An Accurate Evaluation of Water Balance to Predict Surface Runoff and Percolation

An Accurate Evaluation of Water Balance to Predict Surface Runoff and Percolation

National Institute for Water Resources, Water Resources Research Institute Program

3/1/02 - 2/28/05

Hawaii, like many other states, has a number of unlined landfills that are potential groundwater contamination sources. Infiltration control is a major means of reducing leachate generation at unlined landfill sites. Use of synthetic materials for the closure of landfills is quite expensive, especially for small rural communities. Use of alternate capping technologies, such as vegetation caps, may not be suitable in humid areas where the annual precipitation exceeds the evapotranspiration demand of growing crops. However, a combination of natural soil caps and runoff-enhancing structures can be a feasible capping method. Local plants growing on natural soil (clay) caps could transpire a large part of the percolating water. Making a portion of the landfill surface impervious (e.g., by use of rain gutters) and diverting the surface runoff offsite could reduce the entry of water through the landfill cap, thus reducing the potential for leachate generation. A recent demonstration by the U.S. Navy showed that, in tropical areas such as Hawaii, it is possible to cap landfills with natural soil cover if 20% to 40% of the surface area can be covered with rain gutters. However, the amount of error in the prediction was high. In addition to some simplifying assumption for water conduction in capping material, the model used daily water balance for calculating runoff and infiltration. In reality, rainfall in Hawaii occurs over a relatively short period of time. Higher-intensity rains cause significant surface runoff. Averaging a storm event over a day would significantly reduce the intensity, making it appear as if there is no runoff and all water is infiltrating the ground. For groundwater recharge studies, this overestimation of recharge may provide a false sense of security through modeling by implying that a large part of the rainwater is entering the soil in recharge areas and less water is lost through runoff. It is clear that an accurate estimate of the partitioning of rainwater to surface runoff and infiltration components and the subsequent movement of infiltrated water through subsoil media is quite important for a variety of applications.

The objectives of the study were to measure percolate and runoff at frequent intervals at the test plots from natural rain and sprinkler-applied water and to calibrate and test a recharge model and a runoff-producing model against the collected data. These models provided some insight into the mechanisms of percolate and runoff production in response to specific storm events. They also indicated if improvements in modeling strategy were needed to better calibrate these models against the collected data. An additional objective was to validate a regulatory model that is commonly used for the closure of landfill caps. The data also helped in the recalibration of the surface runoff and percolate production models and in the study of chemical transport through the soil. Since this was a multi-year study, the activities this year were closely related the previous two years’ effort.

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