Economic Viability of Ultrasonic Sensor Actuated Nozzle Height Control in Center Pivot Irrigation Systems

: The coverage of center pivot irrigation systems used around the world has increased. One potential factor driving their adoption is improved water application efﬁciency relative to some other sprinkler or surface irrigation approaches. Center pivot irrigation systems may be further improved by dynamic elevation spray application (DESA). DESA systems adjust the nozzle height in response to plant growth and canopy heterogeneities. The DESA approach is relatively new and there is uncertainty in its economic viability and worthiness of further investigation. Thus, an economic scenario analysis was performed to explore the potential economic beneﬁts of DESA based on permutations of irrigation pivot efﬁciency without DESA, water-saving potential of DESA, and water cost. The weighted costs and beneﬁts of the height-adjusted approach for a set of water cost savings scenarios showed the net return price with the water cost savings per season. We show that DESA could have economic viability at current component costs and is worthy of further investigation and reﬁnement. Author Contributions: Conceptualization, C.H., H.A.A.-a., and D.M.M.; methodology, H.A.A.-a., and D.M.M.; H.A.A.-a., D.M.M.; validation, H.A.A.-a., D.M.M.; formal analysis, H.A.A.-a. D.M.M.;


Introduction
Center pivot irrigation systems, invented in the mid-20th century, are now used around the world in field cropping systems. There has been an evolution of the center pivot designs driven by a need to increase irrigation efficiency and reduce costs by minimizing water loss, design, and installation costs [1]. For example, the mid-elevation spray application (MESA), the low-elevation spray application (LESA), the low-energy precision application (LEPA), variable irrigation rate (VIR), and mobile drip systems have been developed to increase application efficiency and reduce energy [1]. Ortiz et al. used two types of sprinkler irrigation systems (rotating spray plate sprinklers and fixed spray plate sprinklers) to study evaporation and drift losses [2]. They installed systems 1 m and 2.5 m above the ground and found significant evaporation and drift losses with fixed spray plate sprinklers at 2.5 m compared to the rotating spray plate sprinklers at 1 m. Another study found lower evaporation and drift losses when the sprinklers are near the ground [3].
The initial cost of specialized systems can be higher than that of traditional center pivot systems [1,[4][5][6][7]. Hall and Lacewell assessed the relative economic costs and benefits of LEPA, surface drip irrigation (SDI), and furrow irrigation (FI) with an annual budgeting method and showed that LEPA was the most economical method [8]. Other studies have concluded that SDI is the most economical approach on smaller farms; the finding has been attributed to lower investments costs per hectare, lower pumping costs, and Agronomy 2022, 12, 1077 2 of 12 increased yields [9,10]. Contemporary efficiency gains were achieved by integrating center pivot systems with next-generation emitters, and by altering the fixed height of those emitters [1,11]. Such adaptations and design improvements demand substantial economic and resource gain [4]. Dhuyvetter, Lamm, and Rogers demonstrated the economic benefits of center pivots over other systems [12,13]. Technology adoption and innovation can be a pathway to support sustainable water management and reduce food shortages [14] and save energy [15]. Center pivot innovation can be a part of a collective effort to close the sustainability gap, which will also include multiple approaches such micro-irrigation adoption [16,17], minimal climate change impacts, reduced food waste, and additional transformative innovations such as smart greenhouses and agrivoltaic systems [18].
In this paper, we evaluate a next step in the evolution of center pivots: dynamically adjusting the emitter position relative to the plant canopy to minimize losses in a real-timedynamic elevation spray application (DESA) [19,20]. We evaluate the economic feasibility of DESA through a cost analysis that considers three water savings assumptions to determine. Economic constraints informed the design decisions and dictate low-cost components to be used. The technical feasibility of DESA is then assessed by exploring the viability of a low-cost sensor to provide the key data input into the feedback-control cycle: nozzle height relative to the crop canopy. We hypothesized that a DESA center pivot with advanced sensing and nozzle height control would improve water distribution and management and has the potential to increase profitability.
The objective of the present study was to evaluate the economic and technical feasibility of DESA. This was achieved by performing an economic analysis for the feasibility of the deployment of such sensors for various cropping systems and comparing the net returns/gains with those of a conventional system.

Cost Estimates
The base case center pivot (CP) is a 305 m device irrigating 33 ha (81 acres) operating at 41.4 kPa. Electrical inputs are evaluated using the state commercial electricity supply (USD 0.1/kW-hr). The total time required to apply 1.27 cm over 33 ha was calculated as 33 h. The financial analysis considered (a) power requirements, (b) set up (CP system) and DESA assembly (present study), (c) sprinkler system, (d) pumps, and (e) bore well. The costs are provided in Table 1. The total calculated annual cost included the total fixed cost (assets and infrastructure) and the total operational and maintenance costs including repair and replacement, water costs, and energy costs. Annual asset depreciation was calculated using straight-line depreciation. The net present value (NPV) sensor cost and its integration into the conventional setup were calculated and recorded annually at a discounted rate of 12.5%. Interest rates for the various assets were based on specified figures and were calculated as the mean of the initial cost plus the salvage value multiplied by the rate of interest (i.e., 8.5% on assets and 1.4% on taxes and insurance). In addition, the cost of the assembly was computed to determine the final cost of the base system with and without DESA. The initial center pivot Agronomy 2022, 12, 1077 3 of 12 system cost to cover 33 ha would be USD 104,000. The next step was to include the water savings to determine the final cost of including DESA into the base system.

DESA Components, and Low-Cost Sensor Test
DESA-enabled pivots would require that each nozzle be associated with a sensor, a microprocessor, and a motorized means to adjust the position of the nozzle head. DESA connects to a microcontroller, like Arduino [21]. Ultrasonic distance sensors (model HC-SR04) have an established work history with Arduino in agricultural and environmental applications. Al-agele et al. [20] presented the fully constructed DESA prototype. The Arduino UNO R3 Mega 2560 (<USD 15) was selected for sensor design and control for the present study. The sensor is positioned 0.2 m above the nozzle to protect the sensor from the water, and all electronic parts are secured inside high-density polymer enclosures. This basic design contains a flange, 0.019 m (45 degree) elbow, 0.077 m (90 degree) elbow, PVC expansion fitting, with additional parts made by a laser cutter and a 3-D printer. The total cost of the components used to construct the prototype was USD 72, with about half of that cost allocated to the DC motor. A DESA design and full breakdown of the component costs is presented in Figures 1 and 2. 8.5% on assets and 1.4% on taxes and insurance). In addition, the cost of the assembly wa computed to determine the final cost of the base system with and without DESA. Th initial center pivot system cost to cover 33 ha would be USD 104,000. The next step was include the water savings to determine the final cost of including DESA into the base sy tem.

DESA Components, and Low-Cost Sensor Test
DESA-enabled pivots would require that each nozzle be associated with a sensor, microprocessor, and a motorized means to adjust the position of the nozzle head. DES connects to a microcontroller, like Arduino [21]. Ultrasonic distance sensors (model HC SR04) have an established work history with Arduino in agricultural and environment applications. Al-agele et al. [20] presented the fully constructed DESA prototype. The A duino UNO R3 Mega 2560 (<USD 15) was selected for sensor design and control for th present study. The sensor is positioned 0.2 m above the nozzle to protect the sensor fro the water, and all electronic parts are secured inside high-density polymer enclosure This basic design contains a flange, 0.019 m (45 degree) elbow, 0.077 m (90 degree) elbow PVC expansion fitting, with additional parts made by a laser cutter and a 3-D printer. Th total cost of the components used to construct the prototype was USD 72, with about ha of that cost allocated to the DC motor. A DESA design and full breakdown of the comp nent costs is presented in Figures 1 and 2.

Calculation of Water Losses in the Center Pivot Irrigation System
A typical 33 ha center pivot irrigation system may apply 160 cubic meters of water per irrigation event [22] and up to 132,000 cubic meters per season. These amounts do vary by crop and local on-the-ground conditions. Water pumping costs also vary depending on water source location with particular differences between rivers and wells [23]. A center pivot applies this water with an approximate water application efficiency of 85% [24] with the 15% water loss caused by wind drift and evaporation. Water loss by wind drift comprises the majority of losses and accounts for 50-70% [25] of the lost water fraction, see Table 2. Thus, our later analysis explores the range of these values, e.g., 50%, 60%, and 70% wind drift loss percentages are explored.
The total irrigation water loss per area was calculated as the water loses percentage from wind draft times application efficiency losses in the center pivot irrigation times total water applied per area [24,25]. The wind drift losses of center pivot irrigation may be reduced by DESA, which would result in higher application efficiency. This is achieved by adjusting the relative position of the droplet nozzle automatically and reducing the distance between the plant canopy and the nozzle. Potential water savings can be projected by assuming a reduction in wind drift losses for each of the considered scenarios: 50%, 60%, and 70%. This exploration of the potential scenarios is presented in Table 3. DESA cannot eliminate the losses due to wind drift, only reduce them. To compute the potential water savings, we explored the potential of DESA to reduce wind drift by 15%, 25%, 35%, 45%, 55%, and 65% to save

Calculation of Water Losses in the Center Pivot Irrigation System
A typical 33 ha center pivot irrigation system may apply 160 cubic meters of water per irrigation event [22] and up to 132,000 cubic meters per season. These amounts do vary by crop and local on-the-ground conditions. Water pumping costs also vary depending on water source location with particular differences between rivers and wells [23]. A center pivot applies this water with an approximate water application efficiency of 85% [24] with the 15% water loss caused by wind drift and evaporation. Water loss by wind drift comprises the majority of losses and accounts for 50-70% [25] of the lost water fraction, see Table 2. Thus, our later analysis explores the range of these values, e.g., 50%, 60%, and 70% wind drift loss percentages are explored. The total irrigation water loss per area was calculated as the water loses percentage from wind draft times application efficiency losses in the center pivot irrigation times total water applied per area [24,25].
The wind drift losses of center pivot irrigation may be reduced by DESA, which would result in higher application efficiency. This is achieved by adjusting the relative position of the droplet nozzle automatically and reducing the distance between the plant canopy and the nozzle. Potential water savings can be projected by assuming a reduction in wind drift losses for each of the considered scenarios: 50%, 60%, and 70%. This exploration of the potential scenarios is presented in Table 3. DESA cannot eliminate the losses due to wind drift, only reduce them. To compute the potential water savings, we explored the potential of DESA to reduce wind drift by 15%, 25%, 35%, 45%, 55%, and 65% to save water from wind drafts during the irrigation event (Equation (1)). Values are presented in Table 3. Total water saving per area = water-saving percentage * total water loss by wind draft per area (1) There is a wide range in the price paid for irrigation water in the United States which ranges from USD 0.005 per m 3 to USD 0.01 per m 3 with some States having elevated prices from irrigation water becoming more expensive from USD 0.02 to more than USD 0.10 per m 3 [26]. D'Odorico et al. determined that the global irrigation water prices for five major crops Wheat, Maize, Rice, Soybean, and Potatoes are USD 0.05/m 3 , USD 0.16/m 3 , USD 0.10/m 3 , and USD 0.67/m 3 , respectively [27]. Water values ranging from USD 0.005 to $0.10 per m 3 are used in the analysis below to determine the value of the lost water in the calculations below (Tables 4 and 5).

Irrigation Water Value Lost by Wind Drift Additionally, Savings with DESA
The value of water lost to wind drift is presented in Table 4. Furthermore, the potential value of the wind drift reduction potential of DESA is presented in Table 5. The values are highly dependent on the situation and circumstances of an individual system and can span across two orders of magnitude.

Climate Change
Climate change may also play a role in wind drift loss in the future. First, climate change affects wind speed which leads to increased evapotranspiration (ETo) from the crops [28] and potential evapotranspiration (PET). This leads to rising crop water requirement (CWR) [29]. Thus, if loss fractions are similar, the water lost by wind drift would increase proportionately in absolute terms. The increased water demands may also lead to increased water costs in the future. This, coupled with the cited literature on localized higher costs of water necessitated the inclusion of scenarios (USD 0.15, USD 0.20, and USD 0.25). The result is presented in Table 6. In addition, water-saving from the DESA system in the same scenarios is presented in Table 7.

Economic Viability of DESA
We hypothesized that the increased cost of DESA can be offset by increased efficiency in either water, fertilizer, pesticide, or herbicide use. In the most conservative case, these savings would come from water use alone. Thus, a series of water savings scenarios (50% to 70%) were explored and the irrigation water saving price of the DESA system per season as a function of hypothetical water savings was calculated. These water-saving prices are presented in Table 3. To calculate the total saving price after using the DESA system during the season, one needs to know the total price of the DESA system per area. The total price of the DESA system can be calculated by Equation (2).
The total cost of DESA system per area = (price per nozzle) * (# nozzles on 33 ha center pivot) (2) Total cost of DESA system per area = USD 72 * 50 = USD 3600 The total price for DESA design is USD 3600 to cover 33 ha with center pivot irrigation. A conservative assumption is that the unit price for DESA should be lower. The systems that control individual nozzles must be made inexpensively from low-cost components for DESA to be economically viable. Improvements in the productivity of the crops due to better water distribution, water use efficiency, and savings in fertilizers and pesticides/herbicides owing to low water inputs could further improve the net returns and profits. However, all potential benefits (including verified water savings potential) are, as of this publication, not yet demonstrated for DESA. Investigations on the effects of DESA are needed for a more robust economic analysis. However, such outcomes would be dependent on nature and the type of crop, the topography, the pedosphere, and the weather conditions.
The DESA profits rise with increased water cost and application efficiency (efficiency gains through reduced wind drift). The net returns per season are calculated by Equation

Calculation of the Installation, Maintenance, and Operation Including DESA System
Fixed costs for the installation, maintenance, and operation of the center pivot (base case) were USD 104,243 (USD 3158.88/ha). The integration of DESA into the base case increases those costs by an amount that depends on the DESA's investment capital cost. The annual fixed cost was calculated based on the initial investment costs. It is assumed that the resale value of the assets at the end of their useful life is negligible. Moreover, depreciation is calculated based on the lifespan of the assets and their utilities only. The lifespan of the DESA assembly was assumed to be 20 years, but other lifespan estimates could also be explored. The lifespans of the other assets range from 10-25 years [25]. The sum of the depreciation cost (1), the rate of interest (8.5%; 2), taxes, and insurance (1.4%; 3) (DITI) are summed (Table 10). For an example calculation, we set DESA unit costs to the current prototype cost (USD 72 each) and show a total fixed cost of USD 107,842 and a depreciation cost of USD 5812, i.e.,~5% of the Investment cost.
Annual operating costs were also tabulated. A 2% repair and maintenance charge for DESA was assigned. The total fuel expenditure (electricity reported as KWH) is USD 29.61 per ha. There are no costs associated with repairs and maintenance of the power delivery. The sum of annual costs (fixed and operating costs) for DESA was computed to be USD 13,169 (USD 399.06/ha). This is 1.1% higher than the USD12,104 (USD 366.79/ha) cost of the conventional center pivot without DESA.

Conclusions
This work illustrates the potential of open, inexpensive, networked hardware to combine with modern precision agriculture through the DESA example. The DESA example was chosen to explore its potential and to determine if such an approach was worthy of further experimentation and refinement. We conclude that it is. DESA makes nozzle-level information and adaptive control possible with off-the-shelf components and a component an assembly costing USD 72. DESA could become economically viable, as shown in the analysis above, in scenarios of higher water cost and climates with multiple annual growing seasons.
Additional DESA benefits, not considered in the economic calculations, could further increase its value and potential. For example, the information and data gathered by the DESA system may also have value to growers as it can be used to create dynamic maps of canopy growth throughout the growing season which could be used to identify locations of reduced productivity. DESA could also adapt to the presence of multiple crop canopies in a single field if a diverse planting layout or intercropping approach were strategically advantageous. Additional economic savings associated with chemigation may also be anticipated. The combination of these potential benefits and the scenario-dependent water savings indicate that DESA is an irrigation approach worthy of future investigation.