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Feature
Microwave Radiometry: A New Tool for Precision Irrigation on Golf Courses
By Madan Sapkota, Chase M. Straw, Weston W. Floyd, and Elia Scudiero
As global demand for freshwater intensifies and the environmental impact of water use becomes more apparent, golf course superintendents face increasing pressure to manage water resources more efficiently. Conventional irrigation scheduling methods may result in overwatering or underwatering, leading to water waste and negatively affecting turfgrass health and playability. Precision irrigation has emerged as an advanced approach that applies water precisely where and when it is needed, using technologies that monitor soil moisture and optimize irrigation practices.
Accurate soil moisture data are crucial for precision irrigation, as they provide real-time information necessary for refining irrigation schedules, minimizing water waste, and maintaining healthy turfgrass. However, current sensor technologies face challenges in covering large areas like golf course fairways. While traditional methods, such as gravimetric techniques, are accurate, they are also labor-intensive and impractical for large-scale use. Time domain reflectometry (TDR) sensors, commonly used by superintendents in the United States, provide real-time data with less labor but are limited to point-specific measurements, making them less efficient for extensive areas. To overcome these limitations, new solutions are needed to improve the accuracy and reliability of soil moisture measurements over large areas, ultimately enhancing water conservation and promoting healthier turfgrass.
Introducing Microwave Radiometry
To address the limitations of current soil moisture sensing technologies, microwave sensing emerges as a promising solution for large-scale, accurate soil moisture measurement on golf courses. Microwave sensing can be categorized into active and passive methods. Active microwave sensing, such as synthetic aperture radar (SAR), involves emitting microwave signals to the ground and measuring the reflected signals. This method is effective for mapping surface features but can be complex and resource-intensive. In contrast, passive microwave sensing, or microwave radiometry, measures the natural microwave emissions from the surface. This passive approach is particularly effective for assessing soil moisture content, as it directly responds to the water present in the soil.
Microwave Radiometry in Golf Course Management
Microwave radiometry is emerging as a promising technology for soil moisture measurement, with significant potential for largescale applications in golf course management. An example of this technology is the Portable L-Band Radiometer (PoLRa), commercially known as turfRad (TerraRad Tech AG, Zurich, Switzerland). Although PoLRa represents a new tool for golf course superintendents, the principles of microwave radiometry could transform how soil moisture is monitored and how high-resolution soil moisture maps are created.
Microwave radiometry detects natural microwave emissions from the surface, allowing for soil moisture measurement up to four inches below the surface. This non-invasive method enables rapid data collection over large areas, making it ideal for managing extensive golf course fairways, where traditional soil moisture-sensing methods are often labor-intensive or limited. The radiometer sensor can be mounted on a mower or strapped to the bed of a utility vehicle, measuring soil moisture about 14 times per second while traveling up to 10 mph. Sensors can also be arranged in an array for broader coverage.
Although microwave radiometry is still relatively new and has not undergone extensive testing in golf course management, it has already been implemented at several golf courses in the United States. Technologies like PoLRa show promise but are still in the early stages of exploration. For precision irrigation, any geospatial sensor technology, including PoLRa, must be rigorously evaluated by assessing soil moisture measurement accuracy and improving soil moisture mapping.
The preliminary research discussed in the next section focuses on the first step: enhancing measurement accuracy through calibration techniques. Calibration involves adjusting the sensor readings to match those from reliable reference measurements, such as those obtained from TDR sensors, to ensure the data collected are accurate and reliable. Factors influencing accuracy include soil moisture content, leaf water levels, brightness temperature (how much microwave radiation is reflected back to the sensor), temperature fluctuations, and surface roughness (how smooth or uneven the surface is, affecting the scattering of microwave signals). Different turfgrass species may also affect readings due to variations in leaf water content and surface characteristics. Therefore, site-specific calibrations are essential for ensuring reliable measurements. Significant effort is needed to develop and apply effective calibration techniques to achieve precise and dependable results.
Preliminary Research at Champions Golf Club, Houston, Texas
On August 14, 2023, Texas A&M University researchers conducted a study at Champions Golf Club (Jackrabbit Course) in Houston, Texas, focusing on fairways 2, 6, and 13, which feature ‘Tifway 419’ hybrid bermudagrass in sandy loam soil. Two methods to measure soil moisture were used: the PoLRa (turfRad) microwave radiometer and handheld TDR 350 sensors (FieldScout TDR 350 Soil Moisture Meter, Spectrum Technologies, Inc., Plainfield, IL, USA). The PoLRa was mounted on a fairway mower about one meter above the ground (Figure 1a) and driven at speeds of 3.5-4.5 mph. The mower made three passes per fairway—two near the edges and one down the center—while data were collected at twelve randomly chosen points per fairway. After each pass, the points were flagged and exact times were recorded using the ‘Unix Time’ app.
Previous research has shown strong correlations between TDR and gravimetric soil moisture measurements, especially in coarse, non-conductive soils. Since TDR is practical for golf course superintendents, it provides reliable data for calibrating the PoLRa sensor. After using PoLRa to take measurements, soil moisture readings were collected at marked points using handheld TDR sensors at depths of 1.5, 3.0, and 4.8 inches (Figure 1b). The PoLRa data were then matched with the TDR readings based on the recorded times to ensure accurate comparisons.
For calibration, we used ANCOVA regression, a statistical method that helps understand the relationship between different variables while controlling for other factors. In this method, TDR readings were treated as the dependent variable (the outcome we are measuring), and the brightness temperature from PoLRa’s vertical polarization was the independent variable (the factor we are testing to see its effect). This method enabled more accurate estimation of soil moisture levels. We assessed the model’s performance using metrics such as R², which indicates how well the model explains the variation in soil moisture, and mean absolute error (MAE), which shows the average size of the prediction errors.
Our initial calibration using PoLRa’s off-the-shelf (i.e., factory) settings showed an R² value of 0.60 (P < 0.01) and MAE of 0.06 (Figure 2a), indicating that 60% of the variability in soil moisture readings could be explained by PoLRa data. While promising, these results highlighted the need for further refinement to improve accuracy. Using an advanced ANCOVA calibration approach that incorporated additional factors such as brightness temperature, the model’s performance significantly improved. The R² value increased to 0.78 (P < 0.01) (Figure 2b), explaining 78% of the variability in soil moisture, and the MAE was reduced to 0.03. These results demonstrate the effectiveness of advanced calibration techniques in enhancing the accuracy of PoLRa’s soil moisture measurements.
Future Direction for Accuracy and Reliability
The improved soil moisture measurement accuracy from using ANCOVA to calibrate microwave radiometry technology highlights its potential to improve golf course irrigation. While effective, ANCOVA requires further refinement. Future research should explore additional factors, such as different soil types, turfgrass species and varieties, management practices, and various climatic environments. Considering temporal factors, including seasonal variations, will also help enhance the model’s year-round accuracy.
Fine-Tuning Microwave Radiometry and Improving Soil Moisture Mapping
After improving soil moisture measurement accuracy, the next step in optimizing precision irrigation is enhancing soil moisture mapping across large areas like fairways. These maps help visualize soil moisture variability, as shown in Figure 3, and hold great potential for precision irrigation. By integrating PoLRa, which utilizes both microwave radiometry and GPS for georeferencing soil moisture data, with digital job board technology, real-time georeferencing of soil moisture readings can be achieved as a PoLRa moves across fairways. The resulting maps can reveal patterns of soil moisture variability, which can be more effectively addressed once the soil moisture calibrations are fully applied (Figure 3a and 3b).
Conclusions
Microwave radiometry holds potential to transform precision irrigation on golf courses. Its ability to provide accurate, large-scale soil moisture measurements could revolutionize water management for superintendents. By overcoming the limitations of current sensors and offering a non-invasive, efficient method for real-time data collection, microwave radiometry could become essential in modern golf course management. Although further research is needed to improve accuracy and reliability, the final step to ease the implementation of precision irrigation is integrating this technology with irrigation systems. This integration could reduce the need for manual adjustments of irrigation schedules by allowing the system to recommend adjustments based on soil moisture and possibly other turfgrass or soil data. Superintendents would then be able to simply approve or adjust these recommendations as needed. This approach could ultimately lead to more sustainable water use, improved turfgrass health, and enhanced overall golf course quality.
M. Sapkota, C.M. Straw, and W.W. FloydDepartment of Soil and Crop SciencesTexas A&M University3100 F and B RdCollege Station, TX 77845
E. ScudieroUniversity of California RiversideWest Big Spring RdRiverside, CA 92507
Acknowledgement
The authors gratefully acknowledge Chris Ortmeier, Director of Agronomy at Champions Golf Club, and Cliff Morris, Superintendent of the Jackrabbit Course, for providing space for our research. We are also thankful to the United States Golf Association and the South Texas Golf Course Superintendents Association for their financial support. Special thanks to Dr. Derek Houtz, Founder and CEO of TerraRad Tech AG, for his technical support with the PoLRa sensor
This article was originally published in Pennsylvania Turfgrass, Spring 2025 and is reprinted with permission.
