Development of Water-Management System and Impact on the Hydrology of Southeastern Florida : Ground-Water Level Changes Over Time

Temporal and spatial water-level changes that occurred during the last 60 years of the 20th century are depicted in figures 42, 43, and 44. Specifically, these maps spatially portray “5-year average” temporal water-level differences that occurred between 1940-44 and 1990-94, 1940-44 and 1970-74, and 1970-74 and 1990-94 during April (dry season) and October (wet season).

Average ground-water levels declined in large inland areas in April 1990-94 and October 1990-94 in comparison to those conditions observed in April 1940-44 and October 1940-44 (fig. 42). The water-level declines are attributable to water-management drainage efforts to control urban and agricultural flooding. The departure in monthly mean average dry-season precipitation from normal conditions in April 1940-44 and 1990-94 and the prior months was similar in magnitude (fig. 45). This contrasts considerably with the departure from normal wet-season (October) rainfall that was considerably below average in 1940-44 when compared to 1990-94. On a spatial basis, however, water levels in 1990-94 appear to have been 1 to 3 ft lower over a wide area in Miami-Dade and Broward Counties. This spatial pattern illustrates that management controls had a far greater effect on ground-water levels than temporal rainfall patterns, and is indicative of the permanent change that occurred as part of a highly controlled drainage system infrastructure.

Figure 42. Average difference in water levels between (A) April 1940-44 and April 1990-94, and (B) October 1940-44 and October 1990-94. Derived from files of the U.S. Geological Survey. [larger image]

Figure 43. Average difference in water levels between (A) April 1940-44 and April 1970-74, and (B) October 1940-44 and October 1970-74. Derived from the files of the U.S. Geological Survey. [larger image]

Figure 44. Average difference in water levels between (A) April 1970-74 and April 1990-94, and (B) October 1970-74 and October 1990-94. Derived from the files of the U.S. Geological Survey. [larger image]

Figure 45. Average departure in rainfall from monthly mean average conditions at 14 sites in southeastern Florida. Data from the National Oceanographic and Atmospheric Administration. [larger image]

Average ground-water levels along the coast were higher in 1990-94 than in 1940-44, which probably is the result of long-term management efforts to reduce saltwater intrusion in the surficial aquifer system. Comparison of average water levels in 1940-44 and 1970-74 indicates similar distributions in water-level change (fig. 43). Generally, canal management and water levels in the surficial aquifer system had been established during the 1970s. The similarity in the distribution of water-level change between maps (figs. 43) suggests that management controls on water levels in the surficial aquifer system had matured by 1990-94.

A comparison of 1970-74 and 1990-94 average ground-water levels (fig. 44) shows that average dry-season ground-water levels in April 1990-94 remained constant or were only 1 or 2 ft higher than 1970-74 ground-water levels in most of Miami-Dade and southern Broward Counties. In northern Palm Beach County, 1990-94 average wet-season ground-water levels were higher than in 1970-74, except near the coast and along a corridor between the L-8 and L-10 Canals (fig. 44).

Differences in average ground-water level conditions in northern Palm Beach County during the dry season in October 1970-74 and 1990-94 are similar to the wet-season differences observed during the same time period (fig. 44), presumably resulting from modifications that were made in water-management practices. Average dry-season water levels that occurred in the 1990s remained unchanged or were slightly lower in developed areas of Miami-Dade, Broward, and southern Palm Beach Counties.

The volume of water lost by long-term drainage of ground water can be estimated by multiplying storativity (specific yield), surface area of the drained aquifer, and the average decline in head (figs. 42 and 43). The specific yield of the Biscayne aquifer has been reported to range from 10 to 35 percent in Broward and Miami-Dade Counties (Schroeder and others, 1958), with a similar estimate that has been assumed for near-surface water-bearing rocks in Palm Beach County. About 2 to 9 billion gallons of ground water were lost permanently from the system through aquifer drainage between 1940-44 and 1970-74. A comparison of April 1940-44 and 1990-94 water levels suggests that the Biscayne aquifer lost between 1.6 and 7.4 billion gallons of water between these periods within the area shown in figures 42 and 43. This water loss only includes drainage of the Biscayne aquifer and does not include water loss resulting from reclamation of overlying wetland areas.

Short-term fluctuations in water levels in the surficial aquifer system result from daily to seasonal variations in recharge and discharge in the aquifer system. Generally, the surficial aquifer system is recharged directly by precipitation through a moderately thin soil cover or by seepage from canals that are hydraulically well connected to the aquifer system. The biannual variation in water levels in the surficial aquifer system can range from 2 to 10 ft/yr in response to seasonal (dry or wet) rainfall patterns. Ground-water levels generally are highest during the latter part of the wet season and the earliest part of the dry season (September-December). Lowest ground-water levels occur during the end of the dry season and the earliest part of the wet season (April-June). Extremely low water levels occur during lengthy periods of below-normal precipitation, or in areas that have been affected by the drawdown of municipal and agricultural wells.

Construction and completion of the Everglades-South Dade Conveyance System served to dampen the seasonal variability of ground-water levels in the 1980s. This conveyance system diminished the seasonal amplitude in ground-water levels, thereby lessening the observed water-level extremes during flood and drought conditions as seen at well S-196A (fig. 46A). Similar relations are observed in nearby well G-789 (fig. 46B).

Figure 46. Dampening of variations in ground-water levels following emplacement of the Everglades-South Dade Conveyance System. Derived from the files of the U.S. Geological Survey. [larger image]