Submitted to: ASAE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 6/15/1998
Publication Date: N/A
Interpretive Summary: Weighing lysimeter-computed evapotranspiration (ET) is sometimes used to assess ET computer-model predictions and/or to compare water use under different management or climatic conditions. For any use of lysimeter-computed ET, the uncertainty associated with this value should be known and reported because an uncertainty too high would bring into question the usefulness of these values. The uncertainty of the Coshocton lysimeters were found to be too great to accurately compute hourly ET thus a quality-improvement plan was initiated. This paper discusses: lysimeter-computed ET measurement uncertainty; the quality-improvement procedure applied to the Coshocton lysimeters; an analysis of the uncertainty involved with the computed ET from Coshocton's lysimeters and; implications of the quality improvements and error analysis. The results indicate that the quality improvements greatly reduced the ET uncertainty. Also, the error analysis indicated that the ET measurement uncertainty of the Coshocton lysimeters was sensitive to various measurement components especially the number of rainfall events and the uncertainty associated with weight and percolation measurements. This research will assist scientists to accurately quantify the uncertainty associated with lysimeter-computed ET and to develop quality improvement plans if needed.
Technical Abstract: Weighing lysimeter computed evapotranspiration (ET) is sometimes used to assess ET computer model predictions and to compare water use under different management or climatic conditions. Therefore, it is important to precisely know lysimeter computed ET error. The error associated with single lysimeter weight measurements has been described, but ET error is a function of water budget variables (percolation, number of rainfall events etc.), measurement component error (error with weight and percolation measurements), and systematic errors (load cell slope bias or potentiometer slope bias). To achieve an acceptable relative ET error (less than 10%) for hourly lysimeter computed ET on the Coshocton lysimeters, the error needed to be reduced. This paper discusses lysimeter computed ET and its error; the quality improvement procedures applied to the Coshocton lysimeters and the effect upon ET error; the component error analysis of the Coshocton lysimeters; and some of the implications of the quality improvements and error analysis. Results indicate that the quality improvements applied to the Coshocton lysimeters reduced the ET error from 0.252 mm to 0.032 mm under most conditions. The ET error of the improved Coshocton lysimeters was found to be sensitive to various components especially the number of rainfall events and the error associated with weight and percolation measurements. The implications of these results are the improved Coshocton weighing lysimeters are sufficiently accurate to compute hourly ET with a maximum relative error of 10% during most of the growing season. But since ET error is sensitive to weight and percolation random measurement errors and sometimes systematic errors, these measurement systems should be monitored regularly.