A prospective study was carried out in which commercial greenhouses that used different water qualities (underground vs. reclaimed) with different production systems (organic vs. conventional) were selected.
2.1. Area Description
The study area was located on the coast of the Almería province, in southeastern Spain, within the 3 main production sub-areas for greenhouse horticultural crops in the province (Figure 1
The 3 study sub-areas included in the trial cover a total of 29,800 ha of greenhouses out of the total of 32,000 ha in the province of Almería [24
]. Sub-area A is the so-called Campo de Dalías region, the largest and oldest greenhouse area on the Spanish Mediterranean coast, concentrating 21,300 ha. Sub-area B is called Bajo Andarax and in this area 3000 ha is concentrated; it has the peculiarity that almost all reclaimed water is used for irrigation. Sub-area C is the denominated Campo de Nijar where 5500 ha is concentrated, and it is where the highest percentage of greenhouses in an organic production system is concentrated. The organic production system is regulated by the European regulation of organic farming (REGULATION (EU) 2018/848).
In the study area the climate is Mediterranean with mild winters and low annual rainfall; the average annual temperature and rainfall are 18 °C and 220 mm, respectively. The greenhouses are Almería-type (low-cost structures covered with plastic film, without active climate control systems, sand-mulched soil, and a drip irrigation system with non-compensating emitters) located on practically flat plots. The average surface area of the greenhouses is 7500 m2
in sub-area A and 7900 m2
in sub-areas B and C [25
2.2. Experimental Design
A stratified random sampling was carried out, classifying the greenhouse surface (number of hectares) into 2 groups according to the production system (conventional/integrated or organic), and in another 2 groups according to the quality of irrigation water used (groundwater or reclaimed water). In both cases, a stratification with proportional allocation was carried out (the number of sample elements of each stratum was directly proportional to the size of the stratum within the population).
To determine the size of each stratum both for the production system and for water quality, we used updated data on the greenhouse surface area in Almería, registering a total of 32,000 ha [24
]. This area is distributed according to the production system, with 3000 ha in organic production and 29,000 ha in conventional/integrated. Furthermore, it depends on the type of irrigation water used, with 2500 ha irrigated with reclaimed water and 29,500 ha irrigated with groundwater.
The sample size was determined in order to achieve a confidence level of 95% and a margin of error of 10%. The 32,000 ha of greenhouses with an average individual greenhouse area of 7500 m2
] were considered, resulting in a total of 42,000 sites. In addition, it was considered that each greenhouse should have an average of 1.5 irrigation units with an average surface of 5000 m2
per unit [26
]. In each unit, the average number of sub-units was 5, with an average area of 1000 m2
per sub-unit [26
], which is the common and representative size, resulting in a total of 320,000 irrigation sub-units.
The average length of drip lines was 25 m, and 90% of the greenhouse irrigation system had a pressure variation coefficient of less than 0.12, with 62% of the installations having a pressure variation coefficient of less than 0.06 [26
]. Regarding the filtering system, 95% of the greenhouses had screen filters, with 105 µm filtration level [26
A total of 88 greenhouses were sampled depending on the production system (80 conventional/integrated and 8 organic), and 88 greenhouses on the basis of the type of irrigation water used (81 with groundwater and 7 with reclaimed water). The choice of greenhouses was carried out randomly with random distribution within the production areas.
Sampling was performed during 4 summer seasons, after the end of the growing cycles, once the harvest remnants were removed from the greenhouse. The last season was fulfilled in the summer of 2018. Due to the large number of greenhouses sampled, it was necessary to distribute it over 4 summer seasons.
The greenhouses chosen are Almería-type (described above) and they had the standard irrigation framework of the greenhouse horticultural production system, 2 emitters m−2 of 3 L h−1, and a drip irrigation system with non-compensating emitters.
The reclaimed water used was provided by the “Cuatro Vegas” Irrigation District, distributor of reclaimed urban wastewater of the city of Almeria. The water source came from the urban wastewater reclaiming plant in the city of Almería (Southern Spain) (36°50′ N, 2°27′ W). It has been estimated that this plant treats 15 hm3
. In this plant, the primary treatment of wastewater is carried out by decanting the solids and breaking down the fatty emulsions. The next stage is the secondary treatment (biological) by activated sludge. After these treatments, the water is sent to the tertiary treatment plant, located 6 km away from the first plant. In this second treatment plant, the water undergoes a treatment using sodium hypochlorite, followed by a treatment a filtration system composed of 20 sand and anthracite filters of 2500 mm diameter that decreases the concentration of suspended solids and turbidity of the water. The chemical characteristics of the reclaimed water were suitable for irrigation (HCO3−
: 6.6 mM, Cl−
: 8.9 mM, N–NO3−
: 0.3 mM, H2
: 0.4 mM, N–NH4+
: 3.3 mM, Ca2+
: 4.8 mM, Mg2+
: 4.2 mM, Na+
: 9.1 mM, K+
: 0.6 mM). The water had low concentrations of inorganic contaminants (Cr: 14.22 µg L−1
, As: 4.84 µg L−1
, Cd: 0.04 µg L−1
, Pb: 1.79 µg L−1
, Ni: 3.08 µg L−1
, Mn: 13.17 µg L−1
, Cu: 9.82 µg L–1
, Zn: 37.76 µg L–1
). The average values of the physical and chemical parameters of the reclaimed water during the evaluation seasons are shown in Table 1
The emitters presented a manufacturing variation coefficient of less than 0.05, complying with the ISO 9261: 2004 standard that establishes that the emitters must have a manufacturing variation coefficient of the analyzed sample that does not exceed 0.07. For this reason, the influence on uniformity is very low.
2.4. Statistical Analysis
The DU data were analyzed by a randomized uni-factorial design, considering the production systems and the water quality used for irrigation (one-way ANOVA). For production systems, 2 treatments (organic and conventional-integrated) were considered, with the farm’s DU being the repetition. For water quality used for irrigation, 2 treatments (groundwater and reclaimed water) were considered, with the farm´s DU being the repetition. The data were also analyzed using a geostatistical analysis for the distribution of irrigation water. Kriging techniques were used to describe and model spatial patterns. With this procedure, we estimated the value of the variable studied in a two-dimensional region that was sampled in 16 locations. For each variable studied, the sample variogram was determined, as well as the estimated model variogram, along with the semi-variance in each lag. Statistical analyses were performed with Statgraphics 18 (2020 Statgraphics Technologies, Inc. The Plains, VA, USA).