Lysimeters versus Water-Content Sensors for Performance Monitoring of Alternative Earthen Final Covers

This paper reviews the relative merits of two approaches for performance monitoring of alternative earthen final covers (AEFCs): (1) direct measurement of percolation via large-scale pan lysimetry and (2) indirect assessment using data from water-content sensors. Large-scale lysimeters account for spatial variability and preferential flow and can resolve percolation rates on the order of 0.1 mm/yr, which is at least ten times lower than typical percolation criteria applied in practice (1–3 mm/yr). However, factors such as the capillary break effect, thermal water fluxes, and leaks can affect the accuracy of percolation rates measured with lysimeters. Lysimeters are also expensive and difficult to install. In contrast, water-content sensors are relatively inexpensive and are relatively simple to install, permitting multiple monitoring points within a cover at relatively low cost. However, demonstrating compliance using water content data is confounded by several factors, particularly spatial variability, scale effects, pedogenesis, and preferential flow. An analysis conducted using Monte Carlo simulation shows that estimates of percolation rate from water content data can vary by as much as six orders of magnitude due to spatial variability alone. For sites where a percolation criterion has been established, the best approach is to combine lysimetry and water-content sensors. Percolation rates measured with the lysimeter can be used for compliance monitoring and water-content data can be used for interpreting the lysimeter data and to assess reliability and representativeness.