1. Introduction
Conventional agricultural practices can cause a wide range of negative impacts on the environment. “Conventional” or “modern industrial agriculture” has been historically defined as the practice of growing crops in soil, in the open air, with irrigation, and the active application of nutrients, pesticides, and herbicides. Some of the negative impacts of conventional agriculture include the high and inefficient use of water, large land requirements, high concentrations of nutrients and pesticides in runoff, and soil degradation accompanied by erosion [
1,
2]. As the world population continues to grow at a rapid rate, so too must the food production. However, approximately 38.6% of the ice-free land and 70% of withdrawn freshwater is already devoted to agriculture [
3,
4]. To sustainably feed the world’s growing population, methods for growing food have to evolve.
The benefits of hydroponic agriculture are numerous. In addition to higher yields and water efficiency, when practiced in a controlled environment, hydroponic systems can be designed to support continuous production throughout the year [
5]. Hydroponic systems are very versatile and can range from rudimentary backyard setups to highly sophisticated commercial enterprises. Various commercial and specialty crops can be grown using hydroponics including tomatoes, cucumbers, peppers, eggplants, strawberries, and many more. Leafy vegetables, such as lettuce can also be grown hydroponically and perform best using the nutrient film technique (NFT) [
6]. Hydroponic NFT production involves the circulation of a nutrient solution through shallow channels in a closed-loop system [
7].
In 2012, in terms of production by weight, head lettuce was the second largest vegetable crop in the Unites States, second only to onions [
8]. A substantial portion of that production (approximately 29% in 2012) occurs in Arizona, primarily in Yuma [
8,
9]. Since Arizona devotes approximately 69% of its current freshwater withdrawals to agriculture [
10], investigations into hydroponic alternatives could be beneficial in reducing the strain on water resources in such regions. There is considerable research available regarding conventional lettuce production and hydroponic lettuce production separately, but few studies that have compared the resource inputs of the two at a commercial level.
Regarding conventional lettuce production, in 2001, the University of Arizona Cooperative Extension developed county-specific crop budgets estimating the operating and ownership costs of producing vegetables in Arizona using representative cropping operations and such resource inputs as water, fuel, and fertilizer [
11]. Realizing that the water and energy use for agriculture is substantial, Ackers
et al. (2008) [
12] performed an “order of magnitude” study and determined a reasonable range of estimates for resources used in the production of Arizona agriculture.
Regarding hydroponics, the Ohio State University developed an enterprise model designed to estimate the revenue, expenses, and profitability associated with a typical hydroponic greenhouse lettuce production system in Ohio [
13]. Various other authors have investigated components of hydroponic lettuce production as it relates to water and energy inputs [
14,
15].
The objective of this study is to determine whether hydroponic lettuce production is a suitable and more sustainable alternative to conventional lettuce production in Arizona. For this study, “a suitable and more sustainable alternative” is one that outperforms (i.e., is more efficient than) conventional agriculture in the metrics of land use, water use, and energy use, normalized by yield.
3. Results and Discussion
In terms of yield per area, the hydroponic production of lettuce in Arizona was found to be 11 ± 1.7 times greater than that of its conventional equivalent. Specifically, hydroponic lettuce production was calculated to result in a yield of 41 ± 6.1 kg/m
2/y (±standard deviation, SD, here and in the following), while conventional lettuce production was projected to yield 3.9 ± 0.21 kg/m
2/y (
Figure 1).
Figure 1.
Modeled annual yield in kilograms per square meter of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods (Error bars indicate one standard deviation).
Figure 1.
Modeled annual yield in kilograms per square meter of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods (Error bars indicate one standard deviation).
Water consumption between the hydroponic and conventional production of lettuce in Arizona was comparable on an area basis, but when normalized by yield the average was 13 ± 2.7 times less water demand in hydroponic production compared to conventional production. Specifically, hydroponic lettuce production had an estimated water demand of 20 ± 3.8 L/kg/y, while conventional lettuce production had an estimated water demand of 250 ± 25 L/kg/y (
Figure 2).
Results for energy consumption found that the hydroponic production of lettuce in Arizona requires 82 ± 11 more energy per kilogram produced than the conventional production of lettuce in Arizona. Dominating the hydroponic energy use are the heating and cooling loads at 74,000 ± 10,000 kJ/kg/y, followed by the energy used for the supplemental artificial lighting at 15,000 ± 2100 kJ/kg/y. The circulating pumps contributed the least to the total energy use at 640 ± 120 kJ/kg/y. In total, the hydroponic energy use was calculated to equal 90,000 ± 11,000 kJ/kg/y (
Figure 3).
The total energy use for the conventional production of lettuce in Arizona was calculated to be 1100 ± 75 kJ/kg/y (
Figure 3). This total was split between the energy use related to fuel usage at 330 ± 20 kJ/kg/y and groundwater pumping at 760 ± 74 kJ/kg/y.
Figure 2.
Modeled annual water use in liters per kilogram of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods (Error bars indicate one standard deviation).
Figure 2.
Modeled annual water use in liters per kilogram of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods (Error bars indicate one standard deviation).
Figure 3.
(a) Modeled annual energy use in kilojoules per kilogram of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods; (b) The energy use breakdown related to the hydroponic production of lettuce; (c) The energy use breakdown related to the conventional production of lettuce (Error bars indicate one standard deviation).
Figure 3.
(a) Modeled annual energy use in kilojoules per kilogram of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods; (b) The energy use breakdown related to the hydroponic production of lettuce; (c) The energy use breakdown related to the conventional production of lettuce (Error bars indicate one standard deviation).
The objective of this study was to determine whether hydroponic lettuce production is a suitable and more sustainable alternative to conventional lettuce production in Arizona. According to the results presented, summarized in
Table 1, while the hydroponic production of lettuce results in higher yields and more efficient water use, the controlled environment from which the hydroponic system produces its higher yields exhibits a higher energy demand.
Table 1.
Summary of modeled annual data with standard deviations (S.D.).
Table 1.
Summary of modeled annual data with standard deviations (S.D.).
Production Method | Yield (kg/m2/y) | Water Use (L/kg/y) | Energy Use (kJ/kg/y) |
---|
Value | S.D. | Value | S.D. | Value | S.D. |
---|
Conventional | 3.9 | 0.21 | 250 | 25 | 1100 | 75 |
Hydroponics | 41 | 6.1 | 20 | 3.8 | 90,000 | 11,000 |
Higher yields of hydroponics result from the controlled environmental conditions maintained within the hydroponic greenhouse, which allow for continuous production year round. These conditions also promote a reduction in the number of days required for each harvest cycle, allowing for multiple crops per year. This benefit of hydroponic production is not unique to lettuce alone, but will vary depending on the operational parameters under which the crop is grown.
Similarly, most hydroponic systems will utilize water more efficiently than conventional farming. The volume of water consumed per plant in a hydroponic system is not different from that grown using conventional methods; however, the hydroponic system delivers the water more efficiently, with a larger percentage of the water going to plant evapotranspiration [
26]. For example, lettuce has shallow roots, but is primarily irrigated through flood furrow irrigation in southwestern Arizona. Water not quickly absorbed by the roots is lost to percolation. Increases in the use of low-flow and more-targeted irrigation techniques could lower the overall water use of conventional farming.
As mentioned previously, most of the energy use for the hypothetical hydroponic greenhouse can be attributed to the heating and cooling loads. This is primarily due to the fact that the greenhouse was sited in Yuma, Arizona, an area which can have average temperatures of 34.7 °C in the summer and 14.1 °C in the winter [
25]. For illustrative purposes, the percent heating and cooling energy demand by month is presented in
Table 2. Greenhouses located in more moderate climates (
i.e., climates closer to the greenhouse set point temperature) will experience a lower energy demand. In fact, in certain climates heating and cooling systems may not be required, but instead replaced by a passive ventilation system, thus reducing the overall energy demand considerably. The feasibility of hydroponic systems is heavily reliant on the climate of farming locations.
Table 2.
Percent heating and cooling energy demand by month.
Table 2.
Percent heating and cooling energy demand by month.
Month | January | February | March | April | May | June | July | August | September | October | November | December |
---|
Average Temperature (°C) | 14.8 | 16.6 | 19.6 | 22.8 | 27.4 | 31.7 | 34.7 | 34.6 | 31.7 | 25.3 | 18.7 | 14.1 |
Percent Energy Demand | 12% | 8% | 6% | 1% | 5% | 10% | 14% | 14% | 10% | 2% | 6% | 13% |
The next highest use of energy for the hypothetical hydroponic greenhouse is for the supplemental artificial lighting, which is used to maximize crop yield and maintain consistent production year round [
7]. Some systems use supplemental lighting to create a 24-h photoperiod, especially during the first few days of plant growth, whereas others may use supplemental lighting for only a few hours a day [
5,
13]. In addition, small and low-output systems may not use artificial lighting at all. This study assumed that maximum yield was desired and did not perform a cost-benefit analysis of reducing or eliminating supplemental lighting. There are various studies that have tried to optimize supplemental lighting systems [
14,
27,
28]. Progress in this area of research could lead to improved energy efficiency for hydroponic lettuce.
Due to the high energy demands, at this time, commercial hydroponics is not a suitable alternative to conventional lettuce production in Yuma, Arizona. One possible way to make commercial hydroponics a more sustainable and suitable alternative would be to relocate the greenhouse to an area where there are cheap and renewable sources of energy, such as solar, geothermal or wind power; though keeping in mind that the initial investment required for such technologies may be cost-prohibitive. For instance, a hydroponic greenhouse could be sited in an area without arable soil, or on previously developed areas with impervious pavement, where land is cheap and could be used for photovoltaic solar panels. Assuming a conversion efficiency of 14% and an average daily solar radiation value of 6.5 kWh per square meter per year [
29] the ratio of greenhouse to solar panel area that would be required to offset the full energy demand of the hydroponic system is approximately 1:3.0, with an understanding that the 24-h production cycle along with seasonal and day-to-day changes in solar radiation would require the system to be connected to the electrical grid. This is still an improvement over conventional production, which uses ten times more land and water on a yield basis and requires arable land. Eliminating the need for arable land has other benefits including versatility in system siting and a potential reduction in the distance in which food must travel. Performing a life cycle assessment that considers the environmental impact of food transportation could show whether this benefit of hydroponics is significant.
An alternative scenario to consider is one in which much of the energy-intensive characteristics of advanced and commercial hydroponic operations are abandoned in favor of simpler systems, a practice known as “simplified hydroponics” or “popular hydroponics.” These operational changes come at the cost of total yield; however, such systems can still out-perform conventional systems by a factor of three to four on an area basis [
30]. The Food and Agriculture Organization of the United Nations maintains that such systems can be as small as one square meter in size, but that most household simplified hydroponic gardens range between 10 to 20 square meters in size. Communities have also adopted simplified hydroponic gardens at sizes greater than 200 square meters—a scale in which it becomes viable to sell excess produce for income [
30].
There are several limitations to this study that need to be considered. To begin, the calculations for the conventional production of lettuce assumed that only one crop of lettuce can be grown in a given year. While this might be true, much of the land used to produce lettuce is dedicated to warm season crops when lettuce is not in production. It’s possible that these crops have water and energy requirements that differ from lettuce. Additionally, the hydroponic greenhouse was assumed to be of commercial scale and was optimized for maximum production. Furthermore, as seen in Equation (2), the greenhouse was assumed to be of a specific size and located in Yuma, Arizona (one of the most productive areas for lettuce grown conventionally in the United States). In reality, hydroponic greenhouses come in a variety of configurations and can be located almost anywhere. As such, it would be reasonable to model multiple hypothetical greenhouses. Changing the assumptions surrounding the hypothetical greenhouses could produce alternative results. In addition, this study solely focused on direct energy inputs to hydroponic and conventional lettuce production and did not consider energy embodied in chemical or material inputs. Performing a full life cycle assessment of hydroponics vs. conventional lettuce production could also produce alternative results, with labor hours potentially figuring prominently into the economic equation. Lastly, this study did not discuss additional factors that might inhibit the successful implementation of hydroponics, such as energy scarcity and higher upfront capital costs.