1. Introduction
Cotton (
Gossypium hirsutum L.) is the main raw material for the international textile industry, the most valuable natural fiber, and the second most important oil-seed crop worldwide [
1,
2]. Cotton yield is affected by edaphoclimatic constraints, genotypes, and crop management practices. Cotton cultivation is predominantly rainfed in most of the producing regions of the world, including Brazil [
3,
4]. Because of the high crop water demand, the water deficit caused by constant droughts in semi-arid regions is the main factor limiting high yields.
Irrigation is important to guarantee the sustainability of production in regions most susceptible to water deficit, especially when associated with efficient water consumption and economic viability [
5,
6,
7,
8]. However, cotton has a relatively long cycle and, when grown under full irrigation, it demands large amounts of water [
9]. The average irrigation requirement for surface-irrigated cotton is reported to be 6000–7000 m
3 ha
−1 depending on soil, weather conditions, and seasonal rainfall [
10,
11]. In the Brazilian semi-arid region, it was determined that the evapotranspiration of cotton (ETc) varied as a function of crop phenology with an average water requirement of 3.8 mm day
−1 during emergence, 5.0 mm day
−1 during vegetative growth, 5.9 mm day
−1 during the reproductive phase, and 5.4 mm day
−1 during the maturation period [
12].
In some parts of the world, as in some cotton producing areas of the United States of America, the irrigation requirement is low or may not be needed, due to the adequate distribution of rainfall during the growing season. However, several producers utilize supplemental irrigation (SI) to minimize drought stress, decrease risk, and improve yield stability across a range of environmental conditions [
13]. The requirement for SI is obviously higher in semi-arid regions, due to recurrent droughts and long periods of dry spells, which tend to worsen because of global climate change [
14].
Water shortage is one of the main factors that contribute to the reduction of productivity of crops, and the use of alternative sources of water for irrigation is an option to minimize water stress and yield losses [
11,
13,
15]. Besides, frequent droughts in semi-arid regions of the planet and excessive pumping of groundwater to meet the water requirements for irrigated crops have compromised the sustainability of agricultural production systems [
16]. These concerns have led fabric manufacturers to adopt environmentally sustainable cotton production technologies to minimize the use of water resources and reduce environmental pollution [
11,
17].
The possibility of using treated wastewater for irrigation of crops with high water demand such as cotton has been evaluated as a viable alternative to preserve natural water resources while mitigating the effects of water scarcity [
18,
19], notably for future climate change scenarios. Given the above, municipal treated wastewater (MTW) is a sustainable strategy to increase irrigated cotton areas, save fresh water, and maintain riparian ecosystems stressed by drought and overuse of underground water. Moreover, MTW represents a promising opportunity to meet the cotton crop mineral nutrient demand [
19] and to mitigate the current world crisis associated with the high prices of fertilizers.
Cotton farming in the Brazilian semi-arid region, even under adverse edaphic and climatic conditions, has greater profitability when compared to other crops [
20]. The main concern for cotton cultivation in semi-arid regions is water scarcity, a determining factor for low productivity rates. In this way, supplemental irrigation in rainfed farming, using the treated wastewater, should become a decisive factor for crop sustainability in semi-arid regions, increasing productivity and improving water use efficiency.
The objective of this research was to evaluate the productivity, fiber quality, and profitability of cotton cultivation in response to supplemental irrigation with treated wastewater, seeking to evaluate the productive and economic sustainability of this crop system under a tropical semi-arid climate. The treatments simulated different water availability scenarios of the rainfed farming system of the Brazilian semiarid region, with and without chemical fertilization.
2. Material and Methods
The experiment was carried out from September to December 2020, in an area adjacent to a municipal wastewater treatment station of CAGECE (Company of Water and Sewerage of the State of Ceará), in the municipality of Russas (4°56′25″ S, 37°58′33″ W, altitude 20.5 m), Ceará, Brazil. Russas (
Figure 1) is situated in a low-precipitation region afflicted by prolonged droughts and prone to desertification with a typical vegetation type classified as
Caatinga (“white forest” or “white vegetation” in Tupi language). The climate of Russas is tropical semi-arid, according to the Köppen’s classification, with an average annual rainfall of 745.7 mm and a mean temperature of 27 °C.
The experimental assay was installed in a randomized complete block design, with treatments arranged in split plots with four replications. Plot experimental design was selected depending on three water scenarios: normal, drought, and severe drought. These scenarios were defined based on a 30-year historical series of precipitation data provided by the Foundation for Meteorology and Water Resources of Ceará (Funceme). This series was defined for the municipality of Russas, Ceará, Brazil, for the rainfed farming season in the region (February to May). The subplots were composed of either the provision or lack of supplemental irrigation with treated wastewater and the sub-subplots by the presence or absence of NPK fertilization. Each experimental unit (sub-subplot) was formed by four rows of plants 4.5 m in length.
Seeds of the cotton cultivar BRS 433, provided by the Secretariat of Economic Development and Labor of the State of Ceará (SEDET), were sown at a spacing of 0.7 × 0.3 m, with two plants per hole. Fertilization was provided by N, P
2O
5, and K
2O applied at doses of 60, 60, and 50 kg ha
−1, respectively [
21], defined according to soil analysis and regional recommendations for cotton. The chemical analysis of the soil before the experimental setup is presented in
Table 1. The determinations were performed according to methodologies recommended by [
22]. The soils were classified as Planosol (Alfisol).
A drip irrigation system was used, consisting of a centrifugal electric pump of 0.5 HP, single-phase current, equipped with double suction with a valve to control the alternate capture of both freshwater and treated wastewater, the first stored in a tank of 2000 L and the second collected in a pre-molded ring well. Irrigation system consisted of a drip tape with a flow rate of 1.6 L h−1 and a spacing of 0.3 m between emitters.
The water sources used were from the municipal supply network of the city of Russas (freshwater; FW) available after the municipal wastewater passes through a series of sedimentation and stabilization ponds belonging to the Company of Water and Sewage Treatment of Ceará State (CAGECE). The chemical characteristics of freshwater and MTW (
Table 2) were obtained employing methodologies recommended by [
23].
The rainfall simulation was carried out with freshwater, while the supplemental irrigation with water from the stabilization pond (MTW). The total water depths applied during the cotton cycle are shown in
Figure 2. According to the historical series of precipitation data, the average for a normal wet-year scenario is 466.2 mm, from February to May (rainy season). For the drought and severe drought scenarios, the average values are 326.2 and 206.2 mm for the same period, reaching average reductions of 30.0% and 55.7%, respectively.
At 80 days after sowing, the stage that begins with the formation of cotton bolls, the shoots’ dry mass and leaf gas exchange parameters were evaluated. Measurements of stomatal conductance (gs, mol m−2 s−1) and photosynthesis rate (A, μmol m−2 s−1) were performed using a portable Infrared Gas Analyzer (model LI-6400XT, LI-COR, Lincoln, NE, USA), with a light intensity of 1800 μmol m−2 s−1. The readings were taken in the morning, between 08:00 and 10:00 a.m., on fully expanded leaves of each plant under ambient conditions of temperature and relative humidity.
After maturation, when reaching the harvest time, two manual harvests were carried out at 106 and 126 days after planting. The following production variables were determined: seed yield (kg ha
−1), lint yield (kg ha
−1), total yield, and physical water productivity (PWP, kg m
−3), obtained by the ratio between the total yield and the total volume of water applied (simulated rain with fresh water plus supplemental irrigation with MTW) in each treatment, according to Equation (1) [
24]:
The efficiency of supplemental irrigation (WUE
SI) was estimated by the ratio between the increment in total yield and the volume of water applied in supplemental irrigation, according to Equation (2) [
25]:
where Y
SI and Y represent the total yields (seeds plus lint) of plots with and without supplemental irrigation, respectively.
The technological quality of cotton fiber was measured through a sample of 20 bolls randomly collected in the middle third of plants in the observation area of the plot. As recommended by [
26], the following variables were analyzed: fiber length (UHM, mm), fiber uniformity (UNF, %), short fiber index (SFI, %), and Micronaire index (MIC, µg in
−1). The technological variables were determined at the Fibers and Yarns Laboratory of Embrapa Algodão (Campina Grande, Paraiba, Brazil), using HVI (High Volume Instruments) model 900 from Spinlab/Zellweger Uster.
Crop yield, PWP, technological quality of fiber, stomatal conductance (gs), photosynthetic rate (
A), and shoots dry mass data were submitted to analysis of variance and the means were compared by Tukey’s test at 0.05 probability. Statistical analyses were performed using the Sisvar software version 5.6 [
27].
Economic analysis was performed using current values (May–July 2022) to estimate gross revenue and costs (fixed, variable, and equipment depreciation), expressed in Brazilian Real (BRL, Brazilian currency). Crop yield data were used to estimate gross revenue using the cotton price (seed plus lint) of 4.67 BRL kg−1 (1 BRL = 0.20 USD). For the economic analysis, fixed and variable costs and equipment depreciation were used. Fixed costs for 1.0 ha were used for all treatments with supplemental irrigation. The total fixed cost amount was divided into ten years, as the farmer has the option to finance the agricultural inputs during this period. The farm equipment depreciation was calculated by dividing the investment needed for implementation by the useful life of each piece of equipment, which was estimated as ten years. The final zero residue method at the end of its useful life was used in the calculation.
It was considered that the costs were financed by
Banco do Nordeste do Brasil (BNB), using the investment credit line called
Pronaf Mais Alimentos (National Program to Strengthen Family-based Agriculture), simulating a ten-year contract with an interest rate of 3% per year with no grace period, seeking to get as close as possible to the reality of the farmer. The calculation of the added value was performed according to the methodology described by [
28,
29]. The added value of the production systems was obtained for 1.0 ha of production and values are expressed in BRL, according to Equation (3):
AV: added value
GVP: gross value of production
FC: fixed costs associated with the production system
VC: variable costs associated with the production system, excluding labor
D: depreciation of equipment and facilities
A linear relationship (AV = ax + b) was used to calculate the added value from 2.0 to 5.0 ha, with the ordinate axis being the added value and the abscissa axis represented by the agricultural area. In the linear model, the marginal contribution per unit of equivalent area is represented by “a” and the fixed capital necessary to implement the production system is represented by “b.
The farmers’ income for 1.0 ha was estimated, according to Equation (4), with all values expressed in BRL:
FI: farmer’s income
AV: added value
I: interest paid to the bank or other financial agent
S: salaries paid to the labor force
T: taxes and tariffs paid to the state in BRL
To calculate the farmer’s income from 2.0 to 5.0 ha, linear models were elaborated (FI = ax + b), which describe the variation of farmers’ income (FI) in the different treatments concerning the planted agricultural area per unit of work. In this model, the marginal contribution of income concerning the area is represented by “a” and the fixed expenses to implement the system of production is represented by “b”.
The level of social reproduction (LSR) of each production unit for a planted area was also accessed. The LSR is related to the income necessary for social reproduction based on the minimum wage, which was adjusted to 1212.00 BRL according to the Provisional Measure of the Federal Government of 2 January 2022. In this case, the LRS value represented in the graph refers to the semiannual (six-month) minimum wage of 7272.00 BRL (US$ 1454.40), considering that in the second semester the farmer will carry out some other activities to obtain his income.
4. Conclusions
Our results show the importance of using treated wastewater in the supplemental irrigation of cotton under scarce-water scenarios in tropical semi-arid regions, as evidenced by improved plant photosynthetic responses, growth, yield, and farmer’s estimated profitability. The frequent dry spells during the drought and severe drought scenarios imply greater use of supplemental water depth and, consequently, a greater addition of nutrients, such as N, P, K, Ca, and Mg, to the soil, ensuring higher yields, high water use efficiency, lower fertilizer costs, and higher income for farmers. As an example, under the Severe Drought scenario, the highest income for the farmer was obtained with the use of supplemental irrigation without mineral fertilization. On the other hand, the lack of supplemental irrigation, mainly during the drought and severe drought scenarios caused decreases in cotton lint yield of 52% and 65%, respectively. The use of supplemental irrigation increased cotton productivity in all three scenarios (normal, drought, and severe drought) by approximately 29%, 255%, and 251%, respectively, indicating that even during a normal average wet year the water demands by the cotton crop are not completely satisfied by rainfall. The use of treated wastewater also increased fiber length, uniformity, and short fiber index. The fact that the electrical conductivity of municipal treated wastewater was approximately 2.5-fold higher than the control freshwater did not seem to hinder the growth of cotton plant or crop yield.
It is worth noting that the acceptance of wastewater as an alternative irrigation source requires not only good results in productive and economic terms, but also the certainty that the existence of environmental risks has been thoroughly evaluated. Thus, future studies need to assess the real water potential of wastewater treatment ponds in the Brazilian semi-arid region, as well as the concentrations of emerging contaminants including heavy metals, antibiotics, and microorganisms that may cause environmental damage or health hazards concerns, if the water is used for the production of consumable products, but not so much for cotton. This set of data will add to the knowledge needed for the evaluation of the productive, economic, and environmental sustainability of supplemental irrigation with treated wastewater under a tropical semi-arid climate.