Greater Everglades

Once a single interconnected system, the modern-day Everglades is a managed system of basins separated by levees and canals and connected by pumps, gates and the ongoing restoration and water management works. | Greater Everglades | Kissimmee | Okeechobee | Caloosahatchee | River of Grass | Big Cypress Swamp | Rainfall | Quick forecast | Drought outlook | Cheatsheets | Return to main blog

Intro - A Sum of Many Parts

One water cycle, many pieces, and lots of moving parts

By Robert V. Sobczak

Introduction

In Central Florida, rain collects in springs and lakes that flow into the Kissimmee River. The water then flows south into Lake Okeechobee to join water already stored. Lake Okeechobee is shaped like a very large, shallow bowl. Historically, when large amounts of water were collected in the lake, water would overflow its southern edge and flow into the Everglades. Today, water flow out of the lake is via canals that pass through the Everglades Agricultural Area. The water then arrives and flows through the Everglades before eventually reaching Florida Bay and the Gulf of Mexico. 

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The Everglades has a unique hydrologic, or water cycle. Throughout most of the continental United States, water levels generally rise and fall in tune with the four seasons. There, water levels typically peak during the spring as snow melts and front-driven storms move through, and ebb in the fall at the end of the hot summer. In contrast, the water cycle of subtropical south Florida and the Everglades is fueled by only two seasons—wet and dry—leading to a reversal of the typical seasonal high and low water levels. Water levels in the Everglades peak in the fall at the end of the wet season, and ebb in the spring when large expanses of wetlands dry out, at the end of the dry season. 

wet season

Summer Wet Season

The wet season typically begins in mid to late May and is characterized by hot and humid weather, daily buildup of spectacular cloud formations and resultant heavy thunderstorms that are often localized and short in duration. Other larger systems—including early season storms enhanced by lingering springtime instability in the upper atmosphere, mid-latitude cyclones and tropical storms—periodically spike the Everglades with regionally expansive rains.

In response to these meteorological inputs, the Everglades becomes flooded with an ankle to waist-deep, slow-moving pool of water throughout summer and fall, leaving only the high-ground tree islands and hardwood hammocks above water. The term sheet flow is used to describe this shallow and spatially expansive wetland plain that, unlike a lake or bog, flows like a stream, only much more slowly, almost imperceptible to the human eye. Spanning from horizon to horizon, this sheet of water flows south through a maze of tree-island-dotted ridges and sinuous low-lying sloughs, giving rise to the name River of Grass coined by Marjory Stoneman Douglas in 1947. 

dry season

Winter Dry Season

As the weather turns milder in October, the regular buildup of afternoon thundershowers and tropical storms ends, ushering in the dry season in mid to late November. As the dry season ensues, water levels continue to drop without much additional rainfall and more land emerges. Water first recedes from the highest elevation pinelands and other tree islands. Drainage of the marl prairies follows next, leading to an eventual retreat of water into the lowest-lying sloughs and marshes. The rate of recession may be slowed or even temporarily reversed by sporadic winter rains that are typically brought on by the descent of cold continental air masses from the north. Lower winter evaporation rates also hinder the rate of recession, though it rapidly picks up again in spring as daylight hours and air temperatures increase evaporation. South Florida is generally considered a wet area by merit of its abundant average annual rainfall, a total of 52.7 inches in the SFMWD region, with a 70/30 percent wet/dry season split.  Despite the often flooded wetland views, drought and wildfire play vital roles in maintaining the region’s unique assemblage of flora and fauna. The ecological health of the Everglades is intimately tied to seasonal and interannual fluctuations of the water cycle.

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Water Years 2021/2022 in Review
A brief comparison of the past two years

1. Water Year 2021 (May 1, 2020 to April 30, 2021)

Water Year 2021 classified as above average in terms or rainfall, recording 46 and 14 inches of wet and dry season rainfall, respectively, for a total of 60 inches.  The summer wet season started early with a June-like rain total in May (8 inches) and ended late with above average rains in October and a surprise storm in November.  Of note, the early start was not enough, or rather in time, to prevent a destructive incineration of an archipelago of hardwood hammocks in the southeast Corner of Big Cypress National Preserve called the Moon Fish Wildfire.  Within a week of the fire ending the May rains swept through.  Near normal rainfall persisted for core four months of the summer wet season (June through September) and were supplemented by a “wet season” like October.  But the real exclamation point came in November in the form of Tropical Storm Eta, filling the Everglades and Big Cypress Swamp to levels last seen in September 2017 from Hurricane Irma, only shifted forward two months to a time when water levels are usually well past their October peak. 

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The late season highwater stand set the stage for or prolonged and steady winter recession that proved to be a boon to wading bird communities in terms of foraging and nesting.  Despite expectations of a wet dry season from the bumper crop of summer and late fall rain, the Big Cypress Swamp dropped into deep drought by April’s end.  Although no similar wildfires occurred, Water Year 2021 proved an important restoration point:  No matter how wet the wet season or the beginning of the dry season, without timely April and May rains the Big Cypress Swamp is especially prone to dropping into deep, unnatural drought due to perimeter and interior canals that stifle the spread of sheet flow and hasten its spring demise. 

2. Water Year 2022 (May 1, 2021 to April 30, 2022)

Despite the previous year’s bountiful rains, Water Year 2022 took started slow thank to subpar rains in May – extending drought conditions into June and even July in some area.  However, the four core seasons of the summer wet season (June through September) and October all charted in with average rainfall.  For a second year in a row, November provided an unexpected boost with twice its normal rainfall amount. Again, despite the surplus of water at the dry season’s start, the Big Cypress Swamp was poised to drop into deep spring drought, imperiling the habits that so vitally depend on natural fire breaks staying wet, when a string of continental fronts at the middle and end of April and start of May boosted the water table just when it needed it most.  South Florida received over 10 inches less annual rainfall and 4 inches less dry season rain than the prior year, but it is as much an issue of timing as it is the total amount. Water Year 2021 classified as low normal in terms or rainfall, recording 36 and 12 inches of wet and dry season rainfall, respectively, for an annual total of 48 inches. 

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Did You Know: Up north on the continent, the water year starts October 1st and ends September 30th in most areas.

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Water Year 2020

Water Year 2020 (May 1, 2019 to April 30, 2020)

WY 2020 started with a normal onset of wet season rains only to be derailed into a drier than normal condition due to a record-low rainfall in September, largely as a result of several large tropical systems that disrupted the summer pattern. Similar to the previous water year, WY 2020 dry season started earlier than normal as a result (Figure 1). However, differing from WY 2019, a series of winter storms failed to materialize. Cold fronts proved too infrequent and lacked sufficient moisture to slow the steady decline of the water table. The virtual lack of any rainfall for the entirety of March sealed the region’s descent into a deep and prolonged drought. Wildfires erupted in the Big Cypress and Everglades in April and May and proved hard to contain due to the loss of surface and shallow ground water from the region’s normally wet soils. The wildfires threatened and, in some cases, significantly impacted, large areas of the ecosystem as well as threatened and endangered species found exclusively in both deep slough and upland habitats. 

The Everglades and Big Cypress Swamp are flood and fire-adapted ecosystems in which every square inch of flora and fauna depend on a regular return interval and dosage of both flood and fire. In WY 2020, a few months proved pivotal in tilting the region in favor of drought and the wildfires that ensued.

Of interest, both WYs 2019 and 2020, as judged by their annual rainfall of 52 and 48 inches, would appear at first glance to have fallen squarely within the normal 45 to 58 inches of rainfall window.  Yet, a closer look reveals a two-year period that was plagued by a continual threat and eventual demise into an ecologically damaging and financially costly drought cycle. 

Water Year 2019
Big expectations and quick shifts

Water Year 2019 (May 1, 2018 to April 30, 2019)

WY 2019 started with prodigious rains in the May, the wet season’s opening month.  Usually a time of transition when both the water table bottoms out and the regular pattern of afternoon thunderstorms begins—resulting in the gradual rise of the water through the Everglades and Big Cypress Swamp—the triple dose of May’s usual rainfall allotment quickly jumped the water table up into the cypress and sawgrass plain. Initial expectations of a flood year resembling the Hurricane-Irma fueled 2017 wet season failed to materialize as summer rains fizzled early, recording only six inches of combined rainfall in September and October compared to the normal 12 inches. The result was an early start to the dry season. Surface water had all but disappeared from the Big Cypress Swamp by January, setting the stage for a deep and prolonged dry season, when a series of storms flooded the cypress back to July levels.

WY 2019 exemplified the seasonally predictable, yet mercurial, nature of the south Florida weather cycle. What was expected to be a “wet” wet season turned dry, and what looked to be a “dry” dry season turned wet. Shifts between flood and drought can occur quickly both within and during the transition between the approximate 6-month long wet and dry seasons.

Water Year 2018

Water Year 2018 (May 1, 2017 to April 30, 2018)

Water Year 2018 was a year of wet and dry extremes: featuring a “record rainy” summer wet season and, in a repeat of the previous year and a “below average” dry season as follows — a whopping 55 inches of rain falling across south Florida during the six-month (May through October) summer wet season (the long-term average is 38 inches) and 8 inches falling in the six month (November through April) winter dry season that followed, for an annual total of 64 inches. 

Despite the bountiful summer rains, a sweltering May actually started Water Year 2018 off on a rather dry note with a continuation and deepening of the drought from the previous water.  Reminiscent of the saying “all droughts end in flood,” an epic three-day onslaught of rain in early June ushered in an “instant” wet season across all of south Florida, and set the stage for the record-rainy wet season to come.  Abundant tropical moisture and regular afternoon storms combined with the exclamation points of Tropical Storm Emily, Hurricane Irma, and Tropical Storm Philippe to produce a wet season that went down in the history books with rarely seen events, including water sheet flowing over a few miles of Turner River and Wagonwheel Roads after the June deluge and a brief overtopping the Tamiami Trail between Forty and Fifty Mile Bend in the days following Irma for the first time since 1995.

The “instant” wet season stayed well above average for most areas of the ecosystem from June into the January, peaking for much of the summer 1-2 feet above normal levels, causing even high-ground tree islands in the Everglades and pine flatwoods of the Big Cypress to submerge for multiple months. 

A series of emergency measures were taken to alleviate the unusual bounty of summer water, some of which had negative connotations ― such as mandatory releases from Lake Okeechobee to the Caloosahatchee and St. Lucie estuaries to protect the integrity of the lakes perimeter levee as it is being repaired ― whereas other were pursed on a positive Everglades Restoration note.  Most notable in that regard were three efforts focused on spreading the water out: (1) to the East, water managers sent water through the new one mile bridge into Everglades National Park’s Northeast Shark River at an unprecedented scale, (2) to the West, the newly-constructed Merritt Pump Station went into action to spread water into downstream Picayune Strand, and (3) in the Center, a series of pumps was utilized for a second straight year in an to send water west across the L-28 levee into Big Cypress National Preserve. 

The meteorologic pendulum swung to the dry side of the spectrum for the winter, producing both good and bad results.  On the positive side, the paucity of winter rains (combined with a high summer climb) set the stage for in a remarkably steady and prolonged water recession that sparked a frenzy of foraging and nesting activity among wading birds across the Everglades.  Super colonies of wading birds were observed for the first time in Everglades National Park in decades.  Wood stork rookeries were reported in Big Cypress National Preserve for the first time since the 1990s.  On the negative side, the Big Cypress half of the ecosystem was plagued by unusually long wildfire season as result of the lack of timely winter rains.  Fires in Picayune and Big Cypress National Preserve generated plume clouds of smoke across the region throughout March, April and into May.