CALIBRATION AND QUALITY CONTROL (QC) OF THE COSHOCTON WEIGHING LYSIMETERS

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

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: Weighing lysimeters are one of the most accurate methods to determine evapotranspiration (ET) from soil. Accuracy is very important for these instruments because the unit of measurement is sometimes less than 0.5 mm, therefore, the error must be maintained at a minimum. To accomplish this, the weighing system of these instruments are generally calibrated. But existing methods of calibration are cumbersome, therefore, we decided to investigate alternative calibration methods. As an additional step to maintain the quality of the measurement process, a quality control (QC) plan was investigated that included the use of control charts. Control charts were chosen because they can: be relatively easy to use; indicate when re-calibration is necessary; identify measurement drift; indicate instrument problems; indicate human mistakes; provide confidence in measurement results; and be adapted to most measurement processes. The objectives of this article are to: present a weighing lysimeter calibratio method; provide a statistically valid QC plan using control charts; and to describe the performance of the Coshocton lysimeters operating under this calibration and QC plan. The results indicate that the error in lysimeter computed ET was about 0.03 mm, therefore, the lysimeters are capable of measuring low ET (e.g., hourly ET). The measurement system was also found to be statistically 'in control' which provides confidence in the measurement process. The Coshocton lysimeter ET measurements are used by other scientists to verify or assess ET mathematical models and our results provide evidence that the weight measurement process is very accurate. Also, the calibration and QC procedure described may be used by other lysimeter research locations to improve their weight measurement process.

Technical Abstract: Many weighing lysimeters, including the Coshocton lysimeters, use load cells to determine evapotranspiration (ET). Electronic changes to the lysimeters affected the slope and the variance of load cell output, therefore, quality control (QC) procedures were investigated to monitor the weighing mechanism. Control charts were chosen because they can: be relatively easy to use; indicate when re-calibration is necessary; identif measurement drift; indicate instrument problems; indicate human mistakes; provide confidence in measurement results; and be adapted to most measurement processes. A calibration procedure was then needed that met the constraints: could be performed by one person; could be performed without covering the lysimeter; and could be used in conjunction with the QC procedure. The objectives of this article are: to present a weighing lysimeter calibration method that works within the constraints often encountered at research facilities and that may be incorporated into a QC plan; to provide a statistically valid quality control plan (using control charts); and to describe the performance of the Coshocton lysimeters operating under this calibration and QC plan. The results show that the Coshocton lysimeters error in lysimeter computed ET was about 0.03 mm indicating that the lysimeters are capable of measuring low ET (e.g., hourly ET). The data (slope estimates and variances) were generally statistically 'in control' which provides confidence in the measurements. Two of the weighing lysimeters were found to be significantly affected by wind, however this affect was negligible for most applications.