Water Management for Sustainable Irrigation in Rice ( Oryza sativa L.) Production: A Review

: In the face of the negative impacts of climate change and the accelerated growth of the global population, precision irrigation is important to conserve water resources, improve rice productivity and promote overall efﬁcient rice cultivation, as rice is a rather water-intensive crop than other crops. For several decades, various water conserving technologies have been studied in order to signiﬁcantly increase water use efﬁciency (WUE). The objective of this paper is to review the main technologies and approaches for assessing the water requirements of rice crop in order to contribute to water saving in irrigated rice production, after clarifying the performance indicators of the irrigated systems. Several scientiﬁc articles from previous studies were consulted and analyzed. These studies showed that irrigation water conservation includes a wide range of practices, staring from the crop irrigation water requirements assessment to the implementation of the water saving practices on the ﬁeld. In addition, irrigation water conservation technologies could be categorized into three groups, namely water-conserving irrigation systems, water-saving irrigation methods, and water-conserving agronomic practices. The inﬂuence of the individual and combined irrigation water use efﬁciency tools was highlighted. This paper will enable researchers to acquire knowledge on water-saving methods for estimating the rice crop water requirements and thus allow them to effectively contribute to improve the performance of irrigated rice cultivation systems using various water conservation technologies.


Introduction
Rice is recognized as the most consumed food in the world, and therefore is indispensable in the fight against food insecurity, especially in countries with weak economies [1].According to Mohidem et al. [2], over half of the world's population depend on rice for food security.Thus, rice production should be improved to meet the ever-increasing demand for rice in view of the growing population in these countries and the change in eating habits.By 2030, the global rice output is expected to increase from 58 to 567 million tons [2].Moreover, world's rice production is predicted to increase to more than 1035 million tons by 2050 through higher productivity, crop intensity, and diversification [3].In addition, rice is an aquatic or semi-aquatic crop, and therefore the most demanding in terms of fresh water use.According to Bin Rahman and Zhang [4], rice consumption accounts for over 80% of all irrigated freshwater resources in Asia, hence, boosting grain productivity whilst saving water is a big challenge for rice cultivation.Unfortunately, the natural resources required for agricultural production, namely water and land/soil, are non-tangible and therefore exhaustible, especially when they are poorly managed due to the negative effects of climate change and population growth [5][6][7][8][9].Therefore, rice farming currently faces certain problems, such as water scarcity, degradation of water and soil quality, degradation of soil fertility, etc. [10][11][12][13].Flooding irrigation with a water depth of 5-10 cm, widely practiced throughout the world for rice production, and continuous flooding are key factors responsible for the depletion of resources and the decreased performance recorded under rice irrigation schemes [14].Numerous studies have been carried out to reduce the environmental impact of rice cultivation and to promote smart and sustainable rice production, such as reducing greenhouse gas emissions, optimizing the water and fertilizers used for rice cultivation, and improving paddy yields and quality of milled rice [15][16][17][18][19][20][21][22][23][24].Thus, several water and soil conservation practices for rice cultivation were developed and implemented on rice farming to improve water and soil management as well as develop efficient irrigation systems [25][26][27][28][29][30][31][32][33][34].The effectiveness of these technologies is assessed using water management indicators, such as water use efficiency/water productivity.However, the meaning and sometimes the method of estimation of these performance indicators vary from one author to another [35].This review focuses on different water management practices under irrigated rice cultivation and their influence on agricultural water productivity and natural resources (i.e., soil and water) conservation.It would enable researchers to distinguish the main concepts related to agricultural water management (WUE, WP, and WSE), and to perceive the various approaches to water-saving in rice cultivation, including methods for assessing the rice crop water requirements, suitable irrigation systems, good irrigation practices and good agronomic practices.

Water Productivity (WP)/Water Use Efficiency (WUE)
Irrigation performance indicators, such as WP/WUE, are used to ensure water conservation and hence support sustainable agricultural water management [35].Thus, increasing water productivity and water use efficiency is a prerequisite for conserving water resources, mitigating water scarcity and achieving sustainable agriculture [36].Unfortunately, both terms have been used without distinction in several recent studies [36].Kilemo [35] and Ragab [37] claimed that water usage efficiency and water productivity are two distinct concepts but overlapping words in research.Additionally, the definitions and applications of these concepts for different scales and domains of water use in irrigation water management are abused [38].Moreover, the term water productivity (WP) is used to more accurately characterize the relationship between the output or income (such as biomass or yield) and the volume of water resource, while WUE measures the ratio or percentage of effective water demand to water consumption [35][36][37][38].According to Kambou et al. [39], the majority of definitions consider WUE as a measure of irrigation efficiency, and WP as a measure of the efficiency of physiological processes of biomass production and crop yield formation related to actual water consumption by crops.
In addition, according to Hamdy [40] and Ragab [37], an efficiency is a percentage and is defined as the ratio of output to input.Hence, irrigation efficiency is the ratio of irrigation water transpired (volume of water used for plant growth) by crops during their growth period to water applied (volume of water supplied) during the same period of time [41].As asserted by Hsiao et al. [42], efficiency (E) of any production process is generally and as often used in economics defined as the ratio of output to input for that process, both measured in quantitative units.Furthermore, Zhang et al. [43] estimate that the value of WUE which varies from zero to one, has no unit (i.e., relative value) and it is most effective when its value is equal to 1.
The "catchiness" of the term "Water Usage Efficiency," according to Kilemo [35], encourages many scientists to use it even in situations where it does not apply, which has led to its incorrect use.WUE measures the ratio or percentage of water input that reaches and is absorbed by the intended crops in an agricultural system [35].According to Zhang et al. [43], the term "Agricultural Water Use Efficiency" (AWUE) refers to the amount of crops that may be produced per unit of water resources, including irrigation water and precipitation.From these two definitions, it can be seen that water use efficiency takes into account irrigation water and rainwater used by the crop for its production and it is expressed in kg/m 3 .Other authors have added technical, socio-economic, and environmental factors to the definition of WUE.Indeed, agricultural water utilization efficiency (WUE), as defined by Liu et al. [44], is the relationship between input and output for water resources that describes the economic and social benefits that result from using a unit of water.
Some authors [45][46][47][48] calculated water productivity (WP), irrigation water productivity (IWP), water consumption productivity (WCP), water use efficiency (WUE) and irrigation water use efficiency (IWUE) using equations mentioned below: Moreover, Zhang et al. [49] evaluated efficiency of the rainwater harvesting system for reducing urban water supply stress using the following equation: where, Y t is the volume of rainwater yield on the tth day, in m 3 , and D t is the water demand on the tth day, in m 3 .On the other hand, the concept of water use efficiency (WUE) indicates a relationship between crop productivity and water use [50].Zhang et al. [43] defined agricultural WUE as the crop production per unit of water resources including irrigation water and precipitation.In the same way, several authors do not differentiate between WP and WUE and calculate WUE using Formula (7) to assess the effectiveness of a water-saving practice [51]: Grain yield water applied (7)

Water-Saving Efficiency
Technically, the term water-saving efficiency is not commonly used and is confused with water use efficiency.Thus, the impact of water saving technology on water conservation is not evaluated.However, Xu et al. [52] used water-saving efficiency index (WEI) to explore the impact of virtual water trade on water withdrawal using Equation (8): where: WEI r is water-saving efficiency index of province r; W r NT is water withdrawal of province r under no-trade; and W r T is water withdrawal of province r under trade.A positive WEI r indicates that province r's water withdrawal under no-trade is higher than it would be under trade, and province r will conserve water through interprovincial virtual water trade; otherwise, province r will lose water resources through interprovincial virtual water trade [52].Furthermore, the water-saving efficiency (WSE) of irrigation system under a water-saving technology compared to the traditional or conventional irrigation technology is calculated using Equation (9) [48]: where: WSE is water-saving efficiency; IW CIT is irrigation water applied under conventional irrigation technology; and IW WST is irrigation water applied under water-saving technology.

Irrigation Water Demand and Rice Crop Water Requirements
The amount of irrigation water demand (IWD) is still high despite the promotion of water-saving irrigation technologies due to poor understanding of irrigation water demands [14].For example, Hossain et al. [53] discovered in Bangladesh that the irrigation water requirements for the rice crop ranged from 358 mm to 445 mm.In China, irrigation water for rice ranged from 135 mm to 288 mm [14].The irrigation water requirements for rice crop in Northern Benin were estimated by Bouraima et al. [54] to be 383 mm in the rainy season and 1148 mm in the dry season.According to de Vries et al. [55], the irrigation water for rice crop in Sahelian regions ranged from 480 mm to 1490 mm.
Rice irrigation water demand (IWD) is defined as the amount of water needed to meet the water losses of a disease-free crop growing under unrestricted soil conditions and achieving full production [56].On the other hand, crop water requirements (CWR) or water consumed by crop representing crop evapotranspiration (ETc) is different from crop irrigation water demand, which is generally low or non-existent during the rainy season in highly rainy areas [14,35].Crop water requirements are influenced primarily by crop factors (e.g., growth stage, cultivars, crop physiology, etc.) and reference evapotranspiration (ETo), while irrigation water demand, in addition to the above factors, is influenced by irrigation management practices (e.g., irrigation methods, irrigation regimes, irrigation scheduling, irrigation technologies, etc.), agronomic practices (e.g., land preparation, irrigation scheme patterns, etc.), soil type (e.g., texture, slope, hydraulic, etc.), season (e.g., rainfall), and farmers knowledge/skills [57].Thus, irrigation water demand can be reduced by acting on the factors that influence it through the adoption of irrigation watersaving management practices, and capacity building of producers on best management practices.In previous studies, different approaches were utilized to estimate rice irrigation water demand (Table 1).
Table 1, presenting the different formulas for assessing rice irrigation water demand (RIWD), shows that Equations (10)-( 12) are irrigation water-saving because water level above the soil is not considered.Nevertheless, Equations ( 10) and (11), by neglecting water losses through percolation and runoff, can underestimate the actual amount of irrigation water demand and consequently lead to crop water stress.

Water-Saving Technologies for Rice Production
Water-saving technologies for rice cultivation are varied and can be classified into three groups: water-saving irrigation systems; water-saving irrigation methods and water-saving agronomic practices.

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 [67,68].According to several authors [69][70][71][72], 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 [73].

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 [29,74].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 [70].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 [75] 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. [76], 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 [77,78].
Several authors [67,79] 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; Previous studies on the contribution of furrow irrigation system in water conservation are reported in Table 2.
Table 2. Previous studies on rice production under furrow irrigation system and its influence on water saving.

Location
Impact on Irrigation Water Use, and Rice Yield References

Jonesboro, USA
In this study, the furrow system received less irrigation water (631 ± 125 mm) compared to the alternate wetting and drying (AWD) practice (696 ± 181 mm) and continuous flooding (824 ± 197 mm), representing a water-saving efficiency of more than 23%.Rice yields under furrow irrigation system were low compared to flooding.

Massey et al. [80]
Mississipi, USA Rice yields under furrow irrigation system were low compared to flooding.Stevens, Rhine and Heiser [76] Mid-South, USA Compared to continuous flooding, the yield of the various cultivars under furrow was low, but the rate of arsenic uptake was very low under furrow.
Aide [77] Southern China, Asia The furrow irrigation system outperformed continuous flooding irrigation, reducing water use by 3130 m 3 , or 48.1%, and increasing grain yield by 13.9% for an early cultivar and by 2655 m 3 , or 40.6%, and by 12.1% for a late cultivar, respectively.
He [79] Elbhera, Egypt In this study, the authors demonstrated higher water savings under the furrow irrigation system as well as higher yields.WUE was improved by 146.44% due to the significant reduction in water use by 56.8%.

Abdallah et al. [81]
Sukamadi, West Java, Malaysia Furrow irrigation system conserved enough water for the next season's irrigation.Setiawana et al. [82] Arkansas, USA In a field trial conducted in 1990, 1991, and 1992 for comparing flooded and furrow-irrigated rice, the furrow irrigation technology, which produced the lowest yields, appeared to have the potential to save water.
Vories et al. [78] South Australia, Australia The study showed that more water can be saved under furrow irrigation system ((WUE = 1.43 kg m −3 ) compared to continuous flooding, although this amount of water remains low compared to subsurface drip (WUE = 2.12 kg m −3 ).

Hassanli et al. [83]
Arkansas, USA Authors showed that WUE (58.1-103.8%)under AWD is higher than WUE under furrow irrigation system.Chlapecka et al. [67] Arkansas, USA The authors did not mention WUE in their study, but the role of the suppression of false smut is highly effective in furrow irrigated rice compared to flooding.

Brooks et al. [84]
Jiangsu, China The furrow irrigation system and AWD considerably boosted the yields by 9.43-11.6%and 6.16-9.94%,respectively, as well as milled rice quality compared to continuous flooding.
Zhang et al. [85] On the contrary, Beesinger et al. [86] and Deliberto and Harrell [87] indicate that furrow irrigation system, compared to flooding irrigation, has huge disadvantages, such as the following: 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 [71].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 [72].However, in a study, Lee [65] 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 [72].

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 3).For Meher et al. [88], 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. [68], 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 [89].Rao et al. [90] 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 [91].He et al. [92] 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 [93].According to Samoy-Pascual et al. [69], 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 [94].
However, the subsurface drip irrigation performed better than the surface drip irrigation system in terms of rice growth, physiology, and yield [68].Among the three irrigation systems (drip irrigation, flood irrigation and sprinkler irrigation), Bansal et al. [95] 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 [96].
Table 3.Previous studies on rice production under drip irrigation system and its influence on water saving.

Nuapada, Inde
Drip irrigation technology compared to traditional irrigation practice can reduce irrigation water costs by 2.0 to 5.6 times by lowering overall water consumption to a biologically sound one Meher et al. [88] Coimbatore, India In comparison with surface irrigation method, the yield of aerobic rice and water savings were increased by 29 and 50%, respectively, using drip irrigation.
Parthasarathi et al. [68] Punjab, India Subsurface drip fertigation, compared to flood irrigation system, reduced irrigation water use for rice by 48-53%.Moreover, subsurface drip fertigation reduced energy consumption while increasing nutrient utilization efficiency.

Sidhu et al. [89]
Bhopal, India, This study showed that, in comparison to conventional irrigation practice, SRI under drip irrigation system provided the greatest plant height, root length, yield, and yield-contributing factors.

Rao et al. [90]
Subang Indonesia Drip irrigation technology used 3864 m 3 /ha/season of water as opposed to the typical 7460-8740 m 3 /ha/season of the conventional irrigation practice under flooded conditions (i.e., the water saving efficiency of 48-56%).

Islamabad, Pakistan
According to this study, the water productivity of a drip irrigation system was higher (249%) than that of conventional flooding and 197% than that of an automatic water delivery system (AWD), which produced higher yields.

Akbar et al. [98]
Mediterranean Region, Turkey Compared to both surface drip and subsurface drip irrigation methods, conventional flooding produced a higher output.Contrarily, water savings of between 50 and 60 percent using surface and subsurface drip methods were made in comparison to traditional flooding.WP values in the first year of study were 0.81 kg/m 3 , and in the second year they were 0.85 kg/m 3 .
Çolak [94] Xinjiang, China Grain yields were decreased by 31.76-52.19%under the drip irrigation method, by 57.16-61.02%under furrow irrigation with plastic mulching, and by 74.40-75.73%under non-mulching furrow irrigation as compared to conventional flooding.However, WUE was significantly greater with the drip irrigation system, measuring 1.52-2.12times higher than with conventional floods, 1.35-1.89times higher than with furrow irrigation with plastic mulching, and 2.37-3.78times higher than with non-mulching furrow irrigation.In comparison to conventional flooding, drip irrigation approaches have a higher capacity for conserving water and narrower yield and economic benefit gaps than furrow irrigation practices.

He et al. [92]
Maharashtra, India Using drip fertigation, water productivity increased across varieties and regions from 0.46 to 0.67 kg/m 3 , and rice yields were greater across varieties (13-28%) than those observed with the corresponding conventional practices.
Padmanabhan [93] Nueva Ecija, Philippines In comparison to surface flooding, drip irrigation systems increased irrigation water productivity to 1.03 kg/m 3 , reduced irrigation water use by 42%, and increased average net revenue by 41-75%.

Samoy-Pascual et al. [69]
Tamil Nadu, India Compared to the traditional irrigation method (4181 kg/ha), grain yield (5389 kg/ha) significantly increased with the subsurface drip irrigation approach.Also, compared to conventional irrigation methods, drip irrigation had a greater capacity to save water (27.0%)without reducing grain yield in aerobic rice production systems and its water productivity was twice as high in subsurface drip irrigation systems.

Parthasarathi et al. [68]
Enez, Edirne, Turkey The authors demonstrated that the amount of irrigation water applied using a subsurface drip irrigation system was lower (751 mm) than the amount applied using a surface irrigation method.
In comparison to conventional irrigation, water was saved by 50 to 69%.

Location Impact on Irrigation Water Use, and Rice Yield References
Beijing, China The study found that water productivity in the furrow irrigation system was 88.9, which was 16.4 and 11.4% higher than in the continuous flooding and drip irrigation systems, respectively.
Hang et al. [100] Chiba, Japan The research showed that drip irrigation with plastic film mulch boosted WUE by 50 to 70 percent, equal to continuous flooding.Fawibe et al. [101] Kanto Area, Japan Although the grain yields under drip irrigation and plastic film mulch were much lower than those under continuous flooding, representing 74% to 85% of the continuous flooding, the percentages of irrigation for these two methods in 2015 and 2016 were 79% and 66%, respectively.

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 [74,103].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 [104].Similar to this, Alvarenga et al. [105] and Spanu et al. [106] 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. [107] 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. [108] 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 [109].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 [110].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 [111].Studies conducted on rice production under sprinkler irrigation are shown in Table 4.
Table 4. Previous studies on rice production under sprinkler irrigation system and its influence on water saving.

Location Impact on Irrigation Water Use, and Rice Yield References
Arkansas, USA This study showed that two rice cultivars grown under center pivot irrigation produced high yields (8.31 Mg/ha in 2009 and 8.2 Mg/ha in 2010), with an irrigation water use efficiency of 2.0 kg/m 3 in 2009 and 1.6 kg m 3 in 2010.Rice cultivation with center pivot irrigation required a total irrigation depth of 414 mm, whereas flood irrigation for rice required depths of 1168 mm.

Vories et al. [74]
Arkansas, USA Researchers have shown that the high-to-low order of total continuous flooding had a greater impact on rice grain content than intermittent flooding or spray irrigation, although neither had a significant impact on production.

Spain
In this study, it appears that sprinkler irrigation, compared to flood irrigation, saved more water, increased soil organic C, and decreased both inorganic and organic arsenic concentration in grain.
Moreno-Jiménez et al. [107] Selviria-MS, Brazil Field studies conducted in 1994/1995 and 1995/1996 came to the conclusion that water levels ranging from 0.5 to 1.5 times the rice crop coefficient, supplied through sprinkler irrigation system provide better conditions for producing rice seeds of upland cultivars with higher physiological quality.
Costa Crusciol et al. [108] Capão do Leão, Brazil According to this study, chemical weed management using herbicide selectivity is more effective with sprinkler irrigation than flood irrigation.

Helgueira et al. [109]
Leão, Rio Grande do Sul, Brazil Scientists found that a soil water tension of 10 kPa was sufficient to control spray irrigation in rice, particularly during the reproductive stage.
Pinto et al. [110] Griffith, Australia The amount of water used for sprinkler irrigation generally appeared to be adequate to meet the crop's evapotranspiration requirements, but the plants may have experienced moisture stress in the intervals between irrigations since data from the soil matric potential at 100 mm revealed little water stress in sprinkler irrigation during the vegetative stage.
Humphreys et al. [111] Monoo, Pakistan Study conducted during 2002-2004 projected that sprinkler irrigation increased rice output by 18% while using 35% less water than the conventional irrigation technique and revealed that adopting sprinkler irrigation for rice is a financially viable choice for farmers.
Kahlown et al. [112] Tamil Nadu, India A field experiment carried out in 2013 and 2014 revealed that sprinkler irrigation used the least amount of irrigation water (329.2mm and 308.7 mm) and surface irrigation used the most (413.6 mm and 428.1 mm) resulting in water savings of 23.1% and 25.4% in 2013 and 2014, respectively.
Kumar et al. [113] Rio Grande do Sul, Brazil Experiments conducted over two years (2012-2013) revealed that sprinkler irrigation used 48% less water than flood irrigation while also reducing water stress and improving the physical and chemical characteristics of the soil.

Sardinia, Italy
Field studies conducted between 2002 and 2006 showed that irrigation water used for rice cultivation utilizing sprinkler irrigation was approximately 6500 m 3 /ha (650 mm).

Spanu et al. [115]
Arizona, USA Authors indicated that flood irrigation used a total of 589 mm of irrigation water, whereas pivot irrigation used 470 mm, resulting in an irrigation water use efficiency of 1.7 kg/m 3 for flood irrigation compared to 2.1 kg/m 3 for pivot irrigation.
Vories et al. [116] India In this review, the author showed that micro-irrigation (drip and sprinkler) potentially contributes to irrigation water savings, but decreases rice yield.

Mandal et al. [117]
Edirne, Turkey The results of this study over the course of three years (1991-1993) revealed that while sprinkler irrigation produced lower yields than continuous flooding, water savings rates ranged from 12.3 to 43.1%.

Cakir et al. [118]
Texas, USA Though it reduces irrigation water use, sprinkler irrigation does not seem to be a practical substitute for traditional flood irrigation, according to the authors, because it decreased plant performance (height by 0.09 to 0.28 m and average yield by 20% to 28%).
McCauley [119] Missouri, USA According to this study's findings, sprinkler irrigation uses 28% less water than conventional flooding.

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 culti-vation for a number of reasons [121][122][123].First, rice cultivation under surface irrigation is an ancestral practice and abandonment of this practice is constrained by socio-psychological barriers [124].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 [94,117].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. [125,126].Previous studies have shown that compared to continuous flooding, all water-saving practices allow for sustainable rice production [127].According to research [126][127][128][129][130], 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) [121,123].Islam et al. [121] 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 5).
Table 5.Previous studies on rice production under alternate wetting and drying (AWD).

China
This study has shown that, compared to conventional flooding-midseason drainage-flooding irrigation (FDF), AWD increased WUE by 40% and resulted in maximum grain production (7808.38 kg/ha) Wang et al. [126] Tripura, India According to the authors, 30% of water can be saved using AWD for rice growing under SRI compared to flooding irrigation.Singh and Chakraborti [131] Carolina, USA Study results showed that AWD method lowered irrigation use hours by around 38% while saving irrigation water and boosting energy without noticeably reducing crop yields and revenues.

Rejesus et al. [132]
Wuhan, China In comparison to other water-saving techniques, the results showed that AWD had the highest average water saving rate of 35.12% and the lowest average yield increasing rate (0.79%) Zhuang et al. [23] Fanaye, Senegal The researchers found that AWD irrigation control at 30 kPa boosted rice production, water use, and nitrogen use efficiency while lowering irrigation applications by 27.3% compared to continuous flooding.

Djaman et al. [133]
Tokyo, Japon This study, carried out from December 2021 to March 2022, found that AWD utilized 25% less water than continuous flooding Bwire et al. [134] Bangladesh According to this study carried out in 2017, AWD conserved 12% to 24% more irrigation water than continuous flooding.Albaji et al. [135] Kushtia, Bangladesh Authors demonstrated that the AWD technique alone saved 20.2% more field water than flooding irrigation practice, and when paired with plastic pipe, 42% more water was saved.
Hossain et al. [136] Pingtung, Taiwan The results indicated that AWD could produce a grain yield that was comparable to the farmers' methods while requiring fewer irrigations.Tapsoba and Wang [137] Table 5. Cont.

Telangana, India
The experiments (2014 and 2015) demonstrated that the alternate wetting and drying strategy of irrigation resulted in lower water usage of about 795 mm to 1180 mm and higher water productivity of 0.52 kg/m 3 to 0.66 kg/m 3 , saving 20.2 to 23.4% more water than the submerged irrigation method.
Rao et al. [138] Tuanlin, China The three-year (1999-2001) study revealed that irrigation water input was 15-18% lower under alternate water distribution (AWD) than under continuous submergence, and water productivity was higher under alternate 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 [123,125,127].Hiya et al. [48] and Massey et al. [80] 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. [142] 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. [121] 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. [139] 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. [135] 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 [51].Additionally, through two-year field experiments at Jiangsu, China, Zhang et al. [140] found that shallow water irrigation used the largest amount of water compared to wet-shallow irrigation, but provided a higher yield.

Water-Saving Agronomic Practices for Rice Production
Various previous studies focused on good agronomic practices (GAP), such as using drought-tolerant rice variety [47,143], plastic mulching [34], straw mulching [144], organic matter application [17,145], minimum tillage [121], 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. [9] 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 evapotranspi-ration) [9].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 [144].The system of rice intensification (SRI) approach helped to lower the need for irrigation water, resulting in immediate advantages of decreased irrigation water demand [146].

Conclusions
In order to increase the effectiveness of irrigation water consumption, this review examined numerous water conservation technologies used in irrigated rice production.Firstly, this review clarified some concepts of irrigation water management, such as water productivity (WP), water use efficiency (WUE) and water saving efficiency (WSE).The clarification was provided as often, the usage of these concepts by several researchers in the literature is confusing.Secondly, this study reviewed various methods used to estimate irrigation water requirements in irrigated rice production, as some methods contribute more to water conservation than others.Thirdly, water conservation technologies in rice production were reviewed.This review distinguished three categories of water conservation technologies: water-saving irrigation systems, water-saving irrigation methods and water-saving agronomic practices.Water-saving irrigation systems including furrow irrigation, drip irrigation, and sprinkler irrigation, conserve water resources in rice cultivation.Water-saving irrigation methods including AWD, dry seeding, etc. are some other practices of applying water to the field that saves irrigation water.Similarly, water-saving agronomic practices, such as mulching, minimum tillage, organic matter using, etc. also improve water use in rice cultivation.Finally, this study analyzed the effect of water conservation technologies on rice yields and water use efficiency.It was found that, compared to the conventional methods, the water conservation technologies lead to much improvement in water use efficiency.However, their contribution to improvements in rice yields remain mixed when compared to the traditional approaches.

Table 1 .
Formulas for calculating rice irrigation water demand.