drought

Popcorn index
Salsa bowls run dry

It’s not quite as dry as popcorn yet …

But it’s getting there.

As show on the diagram above, you know it’s dry in the Big Cypress Swamp when the water table drops below ground surface in the center of a cypress dome. Or in other words, salsa dish is dry other than a thin film. That doesn’t mean the peat still isn’t moist — it is, or that alligator holes aren’t still holding water — they are. But in the next few weeks, we could be entering the “popcorn” dry mode as the water table continues to drop.

Backstory about the popcorn index, and how it became semi-official. About a decade ago we had a major wildfire in the swamp called the Mud Lake Complex. Our chief fire officer asked me to address the new team of fire fighters that were rotating in to get them oriented to the ecology of the swamp. I remember at the time being nervous, and not sure how to talk to such a large crowd without visual aids. The situation is complicated by the fact that fire fighters on any one incident are from all across the country. Thus it was my responsibility, in a 5-minute talk, to dial them into why such a green and lush swamp was so deceptively dry. Our chief fire officer couldn’t have been more clear, and gave me all the reassurance I needed: Use your corn chips and salsa analogy. And thus the popcorn index was born.

dry season

State of drought

It pours in south Florida …

When it rains.

Drought levels in Florida today, a month ago, and a year ago
KBDI drought index history for Collier County

But what about when it doesn’t rain? That’s how Florida got the moniker the Sunshine State. The thing is: Rain or drought doesn’t spread uniformly across the state. Case in point is difference between the panhandle (which is very wet) and the southern part of the state that is starting to dry out (top chart). Unlike the northern part of Florida, and in particularly the panhandle that gets the brunt of the continental winter and spring fronts — south Florida has very distinct wet and dry seasons. In the summer it pours. And then in the winter the rains turn off. In any event, it’s always fun to compare how drought unfolds across the state spatially and back in time over the years.

water table

Sogginess Factor
Soil moisture is the new hydroperiod

Hydroperiod is to the Everglades …

As soil moisture is to the Big Cypress.

Soil moisture: Now, a month ago and a year ago
Historical calendar of soil moisture

The reason? Soggy soils are an indicator that the water table hasn’t dropped too far below the ground. A high water table keeps the pump primed (so to speak) for a rapid return of surface water when the rains return. Bad things happen when the water table drops too low for too long. Peat starts to subside and marl soils become more susceptible to exotic plants. The soils are also more prone to saltwater intrusion near the coast and large wildfires all around. And to bring it full circle (as we do with the water cycle), the summer rains have to work that much harder to raise the water table back up above the land.

We all know that canals drain surface water during the summer, but the worst part is that they continue to drain the groundwater from out under the swamp in the winter even after the sheet flow has long since stopped.

water table

Splashy, mushy and crunchy
The three states of the swamp

Swamps are by definition wet …

But not always, and sometimes very much so.

The chart above shows the annual duration (in months) that Big Cypress National Preserve spends in the “splashy” and “crunchy” mode. Each summer and fall, waters rise high enough to make the swamp “splashy.” That means water is up high enough to splash at the base of the higher-ground slash pine tree trunks. On average that happens around 4 months per year (light blue), with one month on average being high enough to flood into the mesic pines islands (dark blue). Then there’s the crunchier half of the year when even the center of the cypress domes go dry (red). That happens on average about 1.5 months per year, and usually between March and May. Our driest (or crunchiest) spring on record was in Water Year 2010 when the swamp completely dried out for over 4 months. And then there are the years it doesn’t dry out at all (i.e. Water Years 2009 and 2016).

On average, the Preserve inhabits its extreme states of “splashy” and “crunchy” for almost 6 months per year, or about half of the time. As for the other half, the best way to describe it is “mushy:” a little crunchy dry ground, some splashy puddles and a lot of mushy (i.e. muddy) soils in between.

In summary, on average the swamp spends as much time in its extreme modes of flood and drought as it does its normal mushy period in between. The next big question in the swamp: How deep and long will this spring’s crunchy season be? In the meanwhile, we still have a few months of mushy season to enjoy.