Tuesday, November 6, 2007 - 12:30 PM
137-12

Enhancing Accuracy of Weighing Lysimeters by Real-Time Data Filtering.

Peter J. Vaughan and James E. Ayars. San Joaquin Valley Agricultural Sciences Center, USDA-ARS, 9611 S Riverbend Ave, Parlier, CA 93648

Mechanical vibration in the weighing system may be a factor limiting the accuracy in determining evapotranspiration rate using weighing lysimeters. The weighing system used in the lysimeters located at the University of California West Side Research and Extension Center is a counterweighted truck scale fitted with a tension load cell. We determined the mechanical vibration in the system by measuring the power spectral density of the load cell output. The power spectral density, calculated for 8192 load cell output voltage measurements taken over a period of 10 seconds, contained a significant peak at approximately 11 Hz which resulted in degradation of precision in the lysimeter mass measurements. Two methods for improving the data quality were tested. One was a moving average calculation of lysimeter mass and evapotranspiration rate using Savitsky-Golay (S-G) weighting factors. The other was a frequency filtering approach based on discrete Fourier transforms. For data recorded every 10 minutes, a 15-point S-G smoothing provided a noise reduction factor of 0.54. The frequency filtering method applied to 256 data points provided approximately the same noise reduction and is preferred due to the increased logistical complexity of operating a 15-point S‑G filter in real time.

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