Reducing gray, blue, green, and total water footprint in wastewater irrigated silage maize in zero tillage

The agricultural sector, which has a high water demand for irrigation (Fader et al. 2011), consumes 60–70% of the freshwater withdrawn globally (Assouline et al. 2015). Although more water up to an optimal level often increases the income per unit area as a result of greater production, the decrease and pollution of the global freshwater resources do not always make this possible. For this reason, there is pressure to manage irrigation more effectively in agriculture (Van der Laan et al. 2019).

The water footprint has been developed as a paradigm in determining the amount of virtual water (Allan 1998), which describes the amount of water in the entire process contained in a product, thus managing the use of freshwater. The water footprint, unlike other water management models and statistics, reveals the water volume of a good or service during the entire process with different water types (Brindha 2017). The amount of surface and underground freshwater used in the crop production process represents blue water; the amount of precipitation during the vegetation period and the residual amount in soil from precipitation before the vegetation period represent green water; and the amount of polluted water represents gray water (Yerli et al. 2019; Yerli & Sahin 2022a). Blue and green water footprints consider the consumption of physical freshwater resources and soil water gained by effective rainfall, respectively (Song et al. 2023). In fact, the concept of the water footprint is a suitable indicator of water use for all sectors (Kalvani et al. 2020). However, compared with other sectors, the agricultural sector is responsible for more than 90% of the entire water footprint (Aldaya et al. 2012), which has increased the focus of the water footprint on the agricultural sector (Van der Laan et al. 2019).

While the high accuracy data requirements and complexity of analyzes for calculating the agricultural water footprints limit the conduct of studies on water footprints (Muratoglu 2019), the use of estimated data and general climate parameters in determining the water footprint of an agricultural product causes data confusion and imprecise results, resulting in low-resolution water footprint values (Aldaya et al. 2010; Mekonnen & Hoekstra 2011a; Zhang & Anadon 2014). In addition to the water footprint of an agricultural product being directly related to the correct determination of crop evaporation and irrigation water volume and to the clarity of the data obtained (Van der Laan et al. 2019), the fact that the parameters that manage them are complex, the yield can change instantly under all conditions, and many parameters in soil, climate, and crop environment are intertwined with different factors limits the accuracy of the water footprint values produced as a result of the estimation. Accurate estimation of both crop and region-specific water footprints relies on field-scale measurements of evaporation, yield, and irrigation (Van der Laan et al. 2019). Thus, the water footprint calculated with the more real values of the crop produced as a result of a field experiment will provide more clear and accurate data.

It is stated that determining blue and green water footprints may be sufficient to define the water footprint of the crop (Hoekstra et al. 2011; Ma et al. 2021). In general, the gray water footprint is calculated as an output of polluted water production under irrigation conditions with freshwater, while it is input as an irrigation water resource in wastewater irrigation. Therefore, if wastewater is used in irrigation, this requires considering it as an input resource that reduces the use of physical freshwater resources.

The increase in the discharge of recycled wastewater in the world with increased urban populations and thus the increase in the use ratio of this water in agricultural irrigation (Yerli et al. 2023), necessitates an evaluation based on the gray water footprint input providing a decrease for blue and green water footprint outputs. Here, the advantage of zero tillage practice also in preserving soil moisture can make significant contributions to decreasing water footprint (Gozubuyuk et al. 2020).

There is no study on silage maize crop, which has a blue and green water footprint in a very large part of a production area of 201,983,645 ha in 2020 in the world (FAO 2022), showing the contribution of gray water footprint as an input to the reduction of blue, green, and total water footprint outputs under wastewater irrigation conditions. The limited availability of freshwater resources necessitates the use of marginal water resources, primarily treated wastewater, while at the same time, the use of practices such as deficit irrigation and zero tillage will provide water savings. Therefore, this study aimed to determine the effect of recycled wastewater at different levels on the water footprint per unit fresh and dry biomass yield of silage maize under conventional and zero tillage conditions by comparing it with full irrigation with freshwater. Therefore, this study hypothesizes that gray water footprint input can make a significant contribution to reducing blue, green, and total water footprint outputs in silage maize irrigated with wastewater.

The main treatments in the study were two tillage-sowing practices: conventional tillage and zero tillage. The sub-treatments were four irrigation practices: full irrigation with freshwater (F100), full irrigation with recycled wastewater (W100), and 33% (W67), and 67% (W33) deficit irrigation with recycled wastewater. Thus, the study, which was designed with three replications according to the split-plot experimental design, was carried out in a random block experimental plan in a total of 24 plots (3 replications × 2 tillage-sowing practices × 4 irrigation practices), and each plot of 3.5 m × 7.2 m = 25.2 m² was planned as 5 rows with 70 cm × 15 cm row spacing × crop spacing.

While the moldboard plow, cultivator-rotary harrow, and pneumatic seeder were used in the conventional tillage, respectively, in the direct sowing, sowing was performed directly with a direct sowing machine without soil tillage. While weeding was done at two different times (when the crop height was 15–20 and 40–50 cm) under conventional tillage (Cigdem & Uzun 2006), since weeding was not done for weed control during zero tillage, an herbicide suitable for the weed pattern of the experimental field was used. In the first year of the study, the basic fertilization of silage maize (200 kg ha⁻¹ urea and 150 kg ha⁻¹ triple superphosphate) was applied (Celebi et al. 2010; Kusvuran et al. 2015); however, in the second year, to more clearly determine the contribution of recycled wastewater to yield, fertilization was performed only in freshwater plots.

While the freshwater used for irrigation was provided by the university's network, the recycled wastewater was transported to the experimental area by water tankers before each irrigation from the treatment plant located in the Edremit district of Van province. Irrigation was carried out according to the dynamic irrigation program with a surface drip irrigation system with a flow rate of 2.3 L h⁻¹ at 33 cm intervals according to the infiltration rate of the experimental field (≈15 mm h⁻¹). Until the crop height reached 40–50 cm, the current soil moisture determined at a depth of 30 cm in the freshwater plots was completed to the field capacity by irrigation with an equal amount of freshwater with a 0.30 wetting ratio in all plots. After the crop height exceeded 40–50 cm, different irrigation practices (F100, W100, W67, and W33) were started and continued until the harvest period. For this purpose, the current soil moisture determined at a depth of 90 cm in the freshwater plots of the tillage-sowing treatments was completed to the field capacity by applying freshwater and recycled wastewater with a 65% wetting percentage in the F100 and W100 plots (Cakmakci & Sahin 2021). On the same dates, the W67 and W33 treatments were irrigated with 33 and 67% less water than the W100 treatment.

At the end of the study, a total of 30 crops in the central 1.5 m distance from the middle three rows of each plot were cut from the soil surface and weighed, and fresh biomass yield per unit area of silage maize was determined. After the fresh weights of the four crops were weighed from the same samples to represent the plots, they were waited for a certain period to wilt, dried at 78 °C for 48 h, and weighed to obtain their dry weight. The dry matter ratio was determined by dividing the dry weight by the fresh weight, and the dry biomass yield was calculated by multiplying this ratio value by the fresh biomass yield (Kacar 2014).

In this study, the water footprint of silage maize was determined to be blue, green, and gray water. The green water footprint was calculated by the precipitation during the vegetation period of silage maize and the moisture change of the soil during sowing and harvesting, while the blue and gray water footprints were determined by irrigation quantities with freshwater and recycled wastewater, respectively. The total water footprint was determined by the sum of blue, green, and gray water footprints. The blue, green, gray, and total water footprints per unit fresh and dry biomass yields were obtained by proportioning of the blue, green, gray, and total water amounts to the fresh and dry biomass yields of silage maize, respectively.

Statistical analysis of blue, green, gray, and total water footprint per unit of fresh and dry biomass yields was performed in the SPSS program. For this purpose, variables considered as fixed factors (irrigation and tillage-sowing practices) were evaluated with the General Linear Model, and significant means were separated at a 5% probability level with Duncan multiple comparisons.

This study's results indicated that the relationship between the yield of silage maize and the amount of irrigation water was positively linear (Yerli et al. 2023). Similarly, Yolcu & Cetin (2015) pointed out the linear relationship between silage maize yield and both irrigation quantity and nitrogen content. Less wastewater application provides less nutrients and organic matter to the soil (Cakmakci & Sahin 2021). In addition, as soil moisture decreases under deficit irrigation conditions, the increased matrix and osmotic potential in the soil significantly limit productivity by reducing water and thus nutrient uptake. This may explain the significant yield drop in the W67 and especially the W33 treatment. Gheysari et al. (2017) also recorded that a decrease in the irrigation water level decreased the fresh and dry biomass yields of silage maize by causing stress.

Fresh and dry biomass yields were 3 and 7.8% higher in zero tillage than in conventional tillage (Figure 2). The positive contribution of direct sowing to yield can be attributed to the effect of direct sowing on both preserving soil moisture and contributing additional organic matter and therefore nutrients to the soil. In zero tillage, yield increases as the crop residues protect the soil moisture content by shading, and the residues decompose and provide organic matter and nutrients to the soil (Nouri et al. 2019). da Silva et al. (2020) stated that the yield of maize increased by 17% in zero tillage compared to conventional tillage. Idowu et al. (2019) also indicated that the increase in fresh and dry biomass yields of silage maize was due to the stress-reducing effect of zero tillage by preserving soil moisture.

The lower blue, gray, and green water footprint values of silage maize in zero tillage practice compared to conventional tillage could be explained by the reduction in irrigation need due to the mulching effect of crop residues that effectively preserve soil moisture in zero tillage. Similarly, Raes et al. (2013) indicated that preserving soil moisture through various practices reduces the amount of irrigation and therefore crop water consumption, while Chukalla et al. (2015) stated significant effects of irrigation techniques, irrigation strategies, and mulching on green and blue water footprint reduction in irrigated agriculture. Lower evaporation and higher precipitation reduce the crop's need for blue water and increase the green water footprint (Aldaya et al. 2012). Therefore, in this study, the decrease in blue and gray water footprint and the increase in green water footprint in the second year compared to the first year in both tillage-sowing practices could be explained by decreased evaporation and increased precipitation in the second year (Figure 1).

The lowest blue water footprint per unit yield in zero tillage in the 2-year mean was determined in the W100 treatment as 2.8 m³ ton⁻¹ for fresh biomass yield and 9.2 m³ ton⁻¹ for dry biomass yield, while the values in conventional tillage were 2.9 and 9.7 m³ ton⁻¹, respectively (Figures 4 and 5). Considering the means of irrigation treatments, zero tillage had the lowest blue water footprint with 14.8 m³ ton⁻¹ for fresh biomass yield and 49.3 m³ ton⁻¹ for dry biomass yield, and the values were 9.8 and 15% lower than those for conventional tillage, respectively (Figures 4 and 5). This was a result of lower blue water consumption (Figure 3) besides higher fresh and dry biomass yields (Figure 2) in the W100 treatment in zero tillage.

Although the yield was the lowest, the W33 treatment, in which green water obtained from pre-season and during-season precipitation was used at a higher level, provided the highest green water footprint per unit of fresh and dry biomass yield. In zero tillage practice, both a higher yield and conservation of soil moisture gained from precipitation resulted in 4.7 and 16.4% lower green water footprints for per unit of fresh and dry biomass yields, respectively, compared to conventional tillage (Figures 4 and 5).

Since recycled wastewater was not used in F100 treatment, gray water footprint per unit yield could not be calculated; in contrast, W100 treatment, which has the highest gray water consumption and yield (Figures 2 and 3), resulted in the highest gray water footprints per unit fresh and dry biomass yields. Zero tillage reduced gray water footprint for unit of fresh and dry biomass yields, as both applied less amount of gray water in irrigations due to the conservation of water in the soil and higher fresh and dry biomass yields compared to those of conventional tillage. Considering the 2-year means, the lowest total water footprint per unit yield was realized in W100 and zero tillage treatments (Figures 4 and 5), but the total water footprint per unit fresh biomass yield in W100 was found to be statistically similar to the W67 treatment (Figure 4).

In this study, the green water footprints per unit yield under wastewater irrigation practices were determined to be higher than the blue water footprints (Figures 4 and 5). With the increase in blue water consumption in regions where water resources are insufficient, the intense consumption of freshwater resources limits production and agricultural sustainability (Mali et al. 2018). However, the sustainability of blue water resources is supported since the high green water footprint represents rainfall-based crop production (Novoa et al. 2019). Thus, more effective use of green water will ensure the protection of freshwater resources (Mekonnen & Hoekstra 2011a).

Hoekstra et al. (2011) stated that determining blue and green water footprints would be sufficient in the water footprint approach. Similarly, Ma et al. (2021) also reported that blue and green water footprints should be the focus when determining the water footprint of a product in agricultural production. However, Song et al. (2023) pointed out that gray water is one of the water footprint components in agricultural production and therefore should not be ignored. The use of recycled wastewater as a water source in this study supported the importance of determining the gray water footprint because it is an important result that the gray water footprint per unit efficiency in wastewater irrigation is higher than that of blue water. The sustainability of decreasing freshwater resources in the world and continuing agricultural production with higher efficiency is one of the most important issues of today (Yerli et al. 2023). Cakmakci & Sahin (2021) also drew attention to the continuity of blue water resources and agricultural production by reusing wastewater for irrigation.

Wang et al. (2014), in their study in which the water footprint of maize was determined for different regions of China, stated that the blue and green water footprints per unit yield of maize ranged from 10–710 to 510–1110 m³ ton⁻¹, respectively. However, Liang et al. (2020) pointed out that the total water footprint of maize in China ranged from 220 to 580 m³ ton⁻¹. In Buenos Aires, Argentina, the green water footprint per unit yield of maize was calculated as 314 m³ ton⁻¹ under rainfed conditions, while the green and blue water footprints under irrigated conditions were calculated as 224 and 58 m³ ton⁻¹, respectively, (Arrien et al. 2021). Ramezani Etedali et al. (2022) in their study in which they calculated the water footprint of maize in Qazvin Plain with two different approaches, stated that the blue and green water footprints per unit yield of maize ranged from 149–207 to 110–143 m³ ton⁻¹, respectively. While the total water footprint of maize in Brazil was reported to be 533 m³/ha (Abbade 2020), the mean blue and green water footprints per ton in Thailand were determined to be 237 and 894 m³, respectively (Sukumalchart et al. 2013). Esetlili et al. (2022), in their study conducted in the Kucuk Menderes Basin in Türkiye, reported that the blue, green, and gray water footprints per unit yield of silage maize ranged from 483 to 769 m³ ton⁻¹, 192 to 209 m³ ton⁻¹, and 99 to 146 m³ ton⁻¹, respectively. In a different study conducted in the Upper Tigris Basin of Türkiye, the blue and green water footprints per unit yield of silage maize were stated as 23 and 171 m³ ton⁻¹, respectively, (Muratoglu 2019). Mekonnen & Hoekstra (2011b) expressed that, as a world mean, the blue, green, and gray water footprints per unit yield of silage maize are 81, 947 and 194 m³ ton⁻¹, respectively. On the contrary, Chapagain & Hoekstra (2004) pointed out that the global total water footprint of maize is approximately 900 m³ ton⁻¹. The mean blue and green water footprints per unit yield of silage maize in Türkiye were determined as 37 and 24 m³ ton⁻¹, respectively, (Muratoglu & Avanoz 2021). As a result of water footprint calculations in Van province, where this study was conducted, the blue and green water footprints per unit yield of silage maize were 146 and 71 m³ ton⁻¹, respectively, (Yerli & Sahin 2022a), while another study based on different years for the same province revealed blue and green water footprint results of 99 and 48 m³ ton⁻¹, respectively, (Yerli et al. 2019). All these differences can occur with the climatic parameters of the region and changes of agronomic factors, irrigation practices, soil properties, water quality, fertilization, tillage, planting density, main and second crop status, and crop genotype. However, water footprint values obtained by processing numerical data instead of field study data can be seen as another important reason for the differences.

The study results showed that water footprints per unit of fresh and dry biomass yields of silage maize irrigated with wastewater can be reduced with different irrigation levels of wastewater and zero tillage practice. Therefore, it could be concluded that full irrigation with wastewater under zero tillage could be the most effective option for solving wastewater discharge problems and sustainability of freshwater resources by reducing blue, green, and total water footprint with less gray water footprint output per unit of fresh and dry biomass yield in silage maize. In addition, since real water consumption values were used in this study and the effective rainfall amount was not estimated, it can be stated that the accuracy of the values obtained under real production conditions is higher when compared to the estimated water footprint values of silage maize. Therefore, it can be suggested that calculations should be made with field study data in calculating the water footprint, especially in wastewater irrigation conditions.

The study was financially supported by the Scientific and Technological Research Council of Türkiye (TUBITAK) with project number 119O528.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.