transformation of the flow system (USACE and SFWMD, 1999). The impacts of these changes to the landscape and the ecosystem are described in detail in Chapter 2, but the quantitative changes in hydrology are discussed further in this section to provide a basis for additional discussion of improving water flow and distribution.
A comparison between pre- and post-drainage water budgets of the Kissimmee-Okeechobee-Everglades watershed (Figures 4-1 and 4-2) shows how the distributions of water storage and transfers are believed to have changed. Some of the key features of these modeled water budgets are summarized in Table 4-1 according to Natural Systems Model (NSM) version 4.6.2 and the South Florida Water Management Model (SFWMM) version 5.4 (see Box 4-1). Comparable water budgets based on the newer South Florida Regional Simulation Model (RSM) are not yet possible because of model development issues discussed in Chapter 6. The water budget models have considerable uncertainty associated with estimating evapotranspiration and specific values of water flows from one compartment to another, and the models are used here as generalizations rather than as exact accountings.
According to the SFWMM, on average Lake Okeechobee discharges approximately 11 percent less water south under current conditions (554,000 acre-feet/year) compared to pre-drainage flows (622,000 acre-feet/year; see Figures 4-1 and 4-2). Total inflow to the WCAs ranges widely with the models used. The SFWMM v. 5.4 calculates that current water inflows from the north to the WCAs (1.3 million acre-feet [MAF]/year) exceed that which would have occurred via sheet flow in the pre-drainage system (1.06 MAF per year; NSM v. 4.6.2). However, the new Natural System Regional Simulation Model (NSRSM) depicts a wetter pre-drainage Everglades in which 1.5 MAF flowed from Lake Okeechobee into what is now the Everglades Agricultural Area (EAA) and at least 1.7 MAF flowed from the north into the current WCAs, across their northern boundaries (J. Obeysekera, SFWMD, personal communication, 2009).
Roughly 1.9 MAF per year still enters the WCAs across the western, northern, and eastern boundaries under current conditions (see Figure 4-2), but inflow now occurs primarily through canal or stormwater treatment area (STA) discharges, unlike in pre-drainage conditions when direct precipitation and occasional overflows from Lake Okeechobee dominated freshwater inputs (Harvey and McCormick, 2009). Surface-groundwater exchanges were minimal in the relatively flat, peat-covered, pre-drainage landscape. In contrast, peat subsidence, canals, and levees have created local hydraulic gradients that increase seepage and surface-groundwater interactions. As a result, after losses by evaporation, the WCAs now lose nearly half their remaining water through seepage to coastal areas. In addition, the loss of peat through oxidation has accentuated groundwater losses by permitting movement of surface water downward. The thick