Authors: Ruiz, N.1, Miranda, L.2, Gómez-Mora, J.A.2, Benjumea, E.M.2, Cordero, J.2, Gavilán, P.1 1 Institute of Agricultural and Fisheries Research and Training (IFAPA). “Alameda del Obispo” Center. Ave. Menéndez Pidal, s/n. 14004. Córdoba. 2 IFAPA. Huelva Center. Experimental Farm El Cebollar. Hwy. Ermita-Montemayor, HU-3110 km. 4.5. Moguer (Huelva).
Introduction
Strawberry Collection of water samples for analysis of nutrient elements in drainage.
Water and nitrogen are essential resources for agricultural production, but their inefficient use compromises sustainability. In Andalusia, where agriculture consumes around 78% of the available water (Fundación Centro de Estudios Andaluces, 2007), water scarcity and nitrate pollution have increased significantly, reaching 25% of the territory classified as a vulnerable zone to nitrate contamination (Junta de Andalucía, 2020).
The intensive cultivation of berries in Huelva, a strategic sector at the national level with more than 12,500 ha cultivated, faces these environmental challenges despite its high degree of technification and its economic and export relevance (more than 1,400 million euros in 2023/2024, Junta de Andalucía, 2024).
In this context, it is a priority to implement tools that allow evaluating irrigation and fertilization efficiency, as well as life cycle analysis, ISO 14040 (Romero-Gámez and Suárez-Rey, 2020), or the Nitrogen Footprint (Liu et al., 2025). Among the existing indicators, the Water Footprint (ISO-14046, 2014) developed by Hoekstra et al. (2011) stands out.
This indicator quantifies the volume of water used and the impact associated with pollution, differentiating between blue, green, and grey water footprints. The latter estimates the volume of water necessary to dilute pollutants to acceptable levels (Franke et al., 2013).
Global studies on the water footprint have shown that plant production represents 67% of the total water footprint, making it the largest contributor to global water consumption (Hoekstra and Mekonnen, 2012). Extensive studies have been conducted on the water footprint in agriculture in many crops and regions (Mekonnen and Hoekstra, 2011; Chapagain and Hoekstra, 2010; Sarafi et al., 2024), but most of them assume theoretical values, perform simulations (Mialyk et al., 2024), or simply do not consider the grey water footprint (Lovarelli et al., 2016).
Therefore, this study addresses the determination of the water footprint using experimental field data, including direct measurements of nitrogen leaching and crop evapotranspiration, with the aim of evaluating fertigation management in a crop of high economic relevance in Spain.
Methodology
The study used experimental data from IFAPA trials at the Experimental Farm “El Cebollar” in Moguer (twenty-one cases in five seasons; scenario B) and from commercial plots in Almonte (three cases in three seasons; scenario A), with controlled agronomic management and farmer management, respectively. Sandy soils predominated (Almonte) and sandy-loam soils (Moguer). The crop was established under macrotunnel, in 5–6 mulched beds, with drip irrigation, planted in October with two crop rows per bed and a cycle lasting until May–June.
Crop evapotranspiration (ETc) was estimated using water balance (Allen et al., 1998), calculating the difference between irrigation and drainage, since precipitation, runoff, capillary rise, and soil moisture variation were assumed to be zero due to the crop being under tunnel, drip irrigation, sandy soils (and crop located high on the bed), and high-frequency irrigation, respectively.
Moisture probe on a strawberry bed.
The irrigation considered corresponds to the fertigation phase, not including that required for bed formation and planting support. Irrigation was quantified with flowmeters and drainage with buried lysimeters, the latter also used to determine leached nitrogen. Irrigation applied in the trials was scheduled considering water needs based on reference evapotranspiration (ETo) and the strawberry crop coefficient (Kc) (Allen et al., 1998; Lozano et al., 2016).
ETo inside the greenhouse was estimated using a model based on solar radiation (Fernández et al., 2010), applying the methodology proposed by IFAPA, which uses the weather forecast from the Spanish Meteorological Agency (AEMET) (Gavilán et al., 2015). For this, the Riego Berry application was used (Gavilán et al., 2024). At “El Cebollar”, since 2020 an automatic irrigation system was also used, based on measuring volumetric soil moisture with RK520-02 probes (Rika Sensors) at 10 and 30 cm and EP100G-04 probes (EnviroPro, Precision Soil Probes) with sensors at 5, 15, 25, and 35 cm. The system activated irrigation based on preset soil moisture thresholds. In commercial plots, irrigation was ordered by the farmer.
Applied and leached nitrogen was measured with LAQUAtwin portable sensors (Horiba Ltd., Kyoto, Japan). Applied N measured was highly correlated with N incorporated in fertilizer and water (data not shown). Efficiency and productivity indicators were calculated: irrigation efficiency (ETc/R), N efficiency (N leached/N applied), water productivity (yield/irrigation), and N productivity (yield/N applied).
The water footprint was evaluated exclusively for the production period, considering its green, blue, and grey components. The green footprint (rainwater used in evapotranspiration) was considered negligible due to the macrotunnel system. The blue footprint was calculated as the consumptive use of water per unit of yield, and the grey footprint according to leached N (Aldaya et al., 2010; Hoekstra et al., 2011) and the theoretical volume needed to dilute it.
Strawberry
Where: ETc blue is crop evapotranspiration due to Blue Water (mm/day); n is the number of days of the cycle; Nlix is leached nitrogen (kg/ha); Cmax is the maximum allowed N concentration (kg/m³) of 10 mg/l of N according to the US Environmental Protection Agency; Cnat is the natural concentration of the pollutant (kg/m³), using a value of 0 as recommended by Aldaya et al. (2010). Finally, Y is yield (kg/ha).
Results
Volumetric irrigation counter and collection of samples for nutrient analysis in irrigation water.
In commercial plots (A), irrigation applied during fertigation was higher (4,900–6,985 m³/ha) than in trial plots (B), where the average was 4,600 m³/ha over five seasons (Table 1).
Drainage was also higher in A (average 3,180 m³/ha) than in B (731 m³/ha), reflecting greater water inputs. Applied N reached higher values in A (277–293 kg/ha) compared to B (<184 kg/ha), and leached N was slightly lower in B due to lower irrigation and fertilization.
ETc ranged between 2,544 and 4,530 m³/ha (Table 1), influenced by cycle duration, climatic conditions, varieties, and fertigation management (Gavilán et al., 2024).
Commercial plots (A) showed irrigation efficiencies <54% due to excess irrigation (Gavilán et al., 2024), while trial plots (B) showed ≥81% thanks to water balance–based management and automation. Nitrogen fertilization efficiencies were higher in A, although with low representativeness due to lack of replication; in general, N efficiency depended on dose and drainage, being higher with lower percolation.
Yields (Table 1) were lower in A (57–79 t/ha) compared to B (78–104 t/ha), resulting in lower average water and N productivities in A (11.3 kg/m³ and 246 kg/UFN) versus B (19.1 kg/m³ and 627 kg/UFN).
The blue footprint (Table 2) was similar in both scenarios (0.04 m³/kg), depending on fertigation management and efficient water use by variety (Martínez-Ferri et al., 2016). The grey footprint predominated (75% in A and 67% in B), determined by leached N and associated with dose, drainage, and yield; it decreased with higher yields and lower leaching. Its average value was 0.08 m³/kg in B, dropping to 0.07 m³/kg after reducing N in recent seasons (Table 1), and an average of 0.12 m³/kg in A. Total footprint reached 0.16 m³/kg in A and 0.12 m³/kg in B.
Discussion
The study surpasses traditional indicators (productivity) based on applied water, incorporating blue footprint (consumptive use) and grey footprint, allowing the relationship between yield and actual evapotranspiration and evaluating the environmental impact of fertigation. A notable contribution is the direct measurement of leached N and drainage using lysimetry, uncommon compared to estimations (Mialyk et al., 2024), although dependent on correct functioning and annual reinstallation of lysimeters. Variability in irrigation, fertilization, and yield allowed obtaining representative values.
Compared with previous works, the blue footprint values of the study (0.04 m³/kg) are lower than those reported by García-Morillo et al. (2015) (0.06–0.08 m³/kg total footprint without grey). Aldaya et al. (2010) suggested 0.14 m³/kg total without grey footprint and Adams (2009) 0.17 m³/kg. Mekonnen and Hoekstra (2011) estimated 0.109 m³/kg (blue) and 0.037 m³/kg (grey). Discrepancies are attributed to methodological approaches: previous studies rely on public statistics or national-scale estimates, while this work uses empirical field data.
The main strength lies in the use of direct measurements of ETc, yield, nitrates, and drainage through lysimetry, allowing a more precise and representative estimation of real management conditions.
Conclusions
The study quantified the blue and grey water footprint of strawberry cultivation in southern Spain as an indicator of water–fertilizer management efficiency, considering water consumed and water affected by pollution during fertigation. The grey footprint was the predominant component, linked to nitrogen management, and the blue footprint showed similar values in all cases; total footprint ranged between 120–160 l/kg. The approach was based on empirical field data (ETc, yield, nitrates, and drainage through lysimetry), allowing a precise and locally representative estimation of the environmental impact associated with water and fertilization management.
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