The Natural System Model (NSM) simulates the hydrologic response of a pre-drained Everglades system. The NSM does not attempt to simulate the pre-drained hydrology; the data necessary to perform such a simulation do not exist. Rather, the use of recent climatic data, e.g., rainfall, potential evapotranspiration, tidal and inflow boundaries, allows for meaningful comparisons between the current managed system and the natural system under identical climatic conditions.
Prior to major drainage activities in South Florida that began in the late 1800s, the Everglades (south of Lake Okeechobee) consisted of approximately 4 million acres of subtropical wetlands and slow sheetflow that covered much of South Florida. This immense wetland system spread from the south shore of Lake Okeechobee to the mangrove estuaries of Florida Bay and Gulf of Mexico. The Immokalee Ridge to the west and the coastal ridges to the east generally mark the hydrologic boundaries of the historic Everglades, although numerous connections through the coastal ridge overflowed from the Everglades to the Atlantic Ocean.
In the early and mid-1900s, construction changes in the Kissimmee basin removed significant regional storage upstream of Lake Okeechobee. Regional drainage improvements reduced the lake's surface area and lowered storage depth with approximately 3 million acre-feet of storage lost. South of the lake, construction of the east coast protective levee system in the Everglades (water conservation areas) reduced the overall natural area by one-half and constrained water flow south from the lake. Today, water managers are challenged with moving vast volumes of water within a much smaller, managed system.