Field Data and Numerical Modeling of Water Balance of Lysimeter versus Actual Earthen Cap

To evaluate the differences in the hydrological performance of actual earthen cap overlying municipal solid waste (MSW) versus a lysimeter, which is commonly used to measure the hydrologic water balance, a field-scale test section ($30\times 20\times 2\mathrm{m}$) of an earthen cap made up of compacted native glacial clay was constructed and instrumented at a landfill near Detroit. A lysimeter pan was installed within the middle of the test section, and the instrumented area of the test section was expanded upslope and downslope of the lysimeter to monitor water balance parameters within and beyond the lysimeter footprint, in order to evaluate the effect of artificial drainage boundary introduced by the lysimeter. About 50 sensors were installed to monitor meteorological parameters, water content, water potentials, soil temperatures, water levels, gas pressures, percolation, and subsurface lateral flow. The water balance model called Unsaturated Soil Water and Heat Flow Model (UNSAT-H) was used to simulate the water balance of the test section. The UNSAT-H was able to predict the percolation relatively accurately. Relative changes or trends in the soil water storage were generally accurately captured by the numerical model. The numerical model that was validated for the lysimeter was used to simulate the water balance of the actual cap, where the cap was placed on MSW. The net percolation over the monitoring period estimated by the UNSAT-H across the interface between the cap and the underlying landfilled waste was negative (upward), as well as positive (downward into the waste). However, net percolation from the actual cap estimated by the UNSAT-H was relatively close to that measured by the lysimeter. Thus, the field data and modeling results indicate that the soil cap is able to pull moisture from the underlying waste under evapotranspirative gradients.