Evaluation and Control of the Long-Term Water Balance on an Urban Development Site
Ferguson, Bruce K.
Ellington, M. Morgan
Gonnsen, P. Rexford
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The long-term water balance is promising but previously unused technique for evaluating and controlling the hydrologic effects of urban development. The water balance is a summary of all the inflows and outflows, over a period of time, of a land area such as a hillslope, a watershed or a political unit. The long-term water balance refers specifically to the average levels and seasonal fluctuations of flows over a period of years, indicating the overall pattern of interaction of a land area with the hydrologic environment. Although the water balance has long been a prominent concept in geography, where it is used as a summary index of the moisture and energy endowments of regional environments, its application to management of specific urban development projects has not been fully explored. A more traditional approach to urban stormwater management is the design storm. Some design storms are defined by average recurrence intervals. Others are uniform storm phenomena, such as the first one or two inches of runoff from any storm. The application of this concept is appropriate for management of peak flood flows. Interest in flood control has made the design storm essentially the exclusive approach to regulation of stormwater in Georgia. However, a design storm is not a significant part of the total water resources of an area; it does not indicate overall moisture endowments of the environment. For example, in northern Georgia, where the annual precipitation averages roughly 50 inches, a 10 year design storm is only about 6 inches in.24 hours. Thus the design storm lasts only a fraction of one percent of the elapsed time during its recurrence interval, and 500 inches of rain go by while stormwater facilities wait for the 6 inches for which they were designed. A broad interest in water resources demands a more comprehensive management of the stormwater resource. The long-term water balance summarizes average seasonal patterns of hydrologic inputs, outputs and changes in storage. The long-term water balance could be used to evaluate such important specific long-term parameters as baseflow runoff, ground water recharge, and soil moisture levels. Through environmental connections between the hydrologic environment and vegetative and human communities, these parameters indicate potential levels of on-site and downstream water supplies, assimilative capacity, recreational resources, wetlands, and aquatic life. The application of the long-term water balance to a proposed urban development might suggest types of impacts and approaches to stormwater control that would not be considered by applying the design-storm idea alone. One control approach that deserves to be evaluated this way is infiltration, which uses closed basins to force runoff water to enter and be stored in subsurface soil voids (Ferguson, 1990a; Ferguson and Debo, 1990). This paper presents preliminary development of a model for simulating the long-term water balance of urban development sites, and application of the model to a specific development to illustrate potential water-balance effects of urbanization and alternative methods of stormwater control.