| Field data for final covers (both conventional and water balance covers) were used to redefine equivalency criteria and evaluate conditions for equivalency. Data for water balance covers were also analyzed and used to develop methods to estimate required storage capacity (Sr) and available storage capacity (Sa), and to identify percolation rate at locations across the US. Equivalency criteria are defined as a percolation rate of 4 mm/yr for composite covers and 150 mm/yr for covers with compacted clay barrier (CC) or geosynthetic clay liner (GCL). Equivalency of water balance covers to CC or GCL covers can be achieved in all but the most humid climates. Equivalency to composite covers is readily achievable in arid climates, uncertain in semi-arid climates, and unlikely in sub-humid and humid climates. In addition, water balance covers should be designed with minimum Sa/S r > 0.8 to ensure equivalency. Minimum 5-yr monitoring is recommended when evaluating equivalency. Upon review of hydrology of water balance covers, evapotranspiration is > 80% of precipitation, percolation is < 20% of precipitation, and runoff is < 10% of precipitation in most cases. Factors affecting runoff include precipitation intensity, hydraulic conductivity of surface layer, and freezing conditions. Factors affecting evapotranspiration are precipitation, potential evapotranspiration, and leaf area index. Factors affecting percolation are storage capacity and preferential flow. Percolation due to thermal gradient is < 1 % of precipitation. Using the methods for estimating Sa and Sr, Sr can be estimated with bias of 15.6 mm and standard error of 43.1 mm, and Sa can be estimated with bias of 0.1 mm and standard error of 38.7 mm. For adequately-designed water balance covers, percolation rates less than 4 mm/yr can be achieved in the middle part of the western US, except in areas where snowmelt and spring rain occur. In these areas, percolation rates between 4 and 10 mm/yr may be expected. Percolation rates in the western US increase as the Midwest and Pacific coast are approached. Percolation rates exceeding 50 mm/yr can be expected in the Midwestern US and on the Pacific coast, and percolation exceeding 200 mm/yr can be expected in eastern locations. |
