1. Water-Saving Irrigation Systems for Rice Production
The surface irrigation methods are commonly used for rice production. However, the costs and advantages of producing rice under aerobic conditions are an alternative to flooding, which is an efficient technique to decrease weeds and other pests but uses too much water 
. According to several authors 
, majority of the land is irrigated using surface methods in which water is distributed over the field via overland flow. However, rice crop can also be grown under drip or sprinkler irrigation systems to improve water and fertilizer use efficiency, because the flooding practice involves a higher water footprint for rice production than any other crop in the agriculture 
1.1. Surface Irrigation Methods for Rice Production
Despite their low efficiency and uniformity, due to poor design and water management in other irrigation methods, surface irrigation methods are the most used in the world 
. Surface irrigation methods, including basin irrigation, border irrigation, and furrow irrigation are characterized by inefficient irrigation, leading to wastage of water, water logging, salinization and pollution of surface and ground water resources 
. In order to make rice cultivation on gravity-fed irrigated schemes sustainable, several studies focused not only on the water-saving practices, but also on the furrow irrigation.
Hardke and Chlapecka 
showed that furrow irrigated rice production has been increasing in recent years, from less than 1% in 2015 to 10% in 2019 in US due to easy crop rotations, and decrease in time and costs compared to flood irrigated rice production. According to Stevens et al. 
, furrow irrigation method is better than conventional flood irrigation for growing rice with less water and labor and it contributes to reduced arsenic content in irrigated rice grain. Other studies revealed that the yield component and rice yield were low for furrow irrigated rice whereas arsenic concentration is high in rice for flood irrigation 
Several authors 
showed that furrow irrigation system compared to conventional irrigation has several advantages:
- It can significantly reduce irrigation water losses rate through seepage, evaporation, and evapotranspiration;
- It can reduce harmful materials, such as ferrous ion;
- It can reduce the rice field humidity and enhance gas transport in the soil and light penetration;
- It can reduce rice diseases and consequently increase leaf vitality; and
- It can increase grain yield and water productivity.
On the contrary, Beesinger et al. 
and Deliberto and Harrell 
indicate that furrow irrigation system, compared to flooding irrigation, has huge disadvantages, such as the following:
- row rice is not easily made;
- water management uniformity is more difficult;
- fertilizer management uniformity is more difficult;
- weed control is more difficult;
- disease potential is greater; and
- harvest problems are increased in deep furrows if the rice lodges.
Surface irrigation system is subject to several criticisms due to its low efficiency and high water wastage and thus alternative new practices are encouraged to enhance water efficiency and gain both economic and environmental benefits 
. Due to the high water levels in paddy fields, flood irrigation requires a lot of labor, and the farmer often uses a manual device to control the input and output flow 
. However, in a study, Lee 
demonstrated that flood irrigation can be sustainable if it is automated. He contends that the automatic irrigation system can be adequate for water supply automation and sustainable water management and can benefit farmers in saving water and reducing labor demands. An automated system will allow the farmer to control flows, volumes, and water levels in the fields using a special website which controls the gate directly, if necessary, or by configuring the irrigation program in accordance with his agronomic methods 
1.2. Drip Irrigation System for Rice Production
Drip irrigation is almost non-existent in rice production systems. Very few previous studies focused on drip irrigation of rice crops, yet it is possible (Table 1
). For Meher et al. 
, using drip irrigation technology in rice farming is the best way to increase productivity while using the least amount of water possible to produce the highest yields, decrease the cost of irrigation water in rice farming compared to the conventional methods, and reduce the overall water consumption of rice crops to levels that are biologically sound.
According to Parthasarathi et al. 
, drip irrigation improved the aerobic rice yield by 29%, increased water saving efficiency by 50%, and consequently increased water productivity, favored the root oxidizing power, canopy photosynthesis and dry matter partitioning. Studies on reducing pressure on underground water under projected climate due to continuously depleting aquifers came to the conclusion that drip irrigation systems offered real benefits for significant savings in irrigation water and energy as well as an increase in nitrogen use efficiency and net income 
. Rao et al. 
too showed that the drip irrigation system for rice production was more efficient than conventional paddy cultivation under continuous flooding in terms of enhancing water productivity and saving water energy.
In addition, drip irrigation for rice production has enormous agronomic (e.g., reduction in fertilizer use, reduction in leaching of fertilizers, reduction in diseases and pests, weeds control and mulching), economic (e.g., power saving, reduction in manpower and labor) and environmental (e.g., water use reduction, greenhouse gas emission reduction, arsenic uptake and improved rice quality) advantages 
. He et al. 
demonstrated that drip irrigation has greater water saving capacity, lower yield and more economic benefit gaps compared to furrow irrigation and flood irrigation.
Research carried out since 2008 on drip irrigation for rice cultivation (both surface drip and subsurface drip systems and fertigation) showed that rice yields and yield component as well as fertilizers use efficiencies were higher compared to the conventional methods 
. According to Samoy-Pascual et al. 
, drip irrigation is the greatest alternative for minimizing irrigation water usage and boosting water productivity while growing aerobic rice without using as much water as surface flooding, and still having a comparable yield and financial return. On the contrary, though drip irrigation system improved water productivity, it decreased paddy yields 
However, the subsurface drip irrigation performed better than the surface drip irrigation system in terms of rice growth, physiology, and yield 
. Among the three irrigation systems (drip irrigation, flood irrigation and sprinkler irrigation), Bansal et al. 
argued that the drip irrigation method significantly increased rice grain yield and water use efficiency. In spite of their benefits, innovative irrigation technologies, such as drip and subsurface drip irrigation, are expensive and need greater technical expertise; as a result, they are rarely used and are typically seen as last-minute fixes 
1.3. Sprinkler Irrigation System for Rice Production
Research conducted in the USA on rice production under center pivot irrigation showed that sprinkler irrigation can be an alternative to flooding and would improve water use efficiency and soil water tension 
. According to Parfitt et al., rice grain yield, fertilizer use efficacy (FUE) and water use efficacy (WUE) are high for sprinkler-irrigated rice than for rice grown in flood-irrigated lowland. In addition, sprinkler irrigation system, compared to flood irrigation, significantly reduced arsenic in the harvested rice grain 
. Similar to this, Alvarenga et al. 
and Spanu et al. 
reported that rice production that has successfully switched to sprinkler irrigation from the traditional flooding system can save water and reduce the buildup of arsenic and cadmium in the rice grain, thereby allowing to produce safe rice in soils where traditional irrigation might only result in the production of inedible rice.
However, Moreno-Jiménez et al. 
demonstrated that even though, as compared to flood irrigation, sprinkler irrigation saved water, increased organic C in soils, and decreased both inorganic and organic arsenic content, it significantly increased cadmium content in rice grain which is a cause of worry. Costa Crusciol et al. 
showed that sprinkler irrigation system improved the physiological quality of rice seeds produced under upland conditions by reducing water deficiency during the seed development stages. Moreover, sprinkler irrigation facilitates greater weed control as compared to the flood irrigation system 
. In the same way, even when employing cultivars created for flooded conditions, proper management of a sprinkler-irrigated system can retain high levels of output, minimize irrigation water use, and raise soil water tension, which is shown by a drop in plant heights 
. In contrast, when compared to flood irrigation, rice growth was poor under sprinkler irrigation, likely as a result of decreased root activity close to the soil surface due to frequent intervals of soil drying 
. Studies conducted on rice production under sprinkler irrigation are shown in Table 1
Table 1. Previous studies on rice production under sprinkler irrigation system and its influence on water saving.
2. Water-Saving Irrigation Practices for Rice Production
Although experiments on rice production under micro-irrigation systems (drip irrigation and sprinkler systems) are promising, traditional surface irrigation with continuous flooding practices and significant water consumption remains widely dominant in rice cultivation for a number of reasons 
. First, rice cultivation under surface irrigation is an ancestral practice and abandonment of this practice is constrained by socio-psychological barriers 
. Second, surface irrigation systems are more accessible to rice farmers, especially small-scale farmers, given the requirements of the micro-irrigation systems (high costs, unavailable skills and advanced knowledge). Finally, paddy grain yields are low under micro-irrigation systems compared to surface irrigation 
. In view of this situation, the alternative is to develop and adopt irrigation practices that improve water use efficiency without affecting the yield.
In fact, water-saving practices have a positive impact on (i) the environment by conserving water resources and reducing greenhouse gas and (ii) the economy by increasing fertilizers’ use efficiency, agricultural productivity, reducing energy, etc. 
. Previous studies have shown that compared to continuous flooding, all water-saving practices allow for sustainable rice production 
. According to research 
, there are multiple water-saving technologies, including alternate wetting and drying (AWD), soil water potential (SWP), non-flooded mulching cultivation, aerobic rice system (ARS), efficient irrigation regime (EIR), saturated soil culture (SSC), field water level (FWL), intermittent drainage (ID), leaching and flushing methods (LFM), conventional flooding-midseason drainage-flooding irrigation (FDF), etc. The most popular water-saving technologies developed for rice production systems is the alternate wetting and drying (AWD) 
. Islam et al. 
reported that, according to several authors, AWD entails intermittent irrigation events with intervals of non-flooding, wherein the water level drops below the soil surface between each irrigation, and this saves irrigation water by a range of 7–33% without significant impact on yield compared to conventional flooding. Thus, AWD is a water-saving technology promoted in rice cultivation worldwide due to its effectiveness in improving water use efficiency (Table 2
Table 2. Previous studies on rice production under alternate wetting and drying (AWD).
Despite AWD being the most promoted amongst all the water-saving technologies (WST) due to its economic viability and environmental friendliness, it is not extensively used, which is probably because of the complex interactions between agricultural and socioeconomic systems and the absence of institutional backing 
. Hiya et al. 
and Massey et al. 
reported that intermittent flooding with a reasonable depth of water above ground level would be an alternative to improve water use efficiency in rice production. Afifah et al. 
indicated that flooding a field to a depth of 1 cm saved 45% of the water used, with significant improvements in WUE, in comparison to flooding at a depth of 5 cm. On the other hand, flooding at a depth of 5 cm and 1 to 3 cm induced similar rice yield, which was higher than rice yield obtained under AWD. In the Philippines, Islam et al. 
found that seasonal rice water use was 15% lower when utilizing soil water potential (SWP) compared to the water-saving AWD. The studies carried out (2007–2010) by de Avila et al. 
in Rio Grande do Sul, Brazil, revealed that intermittent irrigation reduced runoff water by 56% and irrigation water use by 22–76%, leading to an increase in water use efficiency of 15–346%. (WUE).
In the same way, other avenues have been explored in previous studies to find alternatives to AWD. Albaji et al. 
demonstrated that limited irrigation results in a WUE between 13.3 and 13.9 kg/mm, while flooding irrigation provides an average of 12.48 kg/mm. In India, the experiment conducted in 2018 and 2019 revealed that the yield of saturation was comparable to flood irrigation under non-limited water supply while conserving 27% irrigation water 
. Additionally, through two-year field experiments at Jiangsu, China, Zhang et al. 
found that shallow water irrigation used the largest amount of water compared to wet-shallow irrigation, but provided a higher yield.
3. Water-Saving Agronomic Practices for Rice Production
Various previous studies focused on good agronomic practices (GAP), such as using drought-tolerant rice variety 
, plastic mulching 
, straw mulching 
, organic matter application 
, minimum tillage 
, etc., to assess their influence on water saving in irrigated rice production. In addition, water-saving practices are implemented in combination with these good agronomic practices (GAP). Farooq et al. 
indicated that improved genotype water productivity, different planting times, seeding rates, geometries, improved management of soil fertility, use of mulching to avoid soil evaporation, and weed control will all result in crop plants using water more efficiently.
Moreover, crop canopy is crucial for light interception and light penetration into the soil as well as for plant water consumption (i.e., a dense canopy will shade the soil surface, reduce soil temperature, and therefore limit soil evaporation and reduce crop evapotranspiration) 
. Results of straw returning utilized to improve soil fertility and crop production showed rice yield enhancement on average by 7.9% and 7.5% and irrigation water use efficiency (IWUE) improvement by 6.3% and 8.3% in 2015 and 2016, respectively 
. The system of rice intensification (SRI) approach helped to lower the need for irrigation water, resulting in immediate advantages of decreased irrigation water demand