A review on water use efficiency in different cropping systems

Introduction

Water is a basic production factor in agriculture and it takes an almost 70 percent portion of total freshwater abstractions around the world (FAO, 2021) [10] . Nevertheless, with the rising scarcity of water caused by climate change and population growth coupled with intensive agricultural systems, proper management of water has become a priority (Falkenmark, 2013; Rockstrom et al., 2010) [31] . The use of Water Efficiency (WUE) has emerged as a major measure of sustainable agricultural systems that is used to explain the extent to which water has been used effectively to generate crop yield (Blum, 2009) [3,4] . It is usually defined as a ratio of economic yield to evapotranspiration (ET) (Howell, 2001) [23] . An increase in WUE implies the improved use of available water resources. Cropping systems are also important in determining WUE as it impacts soil moisture dynamics, crop water uptake and evapotranspiration patterns (Passioura, 2006; Jiang et al., 2025) [25,33] . As a result, the concept of WUE in the context of various cropping systems is crucial to enhancing productivity at the expense of the use of the water resources.

Concept of Water Use Efficiency (WUE)

Water use efficiency is a multi-dimensional concept and its application differs according to the level and context of application (Sinclair, 2012) [34] . The yield of the water utilized is referred to as Agronomic WUE whereas the yield of biomass is called physiological WUE. The Irrigation WUE particularly looks at how well the irrigation water applied is used to contribute towards crop yield (Howell, 2001) [23] . WUE is affected by several factors such as climatic conditions, soil characteristics, crop species and management practices (Hatfield & Dold, 2019; Wang et al., 2024) [20,35] . As an example, excessive temperature and low humidity may raise the rate of evapotranspiration, thus lowering the WUE (Monteith, 1977) [36] . On the same note, water retention and availability to plants depends on soil texture and structure (Gregory et al., 2000) [19] . Scheduling of irrigation and nutrient management are efficient management practices that may contribute greatly to the WUE (Fereres & Soriano, 2007; Wang et al., 2024) [13,35] .

Variability in WUE of different crops

Water Use Efficiency (WUE) among crops differs greatly because of the physiological and biochemical differences inherent in crops, especially in their photosynthetic pathways. C3-type crops (wheat, rice etc.) tend to have a lower WUE due to the fact that they are more susceptible to photorespiration, particularly in high temperature and low CO2 conditions, resulting in higher loss of water per unit biomass produced (Blum, 2009; Long et al., 2006) [3,4] . By contrast, C4 crops such as maize and sorghum are able to sequester carbon within a concentration mechanism that reduces photorespiration and maximizes the capability of fix carbon and thus higher WUE is attained with a given environment (Sage & Zhu, 2011; Ghannoum, 2009) [16,37] . This inherent distinction has a direct connection to transpiration efficiency in which C4 crops generate more biomass per unit water transpired than C3 crops (Farquhar et al., 1989) [12] . Though pulse crops are predominantly C3 in nature, they show moderate WUE but work effectively in waterlimited conditions because of their adaptive characteristics of deep rooting systems, osmotic adjustment and reduced water needs (Graham and Vance, 2003; Sinclair and Vadez, 2012) [17,18,34] . In this way, crop-specific WUE is highly regulated by photo-synthetic pathway and transpiration relationship, crop selection is a key factor in enhancing the overall water productivity within various agro-ecological systems.

Water Use Efficiency under Different Cropping Systems 3.1 Monocropping Systems

Monocropping is the common practice that is characterized by growing one type of crop in the same soil, being grown at different periods. It is the easiest and simplest method of farming because it is easy to manage. Nevertheless, such a system may lead to reduced WUE because of unhealthy soils, excessive evaporation losses and inefficient use of resources (Tilman et al., 2002; Lal, 2015) [38,39] . In fact, an instance of continuous monocropping may cause degradation of soil, decrease in organic matter and decrease in root diversity which are disadvantaged in terms of water retention and uptake. Research has indicated that monocropping systems tend to have low WUE relative to diversified systems especially where water is a constraint.

Intercropping Systems

Intercropping is the concomitant planting of two or more crops within a single field and this has been well known to improve the WUE (Lithourgidis et al., 2011) [32] . This system enhances the use of resources using disparity in depth of roots of crops, structure of canopy and the period of growth of crops (Tsubo & Walker, 2004) [41] . Consequently, intercropping minimizes the use of evaporation losses and enhances efficiency of utilizing soil moisture. Indicatively, intercropping systems, specifically maize soybean systems, have been reported to enhance WUE up to 40 percent over monocropping because it provides a better spatial and temporal utilization of resources (Sai et [42,43] .

Crop Rotation Systems

Crop rotation: The sequential growth of other varieties of crops on the same land is important in enhancing the health of the soil and water efficiency (Karlen et al., 1994) [28] . Rotation systems increase the soil structure, improve organic compounds and water absorption and retention (Hobbs et al., 2008) [22] . These transformations lead to the use of water more efficiently. An example is the use of wheat-maize rotation systems, which are found to increase the WUE over that of continuous monocropping systems by 12-32 percent. Crop rotation also contributes to the breaking of the pest and disease cycles reducing stress on crops and enhancing the overall productivity and water use.

Pulse-Based Cropping Systems

Pulse crops can fix nitrogen in the atmosphere and have a low water demand hence they have great significance in sustainable agriculture (Graham & Vance, 2003) [17,18] . Pulses in the systems enhance the structure of the soil and the rate of water penetration and thus increase the WUE (Peoples et al., 2009) [45] . Pulse-based systems are most useful in rainfed and water scarce areas. Researches have revealed the incorporation of pulses with cereals can greatly enhance the WUE by enhancing nutrient and water use effectiveness (Thakur et al., 2025) [46] .

Conservation Agriculture Systems

Conservation agriculture (CA), such as zero tillage, residue retention and crop diversification, have been popularly advanced with regard to enhancing WUE (Kassam et al., 2009) [29] . The practices minimize soil disturbance, increase soil organic content and raise the water percolation and absorption (Farooq et al., 2011) [11] . The retention of the residues in the soil surface decreases the loss of evaporation and the soil is kept moist (Unger, 1990) [47] . Consequently, CA systems tend to be more WUE than traditional tillage systems are (Wang et al., 2024) [35] .

Agroforestry Systems

Agroforestry systems integrate trees with crops, creating a more diverse and resilient production system (Nair, 2012) [48] . Trees play a crucial role in improving soil structure, enhancing water infiltration and reducing evaporation through shading (Jose, 2009) [27] . Deep-rooted trees can access water from deeper soil layers, making it available to crops indirectly (Wallace, 2000) [49] . This complementary use of water resources leads to higher overall WUE in agroforestry systems compared to sole cropping. [21,24] . The presence of extreme weather conditions like droughts and heatwaves also worsen the situation of water stress and cause low crop productivity. The absence of adaptation mechanisms to mitigate the impact of climatic changes on WUE, including climateresilient cropping systems, drought-tolerant varieties and better irrigation timing, is necessary to maintain WUE (Leng and Hall, 2019; Zhao et al., 2017) [50,51] . In addition, the incorporation of weather-based decision support systems can assist farmers in optimizing their irrigation and cropping schedule to enhance water productivity in the climate variability (Raza et al., 2019) [52] .

Soil Health and Microbial Interactions in Enhancing WUE

The health of the soil is an important factor in setting the efficiency of water use, directly influencing the water retention, infiltration, root development. Well-nourished soils full of organic elements also enhance water retention and minimize run-off wastage (Lal, 2020) [40] . Plant water uptake is promoted by soil microbial communities, such as mycorrhizae and plant growth-promoting rhizobacteria (PGPR), which increase root architecture and nutrient availability (Smith and Smith, 2011; Vurukonda et al., 2016) [53,54] . Such beneficial microbes are also useful in enabling plants to endure drought stress by controlling physiological functions. Organic amendments, cover cropping and reduced tillage are some of the practices that increase soil biological activity and consequently, WUE (Bender et al., 2016; six et al., 2004) [2,55] . Therefore, preserving soil health is an important approach to enhancing water productivity in agriculture.

Policy and Socio-economic Aspects of Water Use Efficiency

Policy support and socio-economic conditions have a strong impact on the adoption of water-efficient technologies and cropping systems. Smallholder farmers in most developing countries are hampered by lack of irrigation infrastructure, financial resources and technical expertise (Shiferaw et al., 2014) [56] . The policies of the government that encourage microirrigation, water pricing and subsidies of efficient technologies can also be of great help in improving WUE on a farm level (Rosegrant et al., 2009) [57] . Other promising solutions to enhance water distribution efficiency include community-based water management and participatory irrigation systems (Meinzen-Dick, 2007) [58] . Moreover, the extension services and capacity building are very vital in sharing the knowledge on water saving practices and technologies (Pretty et al., 2018) [59] . Thus, to enhance WUE, it is necessary, not just to undertake technological interventions but also to have robust institutional and policy frameworks.

Agronomic Practices Influencing WUE

There are a number of agronomic practices that have a profound impact on efficiency of water resources in crop systems. The effective irrigation practices like drip irrigation and sprinkler irrigation apply water in the root zone which minimizes the loss through evaporation and water run-off (Fereres & Soriano, 2007; Pereira et al., 2012) [13,60] . The techniques have been cited to enhance the WUE by 10-18 percent over the traditional flood irrigation. Another practice that is significant and enhances WUE is mulching, which lessens soil evaporation, retains moisture and improves soil temperature (Farooq et al., 2011) [11] . The choice of suitable crop varieties, especially the drought tolerant and short life cycle varieties are also important in enhancing the water productivity (Blum, 2011) [5] . Also, the balanced management of nutrients helps to achieve superior root growth and water uptake, which lead to increased WUE (López-Bellido et al., 2005; Wang et al., 2024) [35,63] . [26,44] . Automated irrigation systems and soil moisture sensors can be used to optimize the timing of irrigation so that crops get the appropriate quantity and timing of water. There is an increasing use of artificial intelligence (AI) and decision support systems (DSS) to enhance the management of water (Zwart & Bastiaanssen, 2004; Wang et al., 2024) [35,61] . These technologies promote the efficiency of water utilization on a field and regional basis. (2018) [43] 6. Research Gaps and Future Prospects In spite of the major progress, there are still a number of research gaps associated with the field of water use efficiency.

Long term research is required to determine the viability of various cropping systems to diverse climatic conditions (Pretty et al., 2018) [59] . There are no region-specific recommendations that can be given especially to the small holder farmers in the developing world (Sharma et al., 2015) [62] . Moreover, there is a lack of employment of water-saving technologies because of the economic and infrastructural factors. Future studies must consider combining climate-smart agricultural methods with the use of modern technologies in order to improve WUE (Kijne et al., 2003) [30] .

Conclusion

Efficiency of water use is an important aspect in realization of sustainable agricultural production as water scarcity increases.

Various systems of cropping intercropping, crop rotation, conservation agriculture and agroforestry have revealed a great potential of enhancing WUE as opposed to monocropping systems (Lithourgidis et al., 2011; Kassam et al., 2009) [29,32] . Water productivity can be further increased by adoption of better agronomic practices and modern technologies. The effective use of water in agriculture needs to be holistic and should be a mixture of an efficient cropping system, technological advances and policy support.

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