annual rainfall varied from a low of 

 1,090 mm at Ortona Lock, west of Lake 

 Okeechobee, to a high of 1,691 mm in 

 South Miami (Thomas 1970), on the 

 eastern coastal ridge, from 1960 

 through 1969. In a typical year ap- 

 proximately 75 percent of the rain- 

 fall occurs from May through October. 

 In wet years more than 2,500 mm have 

 been received at some stations; 

 whereas in other years some stations 

 have received less than 1,000 mm. 



The high natural storage capac- 

 ity of south Florida dampens the ef- 

 fect of rainfall variation on runoff 

 to estuaries. Under natural condi- 

 tions, there can be a lag of several 

 months between peak rainfall and peak 

 discharge, because runoff does not 

 commence until certain storage 

 threshholds are reached. This damp- 

 ing effect reduces wet season runoff 

 and increases dry season runoff. 

 Storage of water on the land in shal- 

 low sheets results in high evapo- 

 transpiration, particularly during 

 the wet summer months, which reduces 

 the total quantity of runoff. These 

 relationships are described by the 

 mass balance equation of classical 

 hydrology: 



Runoff = Rainfall - Evapotrans- 

 piration ± "Changes in Storage" 



A main effect of the water man- 

 agement system in south Florida has 

 been to reduce storage capacity. Re- 

 moval of emergent vegetation, drain- 

 age of peat soils, and paving of land 

 in urban areas of south Florida also 

 have reduced the natural storage ca- 

 pacity. Increasing wet season dis- 

 charge and decreasing dry season dis- 

 charge to the estuaries have re- 

 sulted, causing negative impacts on 

 estuarine life (Browder 1977). Prob- 

 ably all south Florida estuaries have 

 been affected. In addition, some 

 estuaries have had the area of their 



watershed increased or decreased, ac- 

 centuating the disruption of the na- 

 tural runoff pattern. 



Everglades National Park Estuaries 



The subtropical estuaries of 

 Everglades National Park^ which cover 

 approximately 1,295 km , are the 

 nursery grounds for the pink shrimp 

 of the Dry Tortugas fishery. They 

 also provide nursery habitat for many 

 commercially and recreationally im- 

 portant fish species. The watersheds 

 of most of these estuaries have been 

 greatly reduced by water management 

 activities. The South Florida Water 

 Management District schedules re- 

 leases of water to the park in an 

 attempt to compensate for discharge 

 losses . 



The freshwater flows of two dis- 

 tinct drainage systems have been mod- 

 ified. One is the Shark Slough sys- 

 tem, which once received water from 

 the Kissimmee-Okeechobee-Everglades 

 drainage basin but now is limited to 

 the Shark Slough area inside the 

 park. The natural watershed was ap- 

 proximately 22,500 km . The present 

 drainage area is approximately 2,059 

 km , or one-tenth its previous size. 

 Receiving basins for this system are 

 Whitewater Bay,, a semi-enclosed estu- 

 ary of 219 km „ and a more open estu- 

 ary of 324 km at the mouths of the 

 Broad, Harney and Shark Rivers. The 

 park staff estimated that rainfall 

 and releases to this system in a re- 

 cent year amounted to 3.45 billion 

 cubic meters. 



The other system is northern 

 Florida Bay, fringed by small embay- 

 ments on its landward sides and a 

 submerged sill on its seaward side 

 that retards exchange with marine 



407 



