Comparison with other studies

In this study, the global average WP of irrigated maize was 18.6 kg ha−1 mm−1, which is substantially higher than the results of other studies (e.g., 14.3 kg ha−1 mm−1 in a study by Liu18 that employed the GEPIC model). This may be attributed to the WP, which significantly increased with time31. In this study, most of our data (78%) were from after the year 2000, whereas Liu18 focused on the year 2000. It was also reported that the average WP increased from 13.5 kg ha−1 mm−1 in 1980 s to 19.8 kg ha−1 mm−1 in 2000 s due to the improvements in fertilizers, cultivars and other management practices32. The WP in the present study was representative of irrigated agricultural systems, whereas that of Liu18 was representative of rain-fed and irrigated systems. Our results are similar to the global average (18 kg ha−1 mm−1) reported by Zwart and Bastiaanssen (2004)13. The latter used a limited dataset with 223 data points from 10 different countries.

WP variation and the effecting factors

A great deal of variation in WP at the country level was observed, from an average of 5 kg ha−1 mm−1 in Uzbekistan to an average of 38.75 kg ha−1 mm−1 in Germany. At the regional scale, WP also varied from 10.3 in Africa to 25.4 kg ha−1 mm−1 in Europe (Fig. 1, Table 1 and Supplementary Table 1). Many previous studies also found large variations in WP5,13,14, owing to differences in climate, site-specific biophysical conditions, agronomic management practices, etc.5,10.

In our study, WP values significantly decreased with increasing irrigation amount, regardless of the level of precipitation, indicating that increases in water consumption through irrigation were not offset by increases in maize yield. This may be because the maize was unable to fully utilize seasonally-available water owing to percolation below the root zone or water remaining in the ground at physiological maturity17,33,34. Maize grain yields have positively and negatively correlated with irrigation amounts when seasonal precipitation was <400 mm and >400 mm, respectively. Similar results were reported that yield increased by about 80% with supplementary irrigation under semi-arid conditions in southern India35. However, supplemental irrigation in high-precipitation regions can decrease yield25,36. These results imply that, for irrigated maize, significant improvements in WP and yield can be achieved in low-rainfall regions because supplemental irrigation helps to reduce water stress37. Conversely, high levels of precipitation may cause nutrients to be leached from the root zone through percolation and runoff, negatively influencing maize yields and WP25.

Deficit irrigation is considered a promising option for improving WP26,38. WP increased without any yield losses when deficit irrigation was >80%. However, the favorable and negative impacts of deficit irrigation on yield and WP depend on water supply and deficit irrigation level. For instance, Kirda28 reported that grain yield was significantly reduced by 10–25% under 50% deficit irrigation compared with full irrigation. In our study, deficit irrigation at levels below 60% resulted in both lower WP and lower yield. Similar results also indicate that deficit irrigation levels should be relatively high for high yield and WP38.

Soil properties significantly affected WP and maize yield. Both yield and WP were positively correlated with SOM. Beneficial effects of increased SOM on yield have also been reported in China39 and India40. Several studies have reported that certain soil management practices increased SOM and soil water-holding capacity, and promoted rooting and increased crop nutrient and water uptake, ultimately increasing yield and WP41,42. In the present study, WP was significantly negatively correlated with soil bulk density (Table 2). In actual production, high soil bulk density is often associated with increased water consumption and reduced yield43,44.

Closing the WP gap for global maize supply and water sustainability

In our study, the estimated potential WP based on the boundary function was 48–56 kg ha−1 mm−1 in different maize production regions. Similar studies reported maximum attainable WP values of 69 kg ha−1 mm−1 for global maize production22, and 60.5 kg ha−1 mm−1 under plastic mulching in the Loess Plateau in China45. The corresponding WP values based on field measurements were 10.3–25.4 kg ha−1 mm−1 for different regions of maize production (Table 1). The large WP gaps (29–41 kg ha−1 mm−1) suggest great potential for improving water productivity to support global food security.

Improvements in WP in maize production are critical for addressing the dual challenges of global maize supply and water sustainability1,10. Our results suggest that maize production could be increased by 100% by 2050 with 20% less planting area and 28.9% less water consumption compared with 2005, if farmers worldwide could approach WPs equivalent to 85% of their potentials. Similar studies have reported that improvements in the WP of irrigated cropland could reduce total water consumption by 8–15% in precipitation-limited regions10.

Many possibilities exist for large gains in WP to close WP gaps. In actual production, WP improved to 57.1 kg ha−1 mm−1 by optimum use of the irrigational water in Bulgaria46, approaching the potential WP. The combination of water irrigation lateral spacing and partial mulching also approach the potential WP in China47. Some caution is required though. Such as, some appropriate strategies for closing WP gaps are region-specific and vary from country to country. Thus, management strategies that aim to increase WP must be tailored to local contexts, depending on factors such as local climate (e.g., rainfall), soil properties, traditional management technologies and economic considerations1,48. Further studies on the interacting effects of such factors are required to inform optimal management strategies in different regions. In addition, improvement in both WP and yield may also depend on policies to educate about and award good management practices, aligning the incentives of producers, resource managers and society, and providing a mechanism for dealing with trade-offs49.

Uncertainty of our analysis

Although global data for WP with irrigated maize production was integrated from individual field results across multiple environments and field management practices, some uncertainties exist in this study. First, other field management practices, such as irrigation techniques50, nutrient management42, varieties and plant populations24,51, tillage and soil mulching12,52,53, may affect maize WP and yield, although those factors were difficult to investigate in our analysis due to a lack of information from the individual studies. Additionally, while critical soil and environmental factors were considered in our analysis, other experimental variables which may affect maize and yield and WP were not provided in many studies, such as soil water supply, effective accumulated temperature, sunshine hours, soil texture, etc.

Second, estimated WP based on experimental field plots do not accurately represent the WP achieved in farmers’ fields. Generally, WP is greater in well-managed research experiments than when the same practices are applied by farmers in production fields. Thus, future research should focus on collecting measurements from farmers’ fields to generate more robust outcomes. Third, potential WP can vary greatly even within a given region, depending on local soil type and management practices. For example, one study found that potential WPs were 47.5 and 60.5 kg ha−1 mm−1 under no mulching and mulching conditions, respectively, in the Loess Plateau of China45. Thus, more effort are needed in evaluating potential WP under specific conditions or on smaller scales to minimize the current large uncertainties in global or regional estimates.