ABSTRAC'I Accurate water balance calculations are essential for water resource and environmental management decisions, but many of the terms used in the equation are difficult to measure. In this study, a method for measuring rates of evapotranspiration and net seepage from a freshwater marsh in southwest Florida is described. The results are compared to evaporation pan estimates as well as to calculations that balanced all the terms in the hydrologic budget. The measured rates of evapotranspiration showed a. distinct seasonal trend ranging from an average high of 0.24 mId during July 1992 to a low of 0.06 mId in January 1993. Evapotranspiration rates were higher than Class A evaporation pan measurements during July and August, indicating transpiration by plants exceeded evaporation by pans. Net ground water seepage flowed out of the marsh except during periods of high water table conditions. When all terms in the hydrologic budget were evaluated, the equation balanced on a yearly basis with an error of 2 percent, on a seasonal basis with errors less than 7 percent, but on a monthly basis errors were as great as 30 percent. Total annual rainfall on the marsh was 45 percent of the total marsh hydrologic input and was approximately equal to the loss by evapotranspiration of 41 percent. (KEY TERMS: water budget; urban hydrology; wetlands; evapotranspiration; stormwater management; surface water hydrology.) 1Paper No. 95038 of the Water Resources Bulletin. Discussions are open until August 1, 1996. 2Environmental Scientist, Southwest Florida Water Management District, 2379 Broad Street, Brooksville, Florida 34609-6899.
A low impact (dispersed) design demonstrates how small alterations to parking lots can reduce runoff and pollutant loads. Storm runoff was treated as soon as rain hit the ground by encorporating a network of swales, strands and a small wet detention pond into the overall design (Figure 1). When the volume of water discharged from all the different elements to the treatment train (the swales, the strand, and the pond) are compared, calculations showed that almost all the runoff was retained on site. The most effective method for reducing pollutant loads is to keep runoff on site and allow time for infiltration as well as for chemical, biological and hydrological processes to take place. Basins paved with porous pavement had the best percent removal of pollution loads with many removal rates for metals greater than 75 percent in the basin with a smaller garden area and greater than 90 percent with larger gardens. More phosphorus loads were discharged from basins with vegetated swales than from basins with no swales. It should be emphasized here that even with some poor removal rates by swales in the parking lot for phosphorus, when the entire system is evaluated, efficiencies are good since the site retained over 99 percent of the storm runoff during the year that it was evaluated. Sediment sampling identified polycyclic aromatic hydrocarbons, chlordane and DDT products as problems. Phosphorus and nitrogen in the sediments increased from year one to year two. Metal and nutrient pollutants in the sediments were not found to be migrating to the deeper strata.
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