Innovations in Clay-Based Irrigation Technologies—A Systematic Review

Abstract

Arid and semi-arid areas are suffering from declines in fresh water availability, making food security in these regions strongly dependent on the adaptability of agricultural production to the minimum usage of irrigation water. In response to this critical need, efforts have been directed towards enhancing irrigation efficiency and exploring innovative clay-based subsurface irrigation systems. These systems use clay materials as porous emitters and operate on the principle of capillary water movement from the pottery to the root zone, effectively reducing water evaporation and demonstrating significant water-saving potential. This article presents the results of a systematic literature review, with a specific focus on identifying recent developments and innovations in clay-based subsurface irrigation technologies, describing cases of applicability and indicating directions for future research. This review follows the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and involved the screening of 233 articles that were found through searches on the databases Web of Science and Science Direct combined with searches of Google Scholar and citation searches. As a result, 58 research articles were investigated. The webtool Rayyan was used for the screening of the articles and the synthesis of the results. The spectrum of clay-based irrigation systems identified in the investigated articles includes traditional methods such as clay pot and clay pipe irrigation as well as more recent advancements in the field such as Subsurface Irrigation with Ceramic Emitters (SICE), Self-regulating Low-Energy Clay-based Irrigation (SLECI), and Ceramic Patch Subsurface Drip Irrigation Line (CP-SDIL) and pottery dripper technologies. This paper offers a comprehensive analysis of each irrigation system, highlighting their main characteristics, advantages, and limitations. Particular attention is paid to the reported outcomes related to yield responses, water use efficiency, and suitability for various agricultural applications. This review indicates as a primary benefit of these systems their potential to allow water conservation, which is especially advantageous in regions with a restricted irrigation water supply. However, a major drawback is the challenge of scaling these systems effectively. Hence, the recommended areas for future research centre on the necessity of substantial economic assessments of and discussion on the potential social impact to promote the scalability of clay-based irrigation systems.

1. Introduction

Progressing climate change exerts enormous pressure on freshwater resources. Extreme weather conditions greatly affect arid and semi-arid areas, causing a decline in fresh water availability. Simultaneously, fresh water demand is rising due to the growth of populations and the expansion of irrigation areas to satisfy the growing demand for food. Access to water is critical for ensuring food security as agricultural production heavily depends on irrigation. The United Nations estimates that 40% of the world’s food is produced in irrigated areas, accounting for 20% of the total cultivated area [1]. While irrigated agriculture accounts for the consumption of 70% of freshwater resources [2], water withdrawal for irrigation purposes largely exceeds the actual requirements due to considerable losses during the distribution and application processes [3]. To reduce the pressure of irrigated agriculture on the available water resources, there is an urgent need for irrigation technologies that are both water-saving and water-efficient, without compromising the quantity and quality of agricultural output.

On the other hand, the imperative of facilitating environmental recovery and climate-neutral social development promotes the utilization of natural and bio-based materials to enhance circularity within production and agricultural systems while also tapping into locally available materials. A series of novel technologies, inspired by the ancient principles of clay pot or pitcher irrigation, have been studied for their potential to enhance irrigation water efficiency and crop yields. While a significant body of knowledge on the traditional irrigation method involving the use of pitchers and buried clay pots has been established, research on recent innovations inspired by these traditional techniques call for investigation of the existing literature. The aggregation of these studies could advance further research and promote the adoption of these novel water-saving irrigation methods. In this study, we carried out a systematic literature review with the following aims:

(1)

Identify existing clay-based irrigation technologies and related available evidence in terms of characteristics, operation principles, and investigated subject matter;

(2)

Analyze the reported findings and determine the applicability of the studied technologies;

(3)

Identify gaps in knowledge on this topic to inform future research efforts.

In this paper, each of the identified irrigation systems is analysed, outlining their main characteristics, advantages, and limitations. The analysis pays particular attention to the reported outcomes related to each system’s hydraulic properties, yield responses, water use efficiency, and suitability for various agricultural applications.

2. Materials and Methods

The present literature review was conducted as a mixed review [4] combining the approaches of a systematic literature review [4,5,6] with a consequent search of the academic search engine Google Scholar and a citation analysis [7] using publications identified as most relevant to the research objectives. The citation analysis provided us with the opportunity to include relevant research publications that were not indexed in the databases.

For the purpose of ensuring the transparency and replicability of the applied review methods, a systematic review protocol was used during the planning and documentation of each step [8]. The guidelines on Preferred Reporting Items for Systematic Reviews and Metanalyses (PRISMA) were used [9,10] (see Supplementary Materials).

2.1. Systematic Search and Information Sources

A database search was conducted based on the pre-defined key words and search strings, as presented in Figure 1. The following sources were used in the information search: Science Direct (https://www.sciencedirect.com, accessed on 31 March 2023), Web of Science (https://www.webofscience.com, accessed on 31 March 2023), and Google Scholar (https://scholar.google.de, accessed on 31 March 2023). These sources were selected due to their reach and large coverage of the scholarly literature.

Consequently, the database search was supplemented with a search conducted on Google Scholar using the same search keywords. In order to track the dynamic development of these technologies, a twofold search was conducted, covering articles published until 2018 and articles published from 2019 to 2023. Finally, a total of 233 articles were identified as per March 2023. As these databases are frequently updated, the number of search results may vary over time.

2.2. Eligibility Criteria, Screening, and Data Selection

The screening and data selection processes were conducted in accordance with the steps presented in the PRISMA flowchart. Firstly, all duplicates were identified and removed. After title and abstract screening, 74 papers were sought for retrival. In total, 58 papers were accessed and assessed for eligibility. Finally, 51 papers were included in the review. In addition, 7 more publications identified through a complimentary citation search qualified as eligible and were added to the review. The automation web tool Rayyan (AI-aided) was used during the screening process.

Relevant articles were then selected based on the following main criteria.

Selection criteria:

• Articles focusing on irrigation systems that report on clay-based materials and components integrated in the irrigation system or method;
• Articles from Agricultural Sciences with a strong focus on irrigation;
• Articles written in English;
• Articles presenting application studies on various crops, reporting findings with regard to yield, water consumption and saving, energy and labour use, and environmental impact;

Exclusion criteria:

• Articles not published in English (as an exception, one research report written in German was identified and included due to the limited number of publications available in English on Self-Regulating Low-Energy Clay-Based Irrigation (SLECI) technology [11]).
• Research articles without an explicit focus on clay-based materials used in the irrigation systems and the corresponding methods;
• Articles on other sub-surface irrigation systems (e.g., Moistube, porous pipe irrigation, subsurface drip);
• Irrigation methods explicitly tailored to potted plants or closed systems.

The results from the selection process are illustrated in a PRISMA flowchart in Figure 2.

3. Results

3.1. Study Selection and Limitations

A substantial amount of the literature identified through this systemic literature review (n = 58) is focused on irrigation with clay pots and pitchers (n = 26), which is the traditional method of sub-surface irrigation. This review also explores other clay-based irrigation technologies derived from this traditional method. Porous clay pipes (n = 9), which are made of unglazed porous ceramics, are the successor of clay pots, and they were mostly the subject of research interest between the early 1990s and 2017 [13,14,15,16]. Porous ceramic emitters are a new version designed to combine the benefits of water efficiency and conservation provided by traditional subsurface irrigation using clay pots with the cost-effectiveness and adaptability required for modern agricultural practices. Two novel subsurface micro-irrigation systems use ceramic or clay-based emitters—the Subsurface Irrigation with Ceramic Emitters (SICE) system (n = 18) in China [17] and the Self-Regulating Low-Energy Clay-Based Irrigation (SLECI) system (n = 3) in Southern Europe [11,18,19]. Two papers (n = 2) indicate two further clay-based irrigation innovations, the Ceramic Patch Type Subsurface Drip Irrigation Line (CP-SDIL) [20] and the pottery dripper [21]. Figure 3 presents an overview of the technologies that have evolved from the traditional clay pot irrigation method.

The identified technologies were systematically analysed in terms of their characteristics, operational principles, and the scope of their application, as reported in the literature. Systematization based on thematic areas revealed that the hydraulic characteristics and soil water dynamics associated with the technologies constituted the most dominant topic, addressed in 43.1% percent of the included articles (n = 25). Consequently, the literature was analysed according to the guiding research questions, synthesizing the findings related to the yield responses, water use efficiency, and applicability across different agricultural contexts reported for each system.

The analysis of the time frame of this research reveals a stagnant interest in porous clay pipes, with most articles published before 2018, and limited availability of recent research findings from the last five years. Conversely, alternative systems that are designed to combine the advantages of clay-based irrigation methods with modern micro-irrigation systems have gained more attention. Notably, Subsurface Irrigation with Ceramic Emitters (SICE) has seen a significant increase in research interest, with a substantial portion of identified papers published after 2019. Similarly, Self-Regulating Low-Energy Clay-Based Irrigation (SLECI) has emerged, with the first scientific evidence published in 2023. Research on traditional clay pot and pitcher irrigation methods has remained stable over the investigated period of time.

A list of the total number of research articles analysed is disclosed in

Table 1. Study characteristics of included articles.

Technology	Nr. of Articles Included	Author	Location	Crop
Clay pot Pitcher	26	Araya et al. [22]; Bainbridge [23,24,25]; Babiker et al. [26]; Daka [27]; Tesfaye et al. [28]; Mondal [29,30,31]; Kefa et al. [32]; Soomro [33]; Woldu [34]; Vasudevan et al. [35]; Abu-Zreig and Atoum [14]; Abu-Zreig [15,36]; Hatungimana et al. [37]; Elavarasan, Govindappa, and Hareesh [38]; Gebru et al. [39]; Mahata et al. [40]; Naik, Panda, and Nayak [41]; Pal et al. [42]; Rajshekar and Armstrong [43]; Tripathi, Sharma, and Meena [44]; Adhikary, R., and Pal, A. [45]; Martínez de Azagra Paredes, Zapata, and Faci [46]	China, India, Pakistan, Malaysia, Indonesia, Iran, Jordan, Ethiopia, Zimbia, Mexico, Morocco, Kenya, Brazil, Rwanda, Iraq	Watermelon, tomato, corn, gourd, cauliflower, okra, sweet orange, avocado, apple, cucumber, eggplant, beans, citrus, cabbage
Porous clay pipe	9	Siyal et al. [47,48,49]; Bhatt, N. J. [50,51]; Bhople et al. [52]; Batchelor, Lovell, and Murata [53]; Dubey, Gapta, and Mondal [54]	India, Sudan, Pakistan	Maize, turnip, okra, eggplant
SICEs	18	Cai et al. [55,56,57,58,59,60]; Liu et al. [61,62]; Yao et al. [63,64]; Wang et al. [65,66]; Zhou et al. [67]; Chen et al. [68]; Huang et al. [69]	China	Tomato, apple, lettuce, wolfberry, persimmon, jasmine (Murraya paniculate)
SLECI	3	Malchev et al. [18]; Pereira et al. [19]; Hansmann and Siering [11]	Bulgaria, Morocco, Malta, Portugal	Sweet cherries, peaches, grapevines, citrus, olives
CP-SDIL	1	Cai et al. [20]	China	Lab-scale experiments
Pottery dripper	1	El-Hagarey [21]	Egypt	Lab-scale experiments

below, summarising the selected articles classified per technology in terms of the respective authors, locations of investigation, and studied crops reported in the articles.

Several limitations related to this review should be considered. Firstly, the differences in study design and outcomes make it challenging to compare and synthesize findings across studies. Key characteristics such as flow rate, water use efficiency, and system performance are highly dependent on environmental factors. Metrics such as water use efficiency vary significantly based on geographical location, climate conditions, soil properties, and the specific crops being irrigated. Additionally, the extent of research on these technologies varies. While technologies like clay pots and SICE have been extensively studied and a substantial number of articles could be identified during the systemic review, there is a lack of comprehensive data on others, like SLECI or CP-SDIL. This disparity in the availability of data limits the ability to provide a thorough and accurate comparison across all technologies. An additional limitation is posed by the time frame of this review. As studies are continually being published, new information with high relevance for this review might have been overlooked in the interpretation of the results. Further on, this review shows that research on traditional clay pipe irrigation has not been updated much since 2018, while newer methods like Subsurface Irrigation with Ceramic Emitters (SICE) and Self-Regulating Low-Energy Clay-Based Irrigation (SLECI) have become more popular recently. This means that important information on older methods might be missing, and the review might focus too much on new technologies. As a result, the conclusions could be biased, giving more importance to new methods and not fully considering older ones.

3.2. Results of Individual Studies on Clay-Based Irrigation Technologies—Types, Main Characteristics, and Operation Principles

3.2.1. Clay Pot Irrigation

(a)

History and process mechanism

Clay pot irrigation is an ancient irrigation method. As noted by Woldu [34], the first records of clay pot irrigation appear in agricultural texts dating back more than 2000 years, as described by Sheng Han [70]. The clay pot irrigation method attracted research interest during the 1970s, with the investigations of Anon [71] in Iran, Mondal [29] in India, and Olguín [72] in Mexico. These pioneering studies laid the foundation for subsequent scientific research in this domain. Notable among these are the contributions of Stein [73] in the domain of agriculture and the extensive work by Bainbridge [23,24,25] in the domains of ecology and forestry, which stand out as particularly significant in this field of study. Consequently, the use of buried clay pot irrigation has been studied in arid and semi-arid areas in countries such as Pakistan [33], Mexico [47], Kenya [32], Ethiopia [22], and Sudan [26]. In the quest for locally available, simple, and low-cost irrigation solutions for small-holder and subsistence farmers in dry areas, the clay pot irrigation method has drawn the attention of development cooperation projects in Eastern Africa [28,34].

Clay pot irrigation is characterized by its self-regulating nature, where water naturally permeates through the unglazed, porous clay pot walls [23], with the rate of flow being directly influenced by the water needs of the plants [23]. The principle of the traditional irrigation method involving the use of unglazed clay pots is schematically presented in Figure 4 (sourced from Woldu [34]). Tesfaye et al. [28] studied clay pot irrigation under semi-arid conditions and reported that this method provided up to 69% water savings in comparison to furrow irrigation. An even higher efficiency of 70–90% water savings compared to conventional surface irrigation has been reported from test fields in Ethiopia [34].

(b)

Case studies

By precisely delivering water to individual plants, this method optimises water usage, making it suitable for areas with constrained water resources or cases where focusing irrigation efforts on specific crops or sections of a field is necessary. The research findings on clay pot irrigation recommend this method for the following cases:

• Arid regions characterised by annual rainfall of less than 500 mm [43];
• Areas where access to sufficient water is a challenge due to scarcity or financial constraints [45];
• Areas characterized by uneven or sloping terrain, wherein levelling the soil for traditional irrigation methods is challenging, ensuring water distribution remains consistent and efficient [25];
• Regions with light or sandy soils that face moisture retention challenges, as clay pot irrigation conserves water by delivering it directly to a plant’s roots [45];
• Situations where the available water supply is limited and cannot cover a wide area [44];
• Regions where the irrigation water is highly saline and unsuitable for crop growth [30,41,45];
• For the initial establishment of horticulture [25]

(c)

Advantages

Clay pot irrigation has demonstrated significant potential in addressing the challenges associated with saline soil conditions and the use of saline irrigation water. Research by Mondal [30] has shown this method to be particularly effective in saline environments, contributing to land restoration efforts in arid areas by maintaining soil moisture and enhancing vegetation stability. Complementing these findings, Adhikary and Pal [45] underscore the effectiveness of clay pot irrigation in saline soils, suggesting that future research focuses on optimising the material properties and configurations of clay pots to further enhance their utility in saline conditions. Moreover, an experimental investigation on pitcher irrigation in alkaline soil with saline irrigation water recommended implementing practical adaptations to allow clay pots to optimise water delivery in saline and alkaline soils [50].

Particularly in resource-constrained settings, several application studies on different crops have demonstrated that clay pots have a positive effect on plant growth and yield while also significantly reducing water usage and increasing water efficiency compared to conventional irrigation systems. The applicability of clay pot irrigation across different crops has been studied in regard to tomato and lettuce [45], Swiss chard [22], cabbage [37,50], ornamental plants [25], peppers [39], watermelon, and cauliflower [54].

A further study on clay pot and clay tube irrigation suggests that these systems significantly influence root development, with roots typically forming a dense mat along the walls of the pitcher when irrigated with fresh water. However, the density of this root mat decreases with increasing water salinity and disappears entirely when salinity levels exceed 15 dS/m, as roots then tend to seek moisture from less-saline surrounding soil. This interaction suggests a potential area for further research, particularly in optimising pot porosity and the clay-to-sand ratio, wall thickness, and firing temperature to maximize irrigation efficiency for various crops. A comparative study of clay pot efficiency showed that clay pot irrigation led to significant increases in biomass water use efficiency and economic water use efficiency when compared to bucket irrigation.

(d)

Disadvantages

Clay pot and pitcher irrigation, while beneficial for water conservation and targeted irrigation in challenging terrains, suffers from several disadvantages that limit its adoption on a large scale [25,56]. The major disadvantages, as outlined by Bainbridge [25], include the cost, size, installation time, flexibility, and frequent breakage of the pots. The pots or pitchers require careful installation and periodic replacement due to breakage or clogging, which increases labour and material costs compared to more conventional methods [45]. These factors limit the application of this system to smaller-scale and more labour-intensive agricultural settings. Additionally, this system’s efficiency heavily depends on the pot’s material properties, which, if not properly calibrated to the specific soil type, can lead to uneven water distribution [25]. However, the manual manufacturing process used to create the pitchers and pots in many developing countries often results in significant deviations in quality and size, making standardisation challenging [35,41]. Siyal et al. [49] suggest that there is a critical need for improvement in the structures and material properties of these irrigation devices to enhance their functionality and reliability. Clay pot and pitcher irrigation faces significant scalability challenges due to its labour-intensive nature [32]. In order to reduce labour efforts for water extraction and the re-filling of the pots, Daka [27] proposes a combination with treadle pumps or a gravity feed system such as a higher-levelled water tank. Still, implementation on a larger scale requires additional infrastructure, such as reservoirs and pipelines, to be practical [46]. Furthermore, research indicates that in certain soil types, particularly those that are very dense, water distribution can be restricted to a very small area around the pot, which may not effectively meet the crop’s water requirements [48].

3.2.2. Subsurface Irrigation with Ceramic Emitters

(a)

Process mechanism

Subsurface Irrigation with Ceramic Emitters (SICE) is a water-saving irrigation system developed by Cai et al. [55] as an advancement of Subsurface Drip Irrigation to address irrigation challenges in Northern China. In this method, ceramic emitters with micron-sized pores (less than 100 μm) crafted from natural materials like clay and quartz sand are used. A typical SICE system comprises six components: a water-harvesting surface, a water tank, a submain pipe, lateral pipes, ceramic emitters, and blow-off valves [59]. A graphical example of this system and its components is presented in Figure 5A. Microporous ceramic emitters are manufactured via the moulded sintering method using a mixture of clay, quartz sand, talcum powder, silica sol, dextrin, and graphite [62]. Depending on the proportion of clay and quartz sand in the mixture, the microporous ceramic emitters can be clay-based or sand-based. The ceramic emitters used in this system are hollow cylinders with specific dimensions and hydraulic conductivity. The SICE system operates with a continuous water supply, maintaining a working pressure head of 20–50 cm. Figure 5B shows a porous ceramic emitter and its operation principle. In practical applications, this system was implemented and reported to enhance crop yields for various crops, such as tomatoes, lettuce, apples, and wolfberries [58,61].

(b)

Case studies

Studies related to the Subsurface Irrigation with Ceramic Emitters irrigation system have covered a wide range of areas to optimize its performance and efficiency. Researchers have examined ceramic emitters’ discharge and flow characteristics, often using HYDRUS-2D simulations to establish empirical flow equations [56]. Additionally, there has been a focus on understanding ceramic emitters’ hydraulic characteristics and parameter optimization, aspects with implications for standardising their fabrication. Investigations into the optimal buried depth of SICE have shown significant improvements in plant growth, yield, and water use efficiency compared to other irrigation methods [58]. Clogging formation and root intrusion have been studied to determine the impact of the working pressure head and emitter types [63]. Researchers have recommended specific working pressure head ranges and ceramic hydraulic conductivity values to meet the water requirements of apple trees and reduce the risk of deep percolation. Furthermore, studies have explored how changes in annual precipitation affect SICE irrigation scheduling and examined the effects of different irrigation levels on fruit quality, particularly in loess areas.

Application studies involving apple trees and greenhouse tomatoes have demonstrated the suitability of the SICE irrigation system for these crops. In the case of apple trees, research findings indicate that burying the SICE system at a depth of 40 cm resulted in significant improvements, leading to a 15.9% increase in new shoot length, a 7.6% boost in yield, and enhancements in both water use efficiency and irrigation water use efficiency by 14.8% and 6.5%, respectively, compared to subsurface drip irrigation [58]. To effectively meet the water needs of trees with root systems extending to depths of approximately 0–100 cm in the Loess Plateau of China while mitigating the risk of excessive water percolation, it is advisable to maintain a working pressure head ranging from 20 to 50 cm of water, and the ceramic hydraulic conductivity should fall within the range of 0.1 to 1.9 cm per hour [56].

(c)

Advantages

Several advantages of SICE over plastic emitters have been reported, including lower operating costs due to this system’s lower operating pressures [56], enhanced clogging resistance [17], effectiveness in conserving irrigation water, increases in crop yields across various crops [62], and environmental friendliness due to the use of natural materials for replacing plastic emitters. The reported disadvantages are associated with high capital costs, the clogging of emitters by roots, and increased maintenance requirements [65].

Out of the 19 publications related to the SICE system, only 4 were published before 2018. The early research focuses on predicting the flow characteristics of the ceramic emitters and investigates soil water movement using software simulations. Consequently, researchers have been dealing with the clogging of ceramic emitters and proposed anti-clogging methods. The latest publications from 2022 and 2023 are application studies involving apple trees and tomatoes and aim to develop optimal parameters to improve yield, fruit quality, and water productivity as well as optimal irrigation schedules for the climate in Loess Plateau of China.

3.2.3. Self-Regulating, Low Energy, Clay-Based Irrigation

(a)

Process mechanism

Self-regulating, Low-Energy, Clay-Based Irrigation (SLECI) is a self-regulating subsurface irrigation technique that uses the actual suction force of the surrounding soil for the regulation of the system’s water release [18]. The irrigation water is delivered to the crop roots by means of hose-connected clay elements. The irrigation principle is based on the capillary effect caused by the porosity of the clay material. The surface tension of the water and the adhesion at the interface between the liquid and solid medium promotes liquid wetting of the capillary vessels [11]. Despite the variability in the size and shape of these pores, they form an interconnected structure that enables the clay to maintain a water equilibrium consistent with a fully humid environment [11]. The unique attribute of the SLECI system is its ability to adjust water output in response to soil moisture levels: as the soil becomes drier, the clay emitters increase water discharge, permitting plants to uptake precisely the amount of water they require [11]. This dynamic facilitates significant water conservation, making SLECI particularly valuable in water-scarce regions. The clay emitters are connected to each other by 6 mm pipes and buried in the soil near the roots of the plants. This system, consisting of clay elements and pipes, is then connected to a water source via connectors, and water moves via gravity, eliminating the need for electrical power. The irrigation system consists of commercially available and inexpensive components and can be adapted to many field and arable conditions [19]. The hoses as well as most of the connectors are composed of polyethylene. The absence of composite materials in its construction simplifies recycling processes, enhancing the environmental sustainability of the system [19]. The clay used in the emitters is derived from natural solids, further affirming the ecological integrity of the SLECI system. The figures below show the operating principle, installation/set-up of the tube with clay elements (Figure 6A), and a practical example of installation for the irrigation of cherry trees (Figure 6B). As illustrated in Figure 6A, the system consists of a water tank (A), a water line (B), clay tubes (C), connectors (D/E), a venting end cap (F), a valve (H), and an UV-C filter (optional) (I).

(b)

Advantages and disadvantages

Despite the limited number of publications on this topic, the SLECI system presents a mix of potential benefits and limitations [19]. On the positive side, the SLECI technology allows significant water conservation, particularly in arid regions, where water scarcity can critically impact agricultural productivity and ecological balance. This technology leverages a natural suction effect created by the capillary pore structure in clay, which can substantially increase crop yields by improving water distribution directly to the plant roots while, at the same time, reducing the energy consumption for irrigation. Furthermore, SLECI has a modular design that allows for flexible adaptation to varying field conditions, enhancing its practicality for diverse agricultural applications. Moreover, this system’s components are recyclable, promoting environmental sustainability through the reuse of materials. The authors also present several challenges associated with the adoption of SLECI technology. This system requires high initial capital due to the costs associated with the manufacturing of the clay bodies and the installation of the system, processes involving underground work such as digging trenches or drilling holes. In particular, the manufacturing process is energy-intensive, largely due to the need to fire the clay, which is similar to the type used in pottery. Additionally, the sustainability of this technology can be compromised by the eventual necessity of excavating and removing the underfloor components at the end of their lifecycles. Another environmental consideration is associated with the use of sealing materials.

3.2.4. Porous Clay Pipe Irrigation

(a)

Process mechanism

In clay pipe irrigation, porous ceramic pipes are used to deliver water directly to the plant roots, enhancing soil moisture management while reducing water wastage through runoff or over-irrigation [53]. In this method, the capillary action of unglazed clay is used to effectively moisturize the soil. Figure 7 shows a graphical presentation and an on-farm installation of this irrigation system. The study by Bhople et al. [52] reports on the fabrication of three types of sub-surface clay pipes, each designed with unique compositions to optimize irrigation efficiency. Type A pipes are crafted entirely from pure clay, Type B mixes 95% clay with 5% fine sand, and Type C combines 90% clay with 5% fine sand and 5% sawdust. These pipes are uniformly manufactured at 50 cm in length, with a 2.5 cm thickness, and feature a 7.5 cm inner diameter and a 10 cm outer diameter. Each pipe includes a coupling head at one end, measuring 10 cm internally and 12.5 cm externally, that facilitates seamless connections between pipe sections [52]. These tubes are buried in the soil, with water continuously supplied from a single source. An elbow fitting is attached at one end with an upright pipe section through which water is added. Water then permeates the root zones either through the joints or directly through the unglazed pipe walls, providing continuous wetting front along the pipe’s length. The cited study also evaluates the hydraulic properties of the three types of pipes, such as their seepage rates and hydraulic conductivity. The authors report water savings between 70 and 80% with 5 to 16% higher yields of okra, eggplant, and turnip compared to surface irrigation methods.

(b)

Advantages

Clay pipe irrigation systems are considered highly efficient in terms of water usage, being capable of reducing water consumption by up to 80% compared to traditional surface irrigation methods [48]. This significant reduction is due to the targeted delivery of water directly to the plant roots, minimising evaporation and runoff. At the same time, the continuous availability of soil moisture at near field capacity ensures that the crops remain adequately hydrated; thus, water stress is reduced [51]. By eliminating surface water evaporation, this system is reported to be able to reduce the occurrence of weeds and disease [26]. This irrigation method also has several environmental benefits. The use of natural materials like clay makes this system environmentally friendly. Moreover, the local manufacturing of clay pipes can reduce the carbon footprint associated with transportation and supports local economies [52].

(c)

Disadvantages

Clay pipe irrigation also has several limitations, as reported in a study by Bhatt [51]. The initial setup of this system is labour-intensive, requiring significant manpower to dig trenches and install the pipes. If the water used contains silt, this silt can accumulate in the pores of the clay pipes, effectively sealing them and preventing water from seeping out. Similarly, if the pipes are left dry for extended periods, salt can accumulate and clog the pores, reducing their effectiveness. The production of clay pipes, while more productive and cost-effective than manufacturing clay pots, still involves a considerable amount of material. Transporting these pipes can be costly due to their weight and volume. Furthermore, sealing the pipe sections in the field requires a specific kind of cement, and any movement of the earth during operations conducted over several seasons can lead to leaks, resulting in significant water losses.

3.2.5. Ceramic-Patch-Type Subsurface Drip Irrigation Line (CP-SDIL)

In 2019, Cai et al. [57] introduced the Ceramic-Patch-Type Subsurface Drip Irrigation Line (CP-SDIL), which represents an approach to subsurface irrigation that combines the advancements in porous ceramic technology and 3D printing to optimize the delivery of water directly to plant roots. This system features a simple but effective design that includes a plastic pipe integrated with a plastic patch and a porous ceramic water-seepage pad, sealed by a rubber leakage-proof gasket. Figure 8 shows photographs and a cross-sectional view of the CP-SDIL. The CP-SDIL operates on the principle of hydraulic performance, which is significantly influenced by the porosity of the ceramic pad and the working pressure applied. This system is designed to be self-regulating, with water discharge from the ceramic patch increasing in response to lower soil moisture levels, thereby enhancing water conservation and ensuring that plants receive water precisely when needed. This irrigation line utilises two types of ceramic pads, burn-free and sand-based porous ceramics, each selected based on their hydraulic properties in order to meet specific soil and crop requirements. Through the porosity of the water-seepage pad, a consistent water flow is maintained. It also prevents clogging from root intrusion, a common issue in subsurface irrigation systems. Based on a theoretical analysis, experiments assessed this system’s efficiency under varying pressures and soil types, using data analysis and mathematical modelling to predict its irrigation capabilities and promote its application.

3.2.6. Pottery Dripper

The pottery dripper is an irrigation device introduced by El-Hagarey [21]. The pottery dripper is designed to utilise saline water effectively, addressing the challenge of saltwater irrigation in Egypt. Developed from local and eco-friendly materials, primarily Aswan clay, these drippers incorporate organic matter like sawdust to vary porosity, which significantly influences their function and effectiveness. These drippers come in three different porosities (10%, 21%, and 31%) and sizes, adapting to various operational pressures and saline water concentrations. A graphical presentation of the pottery dripper is shown in Figure 9 below. The main advantage of using pottery drippers is their ability to reduce saline concentrations in water, which is beneficial for irrigation in salt-affected soils. Additionally, they offer flexibility in installation and operation, catering to specific agricultural needs.

3.3. Results of Syntheses

3.3.1. Hydraulic Characteristics and Soil Water Dynamics

The main factors that have been reported to affect water seepage from pitcher walls encompass the saturated hydraulic conductivity of the pitcher material, wall thickness, surface area, soil type, crop type, and the rate of evapotranspiration [36,49]. The studied literature reveals that the success and sustainability of pitcher irrigation systems depend on these variables, which influence water conductance through the pitcher wall. Abu-Zreig [36] observed that seepage rates increased approximately threefold under constant head conditions compared to those under variable head conditions. Furthermore, when pitchers were buried within the soil rather than exposed to the atmosphere, seepage rates doubled, which underscores the significant influence of the environmental context on irrigation efficacy [36]. Moreover, the increase in seepage rates correlated positively with higher evaporation rates and inversely with soil moisture around the pitcher’s wall [74]. This indicates a dynamic interaction between soil moisture levels, evaporation rates, and the water release characteristics of the pitchers. The definition of the soil wetting front—specifically at matric potentials of −200 cm for fine sand and −763 cm for sandy loam—further illustrates how soil texture influences moisture distribution patterns [74].

Experiments that focused on the impact of wall thickness, porosity, and the saturated hydraulic conductivity of the pitcher on the extent of the soil wetting zone have shown that the porosity of the clay pot wall significantly affects hydraulic conductivity [49]. Notably, surface roughening has a significant impact on clay pot seepage rates. The recorded saturated hydraulic conductivities ranged between 0.5 and 2.3 mm/d, leading to seepage volumes ranging from 0.9 to over 3 L per day [49]. This variability suggests that hydraulic properties can be manipulated to optimize irrigation under different environmental conditions.

Furthermore, temperature has also been observed to qualitatively increase both saturated hydraulic conductivity and conductance. Abu-Zreig [36] studied the self-regulating capability of clay pitcher systems in various applications. He found out that this adaptability is particularly advantageous in arid climates with high potential evaporation rates, as evidenced by the increase in seepage from pitchers with high porosity and hydraulic conductivity from 190 mL/d to as high as 1040 mL/d when the evaporation rate rose from 1 to 16 mm/d. The corresponding increase for pitchers, characterised by low porosity and hydraulic conductivity, was from 60 to 1000 mL/d. The influence of the pitcher’s hydraulic properties on the seepage rate seemed to be strong at low evaporation rate values but weak at high values.

These findings suggest that while clay pitcher irrigation systems have considerable potential for efficiency in arid conditions, their optimal application depends critically on tailoring the system’s parameters, such as pitcher porosity, wall thickness, and soil interaction dynamics, to specific environmental and crop needs.

The significant body of research conducted by Cai et al. [56,57,58,59,60] has contributed extensively to the understanding of the Subsurface Irrigation Ceramic Emitter (SICE) system. Their studies have revealed that the working pressure head and the hydraulic conductivity of these ceramic emitters considerably influence both the discharge rates and the propagation of the wetting front within loamy soils. Notably, increased working pressure heads and hydraulic conductivity reduce the risk of deep percolation, thereby contributing to water conservation. In their 2018 study, Cai et al. [56] explored the optimal conditions for the use of SICE in the Loess Plateau in China, focusing on minimising deep percolation while meeting the extensive water demands of tree root systems. The findings recommended maintaining a working pressure head between 20 and 50 cm of water and a ceramic hydraulic conductivity ranging from 0.1 to 1.9 cm/h. This setup can enhance the efficiency of water distribution to the active root zones, thereby optimising water usage and reducing wastage through deep percolation. In a targeted study examining the efficiency of the SICE system for greenhouse tomatoes, optimal operational parameters were identified to enhance irrigation effectiveness. The study by Liu et al. [62] revealed that a working water head of 0.4 m is most conducive for SICE, effectively maintaining the soil water content within the root zone at 70% to 80% of field capacity. This optimal working head significantly mitigated water losses, reducing actual evaporation by 12.32% and deep percolation by 21.88% compared to a subsurface drip irrigation system. Laboratory experiments conducted by Cai et al. [59] provided an overview of how different working pressure heads and emitter types affect the system’s performance in both air and soil environments. The results indicated that increasing the pressure head enhances discharge rates, improves soil water content uniformity, and reduces discharge deviation, particularly when the pressure head is at or above 20 cm. This threshold appears critical for achieving over 80% uniformity in soil water content, which is essential for consistent crop growth and effective irrigation.

In further investigations of the dynamics of SICE, Cai et al. [60] examined how variations in annual precipitation patterns could affect irrigation scheduling. This study highlighted the complex relationship between event- and annual-scale precipitation and ceramic hydraulic conductivity and its effect on soil water dynamics, underscoring the need for adaptable irrigation strategies that respond to climatic variability.

As described by Malchev et.al. [18], the hydraulic conductivity of the SLECI clay bodies depends on their pore structure, which is determined by the composition of the clay, the grain size distribution of the clay powder, and the firing conditions [18]. The capillarity of the clay bodies leads to the development of suction tension that depends on the pore size distribution. In an evaluation of the SLECI system, the hydraulic conductivity factor of the SLECI tubes was determined to be approximately 45, calculated by considering both the surface area and wall thickness factors of the ceramic materials used. Specifically, the surface area ratio between SLECI and traditional clay pots was 90, and the wall thickness ratio was 0.5 [18]. These calculations, however, do not initially account for the substantial differences in operational hydraulic head, which is about 200 cm for SLECI and just 20 cm for clay pots. Adjusting for these conditions reveals that the hydraulic conductivity of SLECI tube material is actually five times greater than that of the clay pot material [11]. Experiments have shown that a clay tube with a length of 100 mm and a diameter of 20 mm can extend the wetting front by up to 50 cm in a sand/clay loam mix within a single day under a pressure of 0.2 bar. In environments composed of finer sand, the velocity of the wetting front is even greater, demonstrating SLECI’s ability to efficiently distribute water through subsurface layers [18]. Further detailed studies are necessary to fully understand and optimize the application of SLECI in various agricultural irrigation systems.

Field measurements taken using a CPN 503DR HYDROPROBE indicate that traditional drip irrigation may lead to overwatering of the topsoil layer, causing substantial water loss through evaporation in warm weather; only about 10 L per tree per day reaches a soil depth of 60 cm. At 90 cm, the efficiency of drip irrigation decreases further compared to that of clay-tube micro-irrigation. In contrast, the SLECI system, with its underground clay tubes positioned at depths of 40–50 cm, drastically reduces water usage to just 1.60 L per tree per day, maintaining optimal moisture across the root zone and saving between 8.4 L per tree on hot days and 9.23 L on cooler or rainy days. This efficient water distribution not only enhances tree growth but also minimises weed proliferation [18].

A study conducted by Liu et al. [62] introduced a simplified model for estimating the discharge rates of microporous ceramic emitters used in drip irrigation. This model effectively incorporates variables such as pore structure, ceramic material properties, water temperature, and the structural parameters of the emitter, and it has been validated through experimental methods. The findings indicate that water temperature plays a critical role in the discharge behaviour of these emitters, with a notable increase of 39.10% in discharge when the water temperature rose from 20 °C to 35 °C. This temperature sensitivity underscores the need to adjust operating pressures according to seasonal variations to stabilise discharge rates, which were observed to increase linearly with rising operating pressures under constant temperature conditions. This research also explored the impact of construction materials on emitter performance. Emitters made from clay-based materials exhibited lower discharge rates compared to those made from sand-based materials, a phenomenon attributed to the finer particle size of the clay. The composition of these ceramic emitters included a blend of clay or quartz sand, talcum powder as a sintering aid, and silica sol as an adhesive, with dextrin and graphite serving as pore-forming agents. This material composition was specifically designed to lower the sintering temperature and enhance the structural integrity of the emitters.

3.3.2. Application Studies—Water Use Efficiency and Yield Response

Papers reporting application studies have been identified for the clay pot, clay pipe, SICE, and SLECI (for which there was only one) irrigation systems. The crop studies in these papers are summarised according to the type of technology in

Table 2. Crops in application studies of clay-based irrigation technologies.

Clay Pot and Clay Pipe	SICE	SLECI
Tomatoes [28,32,39,54], maize [26,32], bitter gourd [40], pumpkin [42], watermelons [29,54], okra, cucumber, eggplants, coffee [38], cauliflower [54], Swiss chard [22,39], cabbage [37], pepper [39], bitter gourd [40,54]	Greenhouse tomatoes [61], apples [58], lettuce [55], wolfberry [75], persimmon [55]	Cherry trees [18]

.

Application studies of clay pot and clay pipe irrigation have been performed on several crops in mostly arid regions. The planting and establishment of subtropical crops like coffee, oranges, and other tree species require particular attention to the availability of water for irrigation, especially during the first year of establishment and at up to three years. A continued supply of moisture at the root system mitigates the effects of soil temperature and moisture stress shock during a prolonged drought, thereby enhancing the plant survival rate [38]. For young coffee plants, Elavarasan, Govindappa, and Hareesh [38] reported that the dry weight of weeds in crops irrigated by buried clay pots was only 13 percent compared to weeds in control plots irrigated via basin irrigation. Therefore, the adoption and practice of clay pot irrigation in scarce-rainfall areas are considered highly advantageous and could result in maximal establishment of young coffee seedlings, the maintenance of the optimum plant population, the minimisation of crop–weed competition, and better economic returns in coffee plantations.

Mahata et al. [40] demonstrated the efficiency of pitcher irrigation in regard to bitter gourd planting in West Bengal, India. Mulching, combined with pitcher irrigation, was reported to boost water use efficiency by reducing evaporation, conserving soil moisture, promoting microbial activity, and improving soil structure. Utilising various mulching materials in conjunction with pitcher pots, which were buried and filled with water that seeped into the root zone, this method led to a substantial increase in crop yield. For instance, bitter gourd yields with jute fibre mulch reached up to 15.90 tonnes per hectare, significantly higher than the yields for other mulches and the control without mulch, for which the yield was only 9.26 tonnes per hectare.

For maize, the mean yield was higher for clay pipe irrigation compared to clay pot irrigation. However, among the various treatments, pitcher irrigation with jute mulch was found to be the most efficient method for keeping the soil in a favourable state and improving crop production [26].

Pal et al. [42] noted that low-fired pots might degrade in highly saline soils. Saha et al. [76] found that direct pitcher irrigation significantly outperformed pipe irrigation from pitcher and basin systems in pumpkin cultivation. It led to longer vines, more nodes per vine, and thicker stems, with shorter internodes, indicating robust plant growth under this irrigation method.

Tesfaye et al. [28] reported substantial water savings and yield increases with clay pot irrigation for tomatoes in Ethiopia, achieving up to 69% greater efficiency than furrow irrigation [28]. In this study, clay pot irrigation reduced the seasonal water usage to 143.71 mm, which can be compared to the value of 485.50 mm for furrow irrigation, resulting in a water use efficiency of 33.62 kg/m³ (versus 6.67 kg/m³ for furrow irrigation). Additionally, incorporating nitrogen fertilizer with irrigation water in clay pots increased fertilizer use efficiency by up to 52%, making it an advisable method for maximising tomato yields while conserving resources in arid and semi-arid regions.

All the reported application studies confirmed that clay pot irrigation offers advantages in terms of fruit yield, fruit quality, water use efficiency, water savings, and plant safety.

Recent research on the application of the SICE system reports significant improvements in yield responses and water use efficiency across various crops. Liu et al. [61] documented increases in tomato yield from 1.6% to 8.2% and in water use efficiency from 9.9% to 30.5% due to the consistent water delivery provided by SICE. Similarly, Yao and Zhang [64] reported that tomatoes achieved a maximum yield of 1.17 kg per pot when irrigated with SICE. This irrigation method also had positive effects on other crops; lettuce, apple, and wolfberry saw notable increases in yield and water savings, confirming this system’s broad applicability [58,61,69].

Further research conducted by Cai et al. [58] found that when SICE were buried at a depth of 40 cm, there was a significant improvement in new shoot length, yield, water use efficiency, and irrigation water use efficiency for apples, with increases of 15.9%, 7.6%, 14.8%, and 6.5%, respectively, compared to subsurface drip irrigation. This depth also ensured minimal variations in soil water content, enhancing water conservation and supporting stable growth under variable conditions.

In a comparative study, the efficiency of SLECI was assessed against the traditional drip irrigation for sweet cherries from 2019 to 2022 at the Fruit Growing Institute in Plovdiv, Bulgaria. SLECI, in which clay tubes placed around trees at depths of 40–50 cm are used, created 30 cm diameter wet spots at the root zone, significantly reducing water loss through evaporation and percolation [18]. In comparison, drip irrigation led to overwatering of the topsoil layer. Moreover, the effect of drip irrigation decreased at a depth of 90 cm. Water consumption data from 2019 revealed that water use by SLECI varied from 1.46 L per tree on hot summer days to 0.83 L as temperatures cooled and trees prepared for the dormancy period. These data confirm the adaptability of the system to environmental changes. In contrast, drip irrigation consistently used 10–12 L of water per day regardless of the weather conditions. Additionally, soil moisture measurements taken with a CPN 503DR HYDROPROBE showed that SLECI prevented the overwatering that was observed with the drip system, thereby promoting healthier tree growth and reducing weed growth. An assessment of the yield response in this experiment was not possible due to frost in early April 2020, which damaged most of the flowers.

3.3.3. Comparison Studies

The examined clay-based irrigation technologies represent variations of sub-surface irrigation systems. A recent review by Abou Seeda et. al. [77] reported on the optimisation and evaluation of subsurface irrigation systems. The authors summarised several of the advantages of subsurface irrigation systems over other conventional irrigation methods; these advantages were related to water and soil issues, cropping and cultural practices, and the system infrastructure. This review has validated several of these advantages of clay-based irrigation systems, as indicated by the literature reviewed. First, improved water use efficiency through reduced soil evaporation, surface runoff, and deep percolation could be confirmed for the clay pots [45], SICE [60], and SLECI [18] systems. In addition, several studies confirmed that enhanced plant growth and crop yield and quality were advantages some clay-based irrigation systems had over furrow, drip, and subsurface drip irrigation [18,39,75]. During the 2014/2015 cropping season in Northern Ethiopia, yields of Swiss chard, tomato, and pepper increased by up to 51%, 32%, and 30%, respectively, with the bar-shaped clay pot irrigation system compared to furrow irrigation [39]. Water savings were significantly increased by 40.6%, 41.2%, and 41.7% for Swiss chard, tomato, and pepper, respectively, while their water productivities were 10.9, 4.2, and 1.8 kg/m³ [39]. Similarly, the SICE and SLECI systems are also designed to deliver water directly to the root zone through ceramic emitters, ensuring consistent and uniform water distribution while minimising loss through evaporation and surface runoff. The results reported for SICE showed that over a two-year period, the SICE system outperformed surface drip irrigation and subsurface drip irrigation in terms of wolfberry yields, with average increases of 8.0% and 2.3%, respectively [75]. Additionally, SICE showed higher water use efficiency, with 14.6% and 4.5% increases compared to drip irrigation and subsurface drip irrigation, respectively [75]. In a study comparing SLECI and drip irrigation conducted in Bulgaria from 2019 to 2022, SLECI demonstrated superior efficiency for sweet cherries; here, clay tubes were placed at depths of 40–50 cm to create 30 cm diameter wet spots, significantly reducing water loss. SLECI used between 0.83 and 1.46 L of water per tree daily, while traditional drip irrigation consistently used 10–12 L, leading to overwatering [18]. The potential advantage of greater water application uniformity for subsurface irrigation systems has been studied and found to be highly effective for SICE. A recent study on wolfberry crops demonstrated that SICE improved yield and economic benefits due to their ability to maintain stable soil moisture levels and provide uniform water distribution across the field [75,78]. Among the advantages related to cropping and cultural practices that could be confirmed by reviewing the literature is the better weed control reported for clay pot [24] and SLECI [18] irrigation systems. Finally, the advantages of decreased operational costs for energy and labour are assumed to be system-related advantages for all clay-based irrigation systems due to their auto-regulative properties. However, only recent comparative studies of the economic benefits of SICE and subsurface drip irrigation present data that confirm this assumption [75,78].

While clay-based irrigation systems have numerous advantages over other systems, several disadvantages must be considered as well. The effectiveness of using subsurface systems for germination can be limited by installation depth and soil characteristics [77]. High initial investment costs make these systems less accessible compared to alternative irrigation methods [75]. Filtration and other maintenance issues require timely and consistent attention, with leaks caused by rodents being particularly challenging to locate and repair in deeper subsurface systems [18,63,68,77]. Additionally, driplines must be monitored for root intrusion, which can eliminate or reduce water flow, especially in perennial crops, where roots may pinch the driplines [63,68]. Finally, design errors are more difficult to resolve since most of the system is installed underground, complicating troubleshooting and repairs [77].

4. Discussion and Conclusions

This literature review has identified a substantial focus of recent publications on the operational principles, functionalities, and performance of novel clay-based water-saving irrigation technologies. However, there is a notable gap in economic and financial evaluations of these technologies. To address this deficiency, future research should aim to generate comprehensive financial data and cost projections that cover the installation, operation, and maintenance aspects of these irrigation systems. The generation of such data is necessary for assessing the viability and acceptance of these technologies by the end-users.

As a substantial portion of the published research originates from developing countries, there is a need for economic assessments that consider the broader impact of these technologies on farming communities and the environment, particularly considering their impact on agricultural practices under changing climate conditions. The potential social impact of these technologies is significant. By reducing water usage and improving crop yields, clay-based irrigation systems can enhance food security and livelihoods in farming communities, particularly in developing countries. These technologies can contribute to sustainable agricultural practices, thereby supporting community resilience against climate change. Additionally, their adoption could reduce the labour burden on farmers, allowing for greater social and economic development. These assessments would be valuable evidence that could support policymakers tasked with deciding on the funding of such technologies, wherein decisions often extend beyond mere investment costs and encompass a monetary comparison with alternative technologies.

This study also noted that the integration of fertilizers with clay-based irrigation systems is under-researched, with only two of the collected papers addressing this topic. This gap indicates a need for further research to evaluate whether the controlled release of water, aligned with a plant’s water requirements, can also improve fertigation efficiency while minimising soil and groundwater pollution. There is little evidence of a potential for system failures due to the use of incompatible fertilizers. Additionally, as the adoption of treated wastewater for irrigation emerges as an alternative solution to secure irrigation water availability in arid regions, understanding the compatibility of the emerging clay-based irrigation systems with lower-quality irrigation waters represents a topic that aligns with both current research trends and policy discussions.

Additionally, further investigations are required to explore the conditions and requirements necessary for scaling up these technologies in productive conditions. This includes assessing the scalability of these systems in diverse agricultural settings and determining the economic incentives or support needed to promote widespread adoption. Future research should also focus on improving the standardisation and ease of use of clay-based irrigation systems to facilitate broader adoption. This includes developing user-friendly guidelines and training programmes to ensure that farmers can effectively implement and maintain these systems.