Performance Prediction of Evapotranspiration (ET) Cover from Field Monitoring Results

The use of water balance (WB) covers in landfills increased over the last decade because of its enhanced performance and low cost of construction. This type of cover system consists of a vegetated soil layer and a storage layer that stores precipitation until the stored water is either evaporated from the soil surface or transpired through vegetation. Water balance covers rely on the storage capacity of the soil layer, rather than the low permeability of the soil, to minimize percolation. The storage capacity of the soil largely depends on the cover thickness and the unsaturated soil properties of the cover soil. On top of that, the local climatic condition also affects the design of cover thickness. The objective of this study was to analyze the water balance components of the WB cover system and execute a performance prediction evaluation based on field monitoring results. To pursue the objective, six large-scale test sections (lysimeters) were constructed side-by-side with locally available fine-grained soil (CH), followed by field instrumentation including moisture and temperature sensors, tensiometers, dosing siphon, and a weather station at the city of Denton municipal solid waste landfill, Texas. From the field monitoring results, the required soil water storage (S_R) was determined. The results showed that percolation decreases with increasing cover thickness, and cover thickness is significantly influenced by the distribution of precipitation and the unsaturated properties of the cover soil. Based on the field monitoring results and laboratory investigation of the soil, the cover thickness varied approximately between 1,160 mm (3.8 ft) and 1,700 mm (5.6 ft) for widespread annual percolation for the covers constructed with fine-grained soil.