Error analysis and quality improvement of the Coshocton weighing lysimeters

Weighing lysimeter-computed evapotranspiration (ET) is sometimes used to as sess ET computer-model predictions and to compare water use under different management or climatic conditions. For any use of lysimeter-computed ET th e 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 wate r-budget measurements (percolate volume, water-storage change, etc.), measu rement-component uncertainty (uncertainty in mass and percolation measureme nts), and systematic error (e.g., lend-cell and potentiometer slope bias). The Coshocton lysimeters were not sufficiently accurate to compute hourly E T to the desired depth resolution, thus a quality-improvement plan was init iated that utilized Pareto analysis. The objectives of this article are to: (I) derive expressions for ET uncertainty; (2) determine the effect of the quality-improvement procedures applied to the Coshocton lysimeters on ET-m easurement uncertainty; (3) perform an error analysis on the measurement co mponents of the improved Coshocton weighing lysimeters. The quality improve ments reduced the ET uncertainty from 0.36 mm/d to 0.032 mm/d under the con dition of no rainfall (ET and percolation only). The ET-measurement uncerta inty of the improved Coshocton lysimeters was sensitive to various measurem ent components especially the number of rainfall events and the uncertainty associated with mass and percolation measurements. This indicates that the improved measurement systems of the Coshocton weighing lysimeters are suff iciently accurate to compute hourly ET and that lysimeter-mass and percolat ion measurements should be monitored regularly (quality-control procedures should be followed).