armed conflict is limited in many cases by the lack of safe access to affected sites and by possible biases and discrepancies in reporting. However, to a certain extent, consequences of the use or targeting of water systems in conflicts can be estimated on the basis of retrospective analyses of similar impacts on freshwater resources and infrastructure. For example, catastrophic flooding due to damage to the Dnieper HES during World War II and the spread of radionuclides through water as a result of the catastrophe at Chernobyl NPP indicate the spatial extent of potential impacts in cases when large reservoirs or NPPs are affected by military actions. The long-lasting consequences of environmental pollution due to impacts on water infrastructure have been highlighted by an accident of a potash spill into the Dniester River due to overflooding of the Stebnik waste pond in the Lviv region in 198335,36. In this event, more than 3.8 km3 of highly concentrated waste salts were spilled, raising the salinity of the Dniester River to levels higher than seawater. This event disrupted water supply to millions of people in Odessa, Kishinev and the Tiraspol region, killed hundreds of tons of fish and heavily contaminated the sediments of the river35,37.Although modern military technologies can allow precise destruction of localized objects, the damage to industrial targets is not always environmentally local, and many of the attacks have been not precise but general. In highly industrialized Ukraine38, targeting urban and industrial infrastructure leads inevitably to widespread and severe environmental consequences. By the beginning of June 2022, more than 25 big Ukrainian industrial companies were damaged or fully destroyed. Most prominent are the ammonia producer AZOT, the Coke and Chemistry concern in Avdievka and the centre of metallurgy AZOVSTAL in Mariupol39. Port infrastructures in the Black Sea and Azov Sea coastal areas were heavily bombed in Mykolayiv, Odessa and Mariupol.Other impacts on water resources can be only roughly estimated at the moment, including the threat to regional biodiversity. It has been reported that 14 Ramsar wetland sites covering 400,000 hectares along the coastline and lower reaches of the Dnieper River are under threat40. Damage to reservoirs during spring spawning led to mass fish deaths (confirmed for the Oskil Reservoir)41.The need for urgent actionOur study on the impacts of the armed conflict on freshwater resources and water infrastructure in Ukraine highlights diverse and long-lasting consequences not only for local populations and ecosystems, but also for progress towards the global Sustainable Development Goals42.Catchments cut across political borders and pollutants released into the environment from armed conflicts can spread across national borders. Ninety-eight percent of the catchment area of Ukrainian rivers flows to the Black Sea and Azov Sea, and the remaining 2% to the Baltic Sea. Although the international community has already identified the risk of environmental pollution in the Donbass region in the eastern part of Ukraine since 201443, military actions have dramatically intensified and are now taking place in the previously unaffected southern part of Ukraine. This area is important for agricultural activities that depend on an extensive network of irrigation channels. According to the World Food Programme, Ukraine contributed 50% of sunflower oil and 10% of wheat to the total global exports in 2021, being the first and the sixth global producer, respectively44. Due to the armed conflict, agricultural production has been substantially reduced, leading to food shortage on the global scale, with countries of Middle East and Africa most affected11.A lack of access to safe water and the environmental threats urge prompt action. Priority activities should focus on providing safe drinking water for millions of civilians in the affected areas and protecting civilian water supply and treatment systems. A set of international rules related to protection of the environment and civilian water infrastructure during armed conflicts is defined by the Geneva List of Principles, including especially the 1977 Protocols to the Geneva Convention4,45. According to the recent resolution adopted by the United Nations Security Council on 27 April 2021, all parties of the armed conflict are obliged to protect civilians and civilian infrastructure, including water facilities46. Nevertheless, multiple cases of attacks on water technicians since the start of the conflict have been reported in Chernihiv, Kharkiv and Mykolayiv, adding to at least 35 water engineers who have been killed or injured in the Donetsk and Luhansk region since 201447,48. We argue that protection of civilian water technicians should be ensured, providing the so-called ‘green corridors’ for safe access to water infrastructure.Support by international agencies and partners is needed to provide water-treatment systems that can be used by individual households and to provide temporary access to safe drinking water or assistance in rebuilding and replacing destroyed civilian water infrastructure. For places without current access to safe drinking water, sustainable options should be investigated apart from the temporary and costly option of transporting bottled water. In particular, water-treatment systems should be installed at critical locations such as hospitals, schools and community centres. Individual households could be supplied with individual small-scale filtration systems. In the longer term, options such as desalination should be considered because most of the local surface waters in the southeastern parts of the country are characterized by high mineralization49 (for example, the current water supply to Mykolayiv from the Southern Bug to replace the damaged supply system from the Dnieper30). For settlements that were receiving water from the basin of the Siverskyi Donets River, the option for desalination is even more convincing due to both the proximity to alternative water supply sources and the fragility of water-transfer facilities as has been shown by this armed conflict.Importantly, environmental monitoring and data collection efforts to better understand the environmental risks are urgently needed. Unfortunately, in March 2022, the Organization for Security and Cooperation in Europe, the official international conflict monitor, announced closure of its Special Monitoring Mission in Ukraine50. The mission was enabling the repair and maintenance of the critical civilian infrastructure facilities benefitting civilians on both sides of the contact line in eastern Ukraine since 2014.The current crisis demands coordinated action from Ukraine and Russia, mediated and facilitated by other countries of the European Union and the United Nations. We recommend that science and management focus on assessing the dynamic state of the environment and water conditions in the zone of the conflict, with the aim to develop effective and prompt approaches for its post-war rebuilding. Although the conflict is still ongoing, freshwater resources and water infrastructure should be protected and maintained because of their central role in supporting basic human needs, health and well-being. Because access to the sites in the zone of conflict is limited, particular attention should be given to spatial mathematical and cartographic modelling using remote-sensing data, which allow efficient use of limited input information. Such an approach can be applied to simulating flooding due to dam breaching under different hydrological scenarios, spread of pollutants from sunken military monitions, effect of land mines on surface and groundwater, predicting quality of subsurface mine waters and their overflow to geologically connected areas, forecast of quantity and quality of water for drinking and irrigation purposes and assessment of the effect on freshwater biodiversity. From a management perspective, we recommend that future studies focus on assessing financial apparatus and the economic dimensions of sustainable water management, on the enforcement of water-related regulations and on identification and evaluation of current and post-conflict needs, facilitating the recovery of Ukrainian water resources and infrastructure. More

The environmental consequences of armed conflicts have been well documented in the relevant literature. Research has demonstrated the profound and often irrevocable damages of military attacks to life-support ecosystem services1. Freshwater systems are among the most problematic targets, as invading countries can exert substantial pressure on the opponent either directly (by damaging infrastructural assets) or indirectly (by deteriorating water-related services). There have been many instances since the early stages of human civilization where water resources have become the bone of contention among different populations and have been weaponized to weaken the enemy’s position during conflicts or have resulted in collateral damage2,3. It is, however, challenging to estimate the scale of the impacts, and interpret the broader sustainability implications, given the complexity of freshwater systems and the multiple services they provide. More

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.This is a summary of: Shumilova, O. et al. Impact of the Russia–Ukraine armed conflict on water resources and water infrastructure. Nat. Sustain. https://doi.org/10.1038/s41893-023-01068-x (2023). More

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PVWPS operationThe motor and the pump are built in together14 and the motor-pump set is submersed in the borehole under the water43. Control equipment is also installed between the PV modules and the motor-pump and/or integrated to the motor-pump set in the borehole14,17. This equipment allows the motor-pump to stop and also to operate the motor-pump and the PV modules at their best operating points14. Once the water is pumped, it might then be stored in a water tank to mitigate the variability of solar resources14,29. When pumping starts, a cone of depression of radius rc is formed and there is a drawdown Hb,d in the borehole (see Fig. 1). The higher the pumped flow rate, the higher the drawdown Hb,d and therefore the deeper the water in the borehole Hb. If Hb reaches the position of the motor-pump Hmp, the motor-pump automatically switches off, therefore preventing the motor-pump from running dry44. The motor-pump remains shut down during a period Δtshut, after which it makes an attempt to restart44.Input data processingWe observe in Table 1 that the datasets have varying spatial resolutions. In the article, we use the spatial resolution of the irradiance map, 0.2° (~22 km). Indeed, this resolution is sufficient for the purposes of this article and it allows to divide computing time and memory requirements by ~16 in comparison to the 0.05° resolution. At this 0.2° resolution, the total area of Africa of 30 million km2 is divided into 62,000 pixels. We apply this resolution of 0.2° to all datasets by nearest interpolation.No exact value of the static water depth Hb,s, transmissivity T and saturated thickness Hst are provided for each location by the original source but only a range of variation. For instance, for −15.8° (lat) & 21.9° (lon), the saturated thickness Hst is comprised between 25 and 100 m. In most cases, we consider the middle of the range (e.g., 62.5 m in the example). The only two exceptions are: when Hb,s is higher than 250 m, we consider 300 m (same for Hst); and, when Hb,s is between 0 and 7 m, we consider 7 m45. Due to the lack of available information, the input groundwater data provided in Table 1 are considered to remain constant over time.Reference46 provides complete irradiance data with a time step of 15 min from 2013 to 2020 across Africa. In this article, except mentioned otherwise, we use irradiance data from 2020 with a 30-min time step (by taking one point every two 15-min points), instead of all the available complete irradiance data. It divides computing time and memory requirements by ~16. Additionally, it produces reduced and acceptable deviations on the results. Indeed, for 100 randomly chosen locations, we simulated the pumped volume V, for the three considered PVWPS sizes, using (1) irradiance data from 2013 to 2020 with a 15-min time step and (2) irradiance data from 2020 with a 30-min time step. For these locations, the absolute error on volume V is systematically lower than 7.9% and the average absolute error is 2%. These results are coherent with the observed low influence of irradiance on the pumped volume in comparison to groundwater resources. Thanks to the consideration of this reduced irradiance vector, the random access memory (RAM) and the computing time required to obtain a map of final results (such as Fig. 5b) are respectively 38 Gb and 10 h (time for Intel Xeon E5-2643 3.3 GHz processors and 96 GB RAM, running on Debian 4.19.194-2), which is more reasonable.Atmospheric sub-modelFor each location, the irradiance on the plane of the PV modules Gpv at time t can be deduced from satellite data by47,48:$${G}_{{{{{{rm{pv}}}}}}}left(tright)={G}_{{{{{{rm{bn}}}}}}}left(tright){{cos }}left({{{{{rm{AOI}}}}}}left(t,theta ,alpha right)right)+{G}_{{{{{{rm{gh}}}}}}}left(tright)kappa frac{1-{{cos }}left(theta right)}{2}+{G}_{{{{{{rm{dh}}}}}}}left(tright)frac{1+{{cos }}left(theta right)}{2}$$
(1)
where κ is the albedo of the surrounding environment, θ and α are the tilt and azimuth of the PV modules and AOI is the angle of incidence between the sun’s rays and the PV modules. The albedo κ is taken equal to 0.2 because it corresponds to the albedo of cropland, which is a common environment in the rural areas considered49. In any case, additional simulations show that the value of the albedo has a negligible effect on the pumped volume V. AOI is computed using the MATLAB toolbox PVLIB developed by the Sandia National Laboratories50.For each location, the azimuth α and the tilt θ of the PV modules are chosen to maximize the irradiance on the plane of the PV modules Gpv. The azimuth α is taken equal to51:$$alpha =left{begin{array}{c}180^circ quad {{{{{rm{if}}}}}},phi , > ,0\ 0^circ quad {{{{{rm{if}}}}}},phi , < ,0end{array}right.$$ (2) where ϕ is the latitude of the location. The tilt is taken equal to51:$$theta =left{begin{array}{c}{{max }},(10,1.3793+(1.2011+(-0.014404+0.000080509phi )phi )phi )quad{{{{{rm{if}}}}}},phi > ,0\ {{min }},(-10,-0.41657+(1.4216+(0.024051+0.00021828phi )phi )phi )quad{{{{{rm{if}}}}}},phi , < ,0end{array}right.$$ (3) As evidenced by Eq. (3), the tilt should be higher than 10° or lower than −10°, so that the PV modules are tilted enough to be cleaned when it rains.Photovoltaic modules sub-modelConsidering that the maximum power point tracking of the PV modules is correctly performed, a simplified model to compute the power P produced by the modules is used:$$Pleft(tright)=frac{{G}_{{{{{{rm{pv}}}}}}}left(tright)}{{G}_{0}}{P}_{{{{{{rm{p}}}}}}}left(1-{c}_{{{{{{rm{pv}}}}}},{{{{{rm{loss}}}}}}}right)$$ (4) where G0 is the reference irradiance (1000 W m−2), Pp is the peak power of the PV modules in standard test conditions (STC) and cpv,loss is a coefficient that represents the losses (e.g., soiling, temperature, mismatch, wiring52,53) at the level of the PV modules. For the sake of simplicity, and as we consider a generic PVWPS, we consider that cpv,loss is independent of the operating point of the PV modules, of the time, and of the location. We take it constant, equal to a single value (see Table 2).Hydraulic sub-modelThe total dynamic head TDH between the motor-pump and the pipe output is given by54:$${TD}Hleft(tright)={H}_{{{{{{rm{b}}}}}}}(t)+{H}_{{{{{{rm{p}}}}}}}(t)$$ (5) where Hb is the water depth in the borehole and Hp is the additional head due to pressure losses in the pipe.The water depth in the borehole Hb is given by (see Fig. 1)42:$${H}_{{{{{{rm{b}}}}}}}(t)={H}_{{{{{{rm{b}}}}}},{{{{{rm{s}}}}}}}+{H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}(t)$$ (6) where Hb,s is the static water depth and Hb,d is the drawdown. The drawdown is composed of two parts:$${H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}(t)={H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{a}}}}}}}(t)+{H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{b}}}}}}}(t)$$ (7) where ({H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{a}}}}}}}(t)) is the head loss due to aquifer losses and ({H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{b}}}}}}}(t)) is the head loss due to borehole losses.The head loss due to aquifer losses ({H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{a}}}}}}}(t)) depends on the pumping flow rate Q, the aquifer transmissivity T, the borehole radius rb, and a length parameter rc representing the distance of water travel to replace the water pumped out. From dimensional analysis, we expect that (tfrac{{H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{a}}}}}}}left(tright)cdot T}{Qleft(tright)}) should be a function of (tfrac{{r}_{{{{{{rm{c}}}}}}}}{{r}_{{{{{{rm{b}}}}}}}}). We thus propose the following model for ({H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{a}}}}}}}), which is derived from Thiem equation55:$${H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{a}}}}}}}left(tright)=frac{{{{{{rm{ln}}}}}}left(frac{{r}_{{{{{{rm{c}}}}}}}}{{r}_{{{{{{rm{b}}}}}}}}right)}{2pi T}Qleft(tright)$$ (8) where rc can be considered the effective radius of the cone of depression. This model satisfies horizontal, radial and steady Darcy flow in a uniform, homogeneous and isotropic aquifer. It captures the essential features for aquifer losses: ({H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{a}}}}}}}(t)) proportional to pumped flow rate and inversely proportional to transmissivity45. Though the flow is transient, only simplified steady-state models, as the one of Eq. (8), can be applied with the available information as dynamic models would require pumping tests. Furthermore, we consider that the radius of the cone of depression rc is comprised between 100 and 1000 m and, to correlate it to a measured quantity, that it depends linearly on the groundwater recharge R: for the lowest recharge (0 m/year), rc is equal to 1000 m; for the highest one (0.2947 m year−1), rc is equal to 100 m; in-between, rc is obtained linearly from the recharge (rc = 1000–3054 · R). Thus, groundwater recharge R is used to constrain the size of the cone of depression.The head loss due to borehole losses ({H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{b}}}}}}}(t)) is given by56:$${H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{b}}}}}}}left(tright)=beta {Q}{left(tright)}^{2}$$ (9) where β is a coefficient related to the borehole design. For the yields considered in this article, ({H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{b}}}}}}}(t)) usually remains lower than a few meters but, as ({H}_{{{{{{rm{b}}}}}},{{{{{rm{d}}}}}}}^{{{{{{rm{b}}}}}}}(t)) depends on the square of the pumped flow rate, it may be more important for larger abstraction capacities.The additional head due to pipe losses Hp is given by57:$${H}_{{{{{{rm{p}}}}}}}left(tright)={H}_{{{{{{rm{p}}}}}},{{{{{rm{ma}}}}}}}left(tright)+{H}_{{{{{{rm{p}}}}}},{{{{{rm{mi}}}}}}}left(tright)$$ (10) where Hp,ma(t) corresponds to losses that occur along the pipe length (also called “major losses”) and Hp,mi(t) corresponds to losses at junctions such as elbows and curvatures (also called “minor losses”). Hp,ma(t) is given by57:$${H}_{{{{{{rm{p}}}}}},{{{{{rm{ma}}}}}}}left(tright)=frac{8f}{{pi }^{2}g{D}_{{{{{{rm{p}}}}}}}^{5}}{L}_{{{{{{rm{p}}}}}}}Q{left(tright)}^{2}$$ (11) where g is the gravitational acceleration (9.81 m s−2), Dp is the pipe diameter, Lp is the pipe length, Q is the pumped flow rate, and f is the friction coefficient between the water and the pipe. We approximate the pipe length Lp to be equal to the depth of the motor-pump Hmp (see Fig. 1). The expression of f depends on the value of the Reynolds number ({{{{{rm{Re}}}}}}=tfrac{4Q}{pi {D}_{{{{{{rm{p}}}}}}}w}), where w is the water kinematic viscosity (taken equal to 1 × 10−6 m2 s−1)57: for Re More