Date of Award

12-2020

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biosystems Engineering

Committee Member

Dr. Tom O. Owino, Committee Chair

Committee Member

Dr. Dale E. Linvill

Committee Member

Dr. José O. Payero

Committee Member

Dr. Diana C. Vanegas

Abstract

The objective of this thesis was to compare cost-effective methods of measuring crop water use, known as evapotranspiration (ET), in South Carolina’s humid climate. The methods analyzed were the surface renewal method (SR), the Eddy Covariance method (EC), large in-field weighing lysimeters, a newly developed pressure differential device (PDD), a Class A Evaporation pan, and the Penman-Monteith equation. In the first chapter, ET measurements obtained by SR were compared to ET measured by EC and weighing lysimeters. For reference, EC and SR track the energy budget to estimate ET, while the weighing lysimeters used in this study are box-like containers measured continuously for mass changes attributed to water gained or lost. Great agreement was observed between the surface renewal and EC methods (R2≥0.89), while agreement was weak or inconsistent between the surface renewal method and lysimeters. In the second chapter, a PDD was designed, fabricated, and tested in its ability to measure ET. Despite the PDD and its neighboring weighing lysimeter showing agreement in profile moisture changes, inferred PDD ET measurements showed little agreement with the lysimeter (R2<0.2). The PDD appeared to be affected by a delay in measuring rainfall, among other factors, in comparison to the lysimeter. The study suggests that the PDD may not suit ET measurement but could be useful for subsoil measurements in other fields of study. In the third chapter, the Penman-Monteith equation and a Class A Evaporation Pan were analyzed. The two methods measured reference evapotranspiration (ETo), and showed good agreement with each other (R2=0.95). The results of the ETo comparison were further used to develop pan coefficient values (Kp) and compare these to Kp values estimated from equations recommended by the Food and Agriculture Organization (FAO). No significant difference was found in the Kp comparison. The Penman-Monteith ETo measurements were then used a third time with weighing lysimeter data from the cotton field to develop a crop coefficient curve (Kc). The obtained Kc values were compared to FAO recommended Kc values, showing no significant difference. The study suggests that FAO recommendations for ETo measurement, Kp estimation, and Kc values do apply to South Carolina.

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