Performance Monitoring and Model Verification for an Evapotranspirative Landfill Cover

Regulatory approval of evapotranspirative covers is usually based on numerical modeling of cover percolation during the design process, followed by post-closure moisture monitoring to verify adequacy of cover performance and validity of the numerical model. However, there are no well-established guidelines on how moisture monitoring data can be used to calibrate/validate the numerical model and verify adequate cover performance. This paper evaluates data from a Southern California landfill instrumented with TDR moisture probes to develop such guidelines. The TDR readings were found to be a good indicator of moisture trends and fluctuations within the cover. However, in fine-grained soils, TDR probes tend to overestimate the moisture content and location-specific nonlinear calibration functions are required to achieve reasonable accuracy of moisture measurements. The numerical unsaturated flow model UNSAT-H was able to reasonably predict the measured moisture response of the cover, provided near-surface cover degradation was incorporated into the model. Guidelines are provided for calibration of TDR readings in fine-grained soils and the use of near surface cover degradation as a calibration parameter for the unsaturated flow model. The calibrated model is validated by comparisons with observed moisture fluctuations within the cover.