Posted: December 2, 2022

Researchers in the college create and test high-tech instruments to advance "precision irrigation."

Researcher Suat Irmak programming the datalogger in one of his surface water vapor and energy flux towers to measure crop water use, all incoming and outgoing radiation fluxes, and all other climate variables such as air temperature, relative humidity, vapor pressure deficit, net radiation, precipitation, soil moisture, soil temperature on an hourly basis. Credit: Suat Irmak

Researcher Suat Irmak programming the datalogger in one of his surface water vapor and energy flux towers to measure crop water use, all incoming and outgoing radiation fluxes, and all other climate variables such as air temperature, relative humidity, vapor pressure deficit, net radiation, precipitation, soil moisture, soil temperature on an hourly basis. Credit: Suat Irmak

Increasingly, the availability of water is becoming a limiting factor for growing enough crops to feed the world's burgeoning population. Globally, 70 percent of total ground and surface water withdrawal is for agriculture, but due to population growth, urbanization, and climate change, competition for water resources is expected to increase.

Irrigated agriculture, as opposed to rain-fed agriculture, represents 20 percent of land under cultivation, but it contributes 40 percent of the total food produced worldwide, so scientists in the college are experimenting with high-tech methods--especially the use of sensors that monitor soil, water, and plant conditions in real time--to conserve water used in irrigation. The stakes are high because, according to the World Bank, irrigated agriculture is, on average, at least twice as productive per unit of land as rain-fed agriculture, thereby allowing for more production intensification and crop diversification.

Precision irrigation based on sensors is growing increasingly important, according to Blair Siegfried, associate dean for research and graduate education and director of the Pennsylvania Agricultural Experiment Station in the College of Agricultural Sciences. Having the ability to precisely monitor moisture variability and make decisions based on site-specific, in-field data will transform how farmers manage their operations, he suggested.

"For that reason, researchers in our college are leaders in developing the use of sensors and other complex technologies to enhance crop yields while minimizing water-use efficiency," he said. "Technology plays an important role in the sustainable intensification of the agricultural system. The use of sensors in precision irrigation increases crop yields, reduces production costs, and maximizes profitability."

Soil-Based Sensors

One example of precision irrigation research conducted in the college is based on an "Internet of Things"--or IoT--system monitoring real-time data from soil-based sensors to activate an automated precision irrigation setup that can conserve water and boost crop production.

In a study involving fresh-market tomato production conducted at the Russell E. Larson Agricultural Research Center at Rock Springs, the researchers demonstrated that a low-power, wide-area wireless network--called LoRaWAN--is a low-cost, easily implemented online arrangement effective for precision crop irrigation. The system, powered by batteries charged by solar panels, controlled the timing, rate, and distribution of water.

"These results suggest that the Internet-of-Things system can be implemented for precision and automatic irrigation operations for vegetable and other horticultural crops, enhancing those crops' water-use efficiency and sustainability," said research team leader Long He, assistant professor of agricultural and biological engineering. "Overall, the LoRaWAN performed well in power consumption, communication, sensor reading, and valve control."

The field experiment, spearheaded by Haozhe Zhang, who recently graduated with a doctoral degree in agricultural and biological engineering and returned to China to work at a research institute, tested the effects of four irrigation-scheduling treatments on crop yields of fresh-market tomato plants. The team also measured irrigation water-use efficiency.

Researchers Long He, Haozhe Zhang, and Francesco Di Gioia. Credit: Francesco Di Gioia/Penn State

In findings published in Smart Agricultural Technology, the researchers reported that throughout the growing season, the overall water-use efficiencies of the methods ranged from 22 to 28 percent above the control, and they produced 15 to 22 percent higher marketable fruit yield than normal.

Research team member Francesco Di Gioia, assistant professor of vegetable crop science, pointed out that the research is innovative and significant because the LoRaWAN technology it utilized is simple and relatively inexpensive. That will be important going forward, he believes.

"To be adopted, the whole system has to work for smallholder farmers," he said. "So, all of the components are low cost. Our focus was to keep it inexpensive because there's nothing really available at the commercial level that fulfills the need of smallholder farmers and small diversified growers. We spent less than $1,000 putting our IoT system together, and it's something that people without specific knowledge can do. You don't have to be an engineer to develop something similar to work on a small produce-growing farm."

Conventionally, farm managers determine when and how much to irrigate primarily based on their experiences and time availability, which often leads to inefficient water usage and reduction in crop yield and quality either by over-irrigating or under-irrigating, DiGioia explained. Precision irrigation is a management strategy that allows growers to avoid plant water stress at critical growth stages by applying only the necessary amount of water directly to the crop, with rate and duration based on site-specific conditions.

"We are trying to improve the efficiency of resources used in production systems, and the nice thing about this technology is that it could be applied to many crops," he said. "In this case, we are working with vegetables because vegetables are really sensitive to drought stress. This precision technology could be implemented right away by the vegetable industry here in Pennsylvania or elsewhere."

Data logger box connected to the soil moisture sensors installed in an experimental tomato field. Credit: Francesco Di Gioia/Penn State

Plant-Based Sensors

Another precision irrigation research project recently conducted in the college took a different approach. It tested plant-based sensors that measure the thickness and electrical capacitance (the ability to store and release electrical energy) of leaves, which show great promise to alert farmers when to activate their irrigation systems, preventing both water waste and parched plants. When leaves dry out, they get thinner and lose the capacity to conduct electricity.

Continuously monitoring plant "water stress" is particularly critical in arid regions and traditionally has been done by measuring soil moisture content or developing models that calculate the sum of ground surface evaporation and plant transpiration, the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems, and flowers. But potential exists to increase water-use efficiency with new technology that more accurately detects when plants need to be watered.

For this study, published in Transactions of the American Society of Agricultural and Biological Engineers, lead researcher Amin Afzal, who graduated from Penn State with a doctoral degree in agronomy in 2017, integrated into a leaf sensor the capability to simultaneously measure leaf thickness and leaf electrical capacitance, which had never been done before.

The work was done on a tomato plant in a growth chamber with a constant temperature and 12-hour on/off photoperiod for 11 days. The growth medium was a peat potting mixture, with water content measured by a soil-moisture sensor. The soil-water content was maintained at a relatively high level for the first three days and allowed to dehydrate thereafter, over a period of eight days.

The researchers randomly chose six leaves that were exposed directly to light sources and mounted leaf sensors on them, avoiding the main veins and the edges. They recorded measurements at five-minute intervals.

The daily leaf-thickness variations were minor, with no significant day-to-day changes when soil-moisture contents ranged from high to wilting point. Leaf-thickness changes were, however, more noticeable at soil-moisture levels below the wilting point, until leaf thickness stabilized during the final two days of the experiment, when moisture content reached 5 percent.

The electrical capacitance, which shows the ability of a leaf to store an electrical charge, stayed roughly constant at a minimum value during dark periods and increased rapidly during light periods, implying that electrical capacitance reflected photosynthetic activity. The daily electrical-capacitance variations decreased when soil moisture was below the wilting point and completely ceased below the soil volumetric water content of 11 percent, suggesting that the effect of water stress on electrical capacitance was observed through its impact on photosynthesis.

"Leaf thickness is like a balloon--it swells by hydration and shrinks by water stress, or dehydration," Afzal said. "The mechanism behind the relationship between leaf electrical capacitance and water status is complex. Simply put, the leaf electrical capacitance changes in response to variation in plant water status and ambient light. So, the analysis of leaf thickness and capacitance variations indicate plant water status--well-watered versus stressed."

The study is part of a line of research Afzal hopes will end in the development of a system in which leaf clip sensors will send precise information about plant moisture to a central unit in a field, which then communicates in real time with an irrigation system to water the crop. He envisions an arrangement in which the sensors, central unit, and irrigation system all communicate without wires, and can be powered with batteries or solar cells.

"Ultimately, all of the details could be managed by a smart phone app," said Afzal, now a St. Louis resident, who studied electronics and computer programming at Isfahan University of Technology in Iran, where he earned a bachelor's degree in agricultural machinery engineering. He tested his working concept in the field at Penn State.

A few years ago, he led a team that won first place in the College of Agricultural Sciences' Ag Springboard contest, an entrepreneurial business-plan competition, and was awarded $7,500 to help develop the concept. He is still working on it. "I am trying to commercialize the concept of using plant leaf sensors and run a market validation," Afzal said recently. "I have formed a team, and we have made good progress building the prototype but are still a ways away from a commercial product."

In a follow-up study, Afzal evaluated leaf sensors on tomato plants in a greenhouse. The results confirmed the outcomes of the initial study. In the new research, he developed an algorithm to translate the leaf thickness and capacitance variations to meaningful information about plant water status.

Afzal's technology remains promising, noted Sjoerd Duiker, professor of soil management and applied soil physics, Afzal's adviser and a member of the research team. Current methods to determine irrigation are crude, while Afzal's sensors work directly with the plant tissue.

"I believe these sensors could improve water-use efficiency considerably," Duiker said. "Water scarcity is already a huge geopolitical issue. Improvements in water-use efficiency will be essential."

A leaf sensor. Credit: Francesco Di Gioia/Penn State

Sensors for Studying Climate Change Impacts

Another researcher in the college, Suat Irmak, professor and head of agricultural and biological engineering, has been using high-tech sensor arrays for decades to determine the differences between irrigated and rain-fed stands of crops such as corn, soybean, sorghum, watermelon, sweet corn, cover crops, seed corn, alfalfa, and edible beans. Most recently, in a study published in Soil and Tillage Research, he looked at how no-till and reduced-tillage cultivation affect surface heat loss and gain, crop yields, water use (evapotranspiration), and soil-water status in corn production.

Meetpal Kukal, assistant research professor in agricultural and biological engineering, played a role in that research. The team's findings showed that reduced surface evaporation, improved infiltration, and soil-water storage resulting from reduced-tillage or no-till practices resulted in better crop performance or crop water-use efficiency. Tillage decisions not only result in alteration of soil evaporation but crop transpiration, the researchers reported.

In an earlier study published in Agricultural Water Management, Irmak reported that, based on their climate change modeling results, rain-fed corn yields will decline up to 40 bushels per acre in coming decades, whereas irrigated yields are projected to decline only 19 bushels per acre--less than half as much. In many cases, irrigation blunts the impact of a warming climate, he pointed out.

Sensing and decision-making instrumentation and automation will play a crucial role in the future of agricultural operations, Irmak believes, with the technologies offering significant advantages, including minimizing or even eliminating human error associated with decision-making.

"They provide flexibility in farm operations in terms of the timing of different operations, such as making the system work at nighttime, on weekends, and holidays," he said. "And perhaps most significantly, during extreme weather conditions, these technologies provide additional time for producers to spend on other crucial activities."

Implementing climate-smart agricultural-management practices and technology adoption in production fields--coupled with research and science-based policy decision-making--are imperative to counter some of the negative impacts of climate change on agricultural productivity, Irmak contends.

"Thus, Penn State's College of Agricultural Sciences is continuously at the forefront of programs for addressing these challenges," he said. "We are one of the national leaders in terms of developing, testing, and implementing advanced agricultural automation and sensor arrays and data-acquisition systems to help producers to enhance crop production efficiency and farm probability in light of climate change."

A fertigation manifold installed in the experimental tomato field. Credit: Francesco Di Gioia/Penn State

By Jeff Mulhollem