ERROR ANALYSIS AND QUALITY IMPROVEMENT OF THE COSHOCTON WEIGHING LYSIMETERS

Authors: Malone, Robert - Rob; Bonta, James - Jim; STEWARDSON, D - U. OF NEWCASTLE UPON TYNE; Nelsen, Terry

Submitted to: American Society of Agricultural Engineers Transactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: One aspect of studying the hydrologic cycle is looking at the water that evaporates from soil surfaces and the water that transpires through plants. The commonly used term for this combined process is evapotranspiration (ET). Weighing lysimeters (large blocks of soil) can be used to measure or calculate ET, and these results are sometimes used to assess computer models predicted ET and/or compare water use under different management practices or climatic conditions. For any data, the uncertainty or level of precision of the values determines the usefulness of the data. Although the ET data from the Coshocton lysimeters have been quite useful, the uncertainty of those data was found to be too great to accurately compute hourly ET. Thus a quality improvement plan was initiated. The uncertainty of the lysimeter-computed ET from the Coshocton lysimeters was assessed; a quality improvement procedure was developed and described; and the error in nlysimeter computed ET following the improvement was assessed. The results indicated that the quality improvements greatly reduced the uncertainty of the Coshocton lysimeter data and made the data much more useful. Not only does this research greatly increase the quality of the ET data from the Coshocton lysimeters, but it will also assist other scientists who are working with lysimeter-computed ET data and provide a procedure for them to improve their data 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. For any use of lysimeter-computed ET, the uncertainty associated with this value should be known and reported. The uncertainty associated with individual lysimeter mass measurements has been previously presented, but ET measurement uncertainty is a function of water budget measurements (percolation, water change, etc.), measurement component uncertainty (uncertainty with mass and percolation measurements), and systematic error (e.g., load cell and potentiometer slope bias). The Coshocton lysimeters were not sufficiently accurate to compute hourly ET thus a quality-improvement plan was initiated that utilized Pareto analysis. The objectives of this paper are to 1)derive expressions for ET uncertainty; 2)determine the effect of the quality-improvement procedures applied to the Coshocton lysimeters on ET measurement uncertainty; 3)perform an error analysis on the measurement components of the improved Coshocton weighing lysimeters. The quality improvements reduced the ET uncertainty from 0.36 mm to 0.032 mm under the conditions ET and percolation (no rainfall). The ET measurement uncertainty of the improved Coshocton lysimeters was sensitive to various measurement components especially the number of rainfall events and the uncertainty associated with mass and percolation measurements. This indicates that the improved Coshocton weighing lysimeters are sufficiently accurate to compute hourly ET and mass and percolation measurements should be monitored regularly.