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Hydrologic Setting: Characteristics of Pre- and Post-drainage Everglades

A thorough investigation of Everglades hydrology requires an understanding of the pre-drainage hydrologic system. The pre-drainage Everglades received water primarily from direct rainfall, periodic overflow from Lake Okeechobee, and runoff from surrounding pine flatwoods and other upland systems (Gleason and Stone, 1994). In addition, the slough systems of the Everglades probably received ground-water discharge and shallow subsurface runoff from the adjacent low-lying pinelands.

The driving force for water flow in the Everglades is the water-surface slope, which is controlled by the regional topographic gradient. In the pre-drainage Everglades, the topographic gradient was a relatively consistent 2 inches per mile (in/mi), with only minor undulations of the natural landscape affecting water flow. Topography varied across the full (pre-drainage) width of the north-central Everglades (approximately 50 mi). On the western side of the Everglades, a major slough system was present that graded into a broad sawgrass plain in the central area, back into another major slough system on the east side. Microtopography in the sloughs consisted of alternating ridge and slough systems with typical spacing of approximately 0.5 mi.

Changes in topography (for example, because of subsidence or construction of levees) or water levels (because of canal drainage) easily perturb the direction of water flow in the Everglades. Canal construction and drainage began to modify water levels and topography substantially in the northern and north-central parts of the Everglades beginning about 1912. The initial effort was to construct four major north-south canals to drain water to the Atlantic Ocean. Early canal drainage in the Everglades Agricultural Area (EAA) led to excessive oxidation of the peat in the vast sawgrass plain and swamp forest directly south of Lake Okeechobee. Drainage and oxidation eventually caused between 3 and 10 ft of subsidence in the agricultural area over the past century. Subsidence and continual pumping in the EAA to keep agricultural fields dry have had the effect of reversing the horizontal direction of ground-water flow in some areas of the Everglades. Where ground water once flowed toward the southeast, it presently flows toward the northwest (Miller, 1988).

Drainage canals continue to be the primary water-management effort in the north-central Everglades. During excessively wet conditions, the major drainage canals shunt excess water from Lake Okeechobee or the EAA to the Atlantic Ocean (fig. 1). Under more typical wet-season conditions, drainage canals deliver and store water in the WCAs. East of the Everglades the canals have various functions, including drainage of the low-lying pinelands and aquifer recharge to balance losses by ground-water pumping. Interactions between surface flow in canals and ground water have been frequently investigated in south Florida (Miller, 1978: Chin, 1990; Genereux and Slater, 1999; Nemeth and others, 2000, Bolster and others, 2001).

The conversion of wetlands to agriculture compressed the northern part of the Everglades to approximately one-third its pre-drainage width. By the 1950s, it was apparent that the canals were too effective in draining wetlands that were becoming increasingly important for sustaining water supply to the newly formed Everglades National Park, and to the growing population along the Florida Atlantic Coast. Construction of levees during the 1950s and early 1960s began to enclose the large basins now known as the WCAs. These areas are large levee-enclosed basins that encompass only the easternmost part of the pre-drainage system. WCAs were designed for multiple purposes, including storage for later delivery to Everglades National Park, and protection from flooding for the drained areas just outside the wetlands. The WCAs are all that remain of the north-central Everglades and their construction and management have had substantial effects on surface- and ground-water flow in Palm Beach and Broward Counties (fig. 1).

Under pre-drainage conditions, surface flow in the Everglades was augmented by substantial shallow runoff from the surrounding uplands. Under water management, the water levels outside the WCAs normally are maintained at lower levels than inside the WCAs, which causes net recharge from surface water to ground water in the WCAs (Miller, 1988). Seepage losses resulting from flow of recharged water beneath levees represent an important component of water loss from the WCAs. Seepage appears to be greatest along the eastern and northwestern borders of the WCAs, where land is now being managed for a variety of uses, including agriculture, light industry, or suburban development (Miller, 1988). Water losses from the Everglades by seepage were large enough that they became obvious almost as soon as the levee-construction method was tested in the 1950s (U.S. Army Corps of Engineers, 1952). Seepage at many of the Everglades levees began to be widely investigated beginning in the 1960s (Klein and Sherwood, 1961; Swayze, 1988; Genereux and Guardiario, 1998; Nemeth and others, 2000, Sonenshein, 2001). Although often less studied, seepage flow also occurs beneath the levees that separate WCAs (Harvey, 1996; Harvey and others, 2000). Because recharge now occurs at locations where formerly the Everglades gained water from shallow runoff and ground-water discharge, seepage losses have become one of the most important unintended side effects of water management in the north-central Everglades.

Another factor associated with water management that may have affected surface-water and ground-water interactions is the increasing fluctuations of surface-water-levels in the WCAs compared with pre-drainage conditions. From the 30-year comparative simulations of the South Florida Water Management Model (SFWMM) and Natural System Model (NSM), surface-water-level fluctuations under pre-drainage conditions appear to be from 50 to 75 percent of present day fluctuations. As shown in the present study, the increased water-level fluctuations in WCAs drive recharge and discharge in interior areas of the wetlands far from levees.

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