It should be noted that the water shed is almost entirely driven by rainfall. Historically, 75 % of Big Cypress' water came from rainfall, the other 25% flowing from the Kissimmee River and Lake Okeechhobee, on it's slow meander to the Gulf of Mexico and Florida Bay. With water flows now restricted through flood control measures and water delivery systems, what this means today is that there is no giant river or spring that gives us the water to fill the lakes, ponds, sloughs, marshes, and strands. Big Cypress swamp gets an average of 53 inches of rain each year, and the topography and substrate here keep the rain water from flowing quickly out of the watershed. This slow movement of water from inside the watershed to outside the watershed (the 10,000 islands and EVER) is called sheetflow. Because the water moves so slowly across the vast prairies and through the sloughs and strands, water has time to percolate into the limestone substrate, recharging the surficial aquifers. The Big Cypress Swamp forms a recognizable hydrological unit that encompasses a total area of about 1,568,000 acres. Past delineations of the watershed have been based on the area from which flows go south.
Klein et al. (1970) divide the Big Cypress into three reasonably distinct watersheds (subareas A, B, and C), although they recognize that during periods of high water there may be some surface water interchange. They defined the subareas largely on the basis of an analysis of the topographic features and the man-made structures dominating existing drainage patterns. Subarea A comprises nearly 120,000 ha (288,000 acres) and carries flows southeastward from Devil's Garden area in Hendry County into Conservation Area 3A northeast of the BCNP. Subarea B covers about 352,000 acres of the western portion of the Big Cypress and the Naples urban area. Surface water from this basin flows southward and westward, predominately through man-made structures, to the Ten Thousand Islands and the Gulf of Mexico. Since hydrological problems have resulted from rapid development of this area and because it is important to the Naples well fields, this part of the watershed has been the subject of numerous studies by private consultants and federal, state, and local agencies. Subarea C covers about 928,000 acres and lies primarily east of SR 29. It includes most of eastern Collier County and a portion of northern Monroe County, and drains southward through the western part of Everglades National Park. Most of the BCNP is within this subarea.(From "The Big Cypress National Preserve" by Michael J. Duever et al.)