Date of Award

12-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Plant and Environmental Science

Advisor

Khalilian, Ahmad

Committee Member

Privette, Charles

Committee Member

Payero, Jose

Abstract

Irrigation scheduling based on soil moisture sensor readings has gained popularity in the past few decades since it can enhance crop yield while saving water. Such method is limited since the representativeness of an individual soil moisture sensor measurement is questionable in a large field with variable soil type and texture. The optimum location of soil moisture sensors needs to be determined within such a production field for effective sensor-based irrigation scheduling. Therefore, the first object of this study was to investigate the optimum sensor location and the number of moisture sensors required for irrigating cotton in coastal plain soils. Replicated tests were conducted during 2012, 2013, and 2014 growing seasons in a cotton field located at the Edisto Research and Education Center of Clemson University, on a typical coastal plain soil. The test field was divided into different management zones based on soil electrical conductivity (EC) measurements. Soil moisture sensors including AquaSpy, Sentek EasyAg-50, Decagon EC-5, Watermark 200SS, and 503 DR Hydroprobe neutron probe access tubes were installed side by side in plots of each management zone. Irrigation treatments were based on sensor readings from various management zones. Results showed that irrigation based on sensor readings from higher electrical conductivity zones, can stabilize or even enhance yield while increasing water use efficiency (WUE) significantly. The second objective of this study was to evaluate the performance of soil moisture sensors mentioned above to determine the most accurate and affordable sensor technology for irrigation scheduling. Season long soil moisture readings of AquaSpy, Sentek EasyAg-50, Decagon EC-5, and Watermark 200SS sensors were collected and compared to neutron probe readings. The results showed that Sentek EasyAg-50 sensor performed the best among tested sensors compared to neutron probe readings with coefficient of determination, R2 = 0.847 and root mean square error, RMSE= 4.2% for soil profiles up to 50 cm. The performance of Decagon EC-5 sensor was acceptable with R2 of 0.6 to 0.7 and RMSE ranged from 4.9% to 6.7% during the three growing seasons. Further field and lab calibration of Decagon EC-5, reduced RMSE from 4.4% to 3.3% at topsoil (10-30 cm). Compared to Sentek EasyAg and Decagon EC-5 sensors, AquaSpy and Watermark 200SS sensors performances in measuring soil moisture contents, were not satisfactory, as indicated by low R2 of less than 0.45 and high RMSE of 9.5% to 14%. The results of this study suggested that in a field with variable soil type, it would be beneficial to install moisture sensors in management zones with higher EC readings (heavier soil textures) to obtain maximum yield and WUE. The results also indicated that, although the Sentek EasyAg-50 sensor had the highest accuracy among the sensor types tested, Decagon sensor offered more promise for irrigation scheduling than the rest of the sensors tested, since it offered good accuracy and is affordable.

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