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.